Silverado 0a Theory & Operation

2003 ENGINE PERFORMANCE

Theory & Operation - 4.3L Sierra & Silverado

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

AIR INDUCTION SYSTEMS

AIRFLOW SENSING

Speed Density

Engine is equipped with a Mass Airflow (MAF) sensor, a Manifold Absolute Pressure (MAP) sensor and an Intake Air Temperature (IAT) sensor. See Fig. 1 or Fig. 6 .

The MAF sensor is an airflow meter that measures the amount of air entering the engine. The PCM uses the MAF sensor signal to provide the correct fuel delivery for all engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle condition. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit and a signal circuit.

The MAP sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits: 5-volt reference, low reference circuit and MAP sensor signal circuit. The PCM supplies 5 volts to the MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. MAP sensor provides a signal to PCM on MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off, or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when the ignition is on, with engine off. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range.

The IAT sensor is a variable resistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose DTC, go to SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

2003 Chevrolet Silverado 1500

2003 ENGINE PERFORMANCE Theory & Operation - 4.3L Sierra & Silverado



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April-29-08 3:52:37 PM Page 1 © 2005 Mitchell Repair Information Company, LLC.

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Fig. 1: Locating Intake Air Temperature & MAF Sensors Courtesy of GENERAL MOTORS CORP.

COMPUTERIZED ENGINE CONTROLS

The computerized engine control system monitors and controls a variety of engine/vehicle functions. The computerized engine control system is primarily an emission control system designed to maintain a 14.7:1 air/fuel ratio under most operating conditions. When the ideal air/fuel ratio is maintained, the Three-Way Catalytic (TWC) converter can control Oxides Of Nitrogen (NOx), Hydrocarbon (HC) and Carbon Monoxide (CO) emissions.

The computerized engine control system consists of engine PCM, input devices (sensor and switch input signals) and output signals.

POWERTRAIN CONTROL MODULE



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The PCM has a "learning" ability which allows it to make minor corrections for fuel system variations. PCM is located on left front of engine compartment, behind radiator. See Fig. 2 . If battery power is interrupted, a vehicle performance change may be noticed. PCM module corrects itself, and normal performance returns if vehicle is allowed to "relearn" optimum control conditions. "Relearning" occurs when vehicle is driven at normal operating temperature under part throttle, moderate acceleration and idle conditions.

Fig. 2: Locating PCM Courtesy of GENERAL MOTORS CORP.

INPUT DEVICES

NOTE: Components are grouped into 2 categories. The first category is INPUT DEVICES, consisting of components which control or produce voltage signals



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A/C On/Request Signal

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a class 2 message to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil.

Battery Voltage

Battery voltage is monitored by PCM. If battery voltage swings low, a weak spark or improper fuel control may result. To compensate for low battery voltage, PCM may increase idle speed, advance ignition timing, increase ignition dwell or richen the air/fuel mixture. If voltage swings high, PCM may set a charging system fault code and turn on Malfunction Indicator Light (MIL). If voltage signal swings excessively low (less than 9 volts) or excessively high (16 volts), PCM shuts down for as long as condition exists. If condition is short-term, MIL flickers and vehicle may stumble. Vehicle stalls if condition persists.

Brake Switch Feedback

On models equipped with cruise control system, PCM may monitor the brake switch circuit to determine when to engage and disengage cruise control.

Camshaft Position Sensor

The Camshaft Position (CMP) sensor is a hall-effect sensor located in the ignition distributor base and uses the same type of circuits as the CKP sensor. See Fig. 3 . The CMP sensor signal is a digital ON/OFF pulse, output once per revolution of the camshaft. The CMP sensor information is used by the PCM to determine the position of the valve train relative to the CKP. If PCM does not detect the CMP signal while the engine is running, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

monitored by the control unit. The second category is OUTPUT SIGNALS, consisting of components controlled by the PCM.



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Fig. 3: Locating Camshaft Position Sensor & Knock Sensor Courtesy of GENERAL MOTORS CORP.

Cranking Signal

Cranking signal is a 12-volt signal monitored by the PCM. Signal is present when ignition switch is in the START position. The PCM uses this signal to determine the need for starting enrichment. PCM also cancels diagnostics until engine is running and 12-volt signal is no longer present.

Crankshaft Position Sensor

The Crankshaft Position Sensor (CKP) sensor is a 3-wire sensor based on the magneto resistive principle. A magneto resistive sensor uses 2 magnetic pickups between a permanent magnet. As an element such as a reluctor wheel passes the magnets the resulting change in the magnetic field is used by the sensor electronics to produce a digital output pulse. The PCM supplies a 12-volt, low reference, and signal circuit to the CKP sensor. The sensor returns a digital ON/OFF pulse 4 times per crankshaft revolution. The CKP sensor reads the crankshaft mounted reluctor wheel to identify pairs of cylinders at Top Dead Center (TDC). See Fig. 4 . If PCM detects that the CKP sensor signal is incorrect, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.



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Fig. 4: Locating Crankshaft Position Sensor Courtesy of GENERAL MOTORS CORP.

Engine Coolant Temperature Sensor



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The Engine Coolant Temperature (ECT) sensor is a variable resistor, that measures the temperature of engine coolant. ECT sensor is located on side of left cylinder head. See Fig. 5 . The PCM supplies 5 volts to the ECT signal circuit and ground for the ECT low reference circuit. When ECT is cold, sensor resistance is high. When ECT increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on ECT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the ECT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

Fig. 5: Locating Engine Coolant Temperature Sensor Courtesy of GENERAL MOTORS CORP.

Fuel Pump Feedback



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The PCM monitors the voltage on the fuel pump relay control circuit. If control module detects an incorrect voltage on the fuel pump relay control circuit, a fuel pump relay control DTC sets. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

Fuel Tank Pressure Sensor

See EMISSION SYSTEMS & SUB-SYSTEMS .

Intake Air Temperature Sensor

The Intake Air Temperature (IAT) sensor is a variable resistor. IAT sensor is located in fresh air intake tube. See Fig. 1 . The IAT sensor has a signal and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, the sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. Failure in IAT sensor circuit should set a related DTC. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

Knock Sensor

The Knock Sensor (KS) produces an AC voltage at all engine speeds and loads. KS is located on top rear of engine. See Fig. 3 . The PCM adjusts the spark timing based on the amplitude and frequency of the KS signal. PCM uses the KS signal to calculate the average voltage. Then PCM assigns a voltage value. PCM checks the knock sensor and related wiring by comparing the actual knock signal to the assigned voltage range. A normal KS signal should stay within the assigned voltage range. A fault in the KS circuit may set a DTC. See SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

Manifold Absolute Pressure Sensor

The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. MAP sensor is mounted on top of intake manifold. See Fig. 6 . The pressure changes occur based on engine load. The MAP sensor has a 5-volt reference circuit, a low reference circuit and a signal circuit. The PCM supplies 5 volts to MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when ignition is on, with engine off. The BARO reading may also be updated whenever engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If PCM detects a MAP sensor signal voltage that is out-of-range, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

If MAP sensor fails, the PCM substitutes a fixed MAP value, and uses the TP sensor to control fuel delivery. A fault in the MAP circuit should set a related diagnostic trouble code.



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Fig. 6: Locating EGR, EVAP Canister Purge Solenoid & MAP Sensor Courtesy of GENERAL MOTORS CORP.

Heated Oxygen Sensor

Heated Oxygen Sensor (HO2S) is used for fuel control and post catalyst monitoring. Each HO2S compares oxygen content of he surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The PCM supplies the HO2S with a reference or bias voltage of about 450 mV. When engine is first started, PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperatures and closed loop is achieved, the HO2S generates a voltage within a range of 0-1000 mV that fluctuates at greater than or less than bias voltage. High HO2S voltage indicates a rich exhaust stream. Low HO2S voltage indicates a lean exhaust stream.

CAUTION: Measure oxygen sensor voltage with a digital volt-ohmmeter (minimum 10-megohm impedance) only. Current drain of a conventional voltmeter could damage sensor.



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Throttle Position Sensor

The Throttle Position (TP) sensor is used by the PCM to determine the throttle plate angle for various engine management systems. TP sensor is mounted on throttle body assembly. See Fig. 7 . The TP sensor is a potentiometer type sensor with a 5-volt reference circuit, a low reference circuit and a sensor signal circuit. PCM provides the TP sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to Wide Open Throttle (WOT) position provides the PCM with a signal voltage from less than one volt to greater than 4 volts through the TP sensor signal circuit. When TP sensor signal voltage is not within the predicted range, a DTC sets. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

Fig. 7: Locating IAC Valve, Ignition Coil, Knock Sensor & TP Sensor



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Courtesy of GENERAL MOTORS CORP.

Transmission Fluid Temperature Sensor

The automatic Transmission Fluid Temperature (TFT) sensor is part of the automatic Transmission Fluid Pressure (TFP) manual valve position switch. The TFT sensor is a resistor or thermistor, which changes value based on temperature. The sensor has a negative temperature coefficient. This means that as the temperature increases, resistance decreases and as temperature decreases, resistance increases. The PCM supplies a 5-volt reference signal to TFT sensor and measures voltage drop in the circuit. When transmission fluid is cold, sensor resistance is high and PCM detects high signal voltage. As fluid temperature warms to a normal operating temperature, resistance becomes less and signal voltage decreases.

PCM uses TFT sensor input to control Torque Converter Clutch (TCC) application and shift quality. Sensor circuit problem should set a related DTC.

Transmission Range Switch

The Transmission Range (TR) switch is part of the Park/Neutral Position (PNP) and backup light switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains 4 internal switches that indicate the transmission gear range selector lever position. The PCM supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. The PCM detects the selected gear range by deciphering the combination of the voltage signals. PCM compares actual voltage combination of the switch signals to a TR switch combination chart stored in memory.

Vehicle Speed Sensor

The Vehicle Speed Sensor (VSS) assembly provides vehicle speed information to the PCM. The VSS assembly is a Permanent Magnet (PM) generator. The PM generator produces a pulsing AC voltage as rotor teeth on the transmission output shaft pass through the sensor's magnetic field. The AC voltage level and the number of pulses increase as the speed of the vehicle increases. Output voltage varies with speed from a minimum of 0.5 volt at 100 RPM to more than 100 volts at 8000 RPM. PCM converts the pulsing voltage to vehicle speed and uses vehicle speed signal to determine shift timing and TCC scheduling.

OUTPUT SIGNALS

A/C Clutch Relay

See MISCELLANEOUS PCM CONTROLS .

Cruise Control


Electronic Ignition



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See IGNITION SYSTEMS .

EVAP Canister

See EVAP CANISTER under EVAPORATIVE EMISSION SYSTEM.

EVAP Purge Solenoid

See EVAP PURGE SOLENOID under EVAPORATIVE EMISSION SYSTEM.

EVAP Vent Solenoid

See EVAP VENT SOLENOID under EVAPORATIVE EMISSION SYSTEM.

Fuel Injectors

See FUEL CONTROL under FUEL SYSTEMS.

Fuel Pump & Fuel Pump Relay

See FUEL DELIVERY under FUEL SYSTEMS.

Idle Air Control Valve

See IDLE SPEED under FUEL SYSTEMS.

Malfunction Indicator Light

See SELF-DIAGNOSTIC SYSTEM .

Self-Diagnostics


Serial Data


Shift Solenoids (Electronic Transmission)

See MISCELLANEOUS CONTROLS .

Torque Converter Clutch


FUEL SYSTEMS



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FUEL DELIVERY

Fuel Pressure Regulator

Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator maintains a pressure of 55-62 psi (379-427 kPa) under all operating conditions. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.

Fuel Pump

An in-tank, electric fuel pump delivers fuel to injector(s) through an in-line fuel filter. The pump is designed to supply fuel pressure in excess of vehicle requirements. The pressure relief valve controls maximum fuel pump pressure. Pressure regulator keeps fuel available to injector(s) at a constant pressure. Excess fuel is returned to fuel tank through pressure regulator return line.

When ignition switch is turned to ON position, PCM turns on electric fuel pump by energizing fuel pump relay. PCM keeps pump on if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off within 2 seconds after ignition is turned on.

Fuel Pump Relay

When ignition switch is turned to ON position, PCM turns electric fuel pump on by energizing fuel pump relay. PCM keeps relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off.

For additional information on fuel pump activation, see SYSTEM & COMPONENT TESTING - 4.3L SIERRA & SILVERADO article.

FUEL CONTROL

The PCM, using input signals, determines adjustments to the air/fuel mixture to provide the optimum ratio for proper combustion under all operating conditions. Fuel control systems can operate in the open loop or closed loop mode.

Closed Loop Mode

When HO2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, PCM operates in closed loop mode. In closed loop mode, PCM controls air/fuel ratio based upon HO2S signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If HO2S cools off (due to excessive idling) or a fault occurs in HO2S circuit, vehicle will re-enter open loop mode.

Fuel System Operating Modes

Internal PCM calibration controls fuel delivery during starting, clear flood mode, deceleration and heavy acceleration.



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Starting When ignition is first turned ON, the control module energizes the fuel pump relay for 2 seconds. This allows fuel pump to build pressure in the fuel system. The control module calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system stays in starting mode until engine speed reaches a predetermined RPM.

Clear Flood If engine floods, clear engine by pressing the accelerator pedal down to floor and then crank engine. When TP sensor is at Wide Open Throttle (WOT), the control module reduces the fuel injector pulse width in order to increase the air to fuel ratio. The control module holds this injector rate as long as the throttle stays wide open and engine speed is below a predetermined RPM. If throttle is not held wide open, the control module returns to starting mode.

Run Mode The run mode has 2 conditions called open loop and closed loop. When engine is first started and engine speed is above a predetermined RPM, the system begins open loop operation. The control module ignores the signal from the HO2S. The control module calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP, and TP sensors. The system stays in open loop until meeting the following conditions: The HO2S has varying voltage output, showing that the HO2S is hot enough to operate properly, ECT sensor is above a specified temperature and specific amount of time has elapsed after starting engine.

Specific values for the conditions exist for each different engine and are stored in the Electrically Erasable Programmable Read-Only Memory (EEPROM). The system begins closed loop operation after reaching these values. In closed loop, the control module calculates the air/fuel ratio, injector ON time, based upon the signal from various sensors, but mainly from the HO2S. This allows air/fuel ratio to stay very close to 14.7:1.

Acceleration When driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly. To prevent possible hesitation, the control module increases the pulse width to the injectors to provide extra fuel during acceleration. This is also known as power enrichment. The control module determines the amount of fuel required based upon TP, ECT, MAP, MAF and engine speed.

Deceleration When driver releases the accelerator pedal, air flow into the engine is reduced. The control module monitors the corresponding changes in the TP, MAP and MAF. The control module shuts OFF fuel completely if the deceleration is very rapid or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.

Battery Voltage Correction PCM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM.



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PCM is able to perform these commands because of a built-in memory/learning function.

Fuel Cut-Off When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Some models may also cut off fuel injector signals during periods of sudden, closed throttle deceleration (when fuel is not needed).

Fuel Trim The control module controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The control module monitors the HO2S signal voltage while in closed loop and regulates the fuel delivery by adjusting the pulse width of the injectors based on this signal. The ideal fuel trim values are around zero percent for both short and long term fuel trim. A positive fuel trim value indicates the control module is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the control module is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The control module selects the cells based on the engine speed and engine load. If the control module detects an excessively lean or rich condition, the control module will set a fuel trim DTC.

Open Loop Mode

When engine is cold and engine speed is greater than 400 RPM, PCM operates in open loop mode. In open loop mode, PCM calculates air/fuel ratio based upon coolant temperature and MAP sensor readings. Engine remains in open loop mode until oxygen sensor reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has elapsed after engine starts.

Sequential Fuel Injection System

Fuel injectors are pulsed sequentially in spark plug firing order. Constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by amount of time injector stays open (pulse width). Various sensors provide information to the PCM to control pulse width.

IDLE SPEED

PCM controls engine idle speed depending upon engine operating conditions. PCM senses engine operating conditions and determines best idle speed.




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The engine idle speed is controlled by the Idle Air Control (IAC) valve. IAC valve is mounted on throttle body. IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is supplied current by two circuits from the PCM. When PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition is turned off. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If engine idle speed is out of range for a calibrated period of time, an idle speed related DTC will set. To diagnose, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

IGNITION SYSTEMS

DISTRIBUTOR IGNITION SYSTEM

The Distributor Ignition (DI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy and control of exhaust emissions. This ignition system consists of a single ignition coil and Ignition Control Module (ICM). Spark energy is delivered via a distributor cap, rotor, and secondary spark plug wires. The driver module within the ICM is commanded to operate the coil by the PCM, that has complete control over spark timing. The DI system consists of the CKP, CMP, ignition coil and ICM, secondary ignition and PCM.

IGNITION TIMING CONTROL

Ignition spark timing and ignition dwell time are controlled entirely by the PCM. The PCM monitors information from various engine sensors, computes the desired spark timing and dwell, and firing of the ignition coil via ignition control circuit to the coil driver.

EMISSION SYSTEMS & SUB-SYSTEMS

CATALYTIC CONVERTER

A Three-Way Catalytic (TWC) converter is used to reduce exhaust emissions. This type of converter can reduce Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides Of Nitrogen (NOx).

EVAPORATIVE EMISSION SYSTEM

The Evaporative Emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, fresh air is



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drawn through the vent solenoid and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

EVAP Canister

The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.

EVAP Purge Solenoid

The EVAP purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. EVAP purge solenoid is located on top of intake manifold. See Fig. 6 . This normally closed solenoid is Pulse Width Modulated (PWM) by the control module to precisely control the flow of fuel vapor to engine. The solenoid will also be opened during some portions of EVAP testing, allowing engine vacuum to enter the EVAP system.

EVAP Vent Solenoid

The EVAP vent solenoid controls fresh air flow into the EVAP canister. EVAP vent solenoid is mounted on frame rail, near fuel tank. See Fig. 8 . The solenoid is normally open. The control module will command the solenoid closed during some EVAP tests, allowing the system to be tested for leaks.



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Fig. 8: Locating EVAP Canister & EVAP Vent Solenoid Courtesy of GENERAL MOTORS CORP.


The Fuel Tank Pressure (FTP) sensor measures the difference between the pressure or vacuum in fuel tank and outside air pressure. The FTP sensor is located on top of fuel tank. The control module provides a 5-volt reference and a ground to FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. As FTP increases, FTP sensor voltage decreases, high pressure equals low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum equals high voltage.

POSITIVE CRANKCASE VENTILATION

The Positive Crankcase Ventilation (PCV) system provides effective evacuation of crankcase vapors. Fresh air from the air filter housing is supplied to the crankcase, where it is mixed with blow-by gases and passed through the PCV valve and into the intake manifold. This mixture is then passed into the combustion chamber



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and burned.

The PCV valve provides primary control in this system by metering the flow (according to manifold vacuum) of the blow-by vapors. When manifold vacuum is high (at idle), the PCV valve restricts the flow to maintain a smooth idle.

Under conditions in which abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), system is designed to allow excess gases to flow back through crankcase vent hose into air inlet.

Spring pressure holds PCV valve closed when engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, a condition which could result in hard starting.

During engine operation, manifold vacuum pulls the valve closed against spring pressure. As vacuum decreases with increased engine load, spring pressure begins to overpower vacuum strength. This allows PCV valve to open proportional to engine load and evacuation requirements. Should the engine backfire, the PCV valve closes to prevent ignition of fumes in the crankcase.

SELF-DIAGNOSTIC SYSTEM

The PCM is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, PCM will illuminate the MIL located on instrument cluster. When a malfunction is detected and MIL is turned on, a corresponding DTC will be stored in PCM memory. Malfunctions are designated as either "emission related" or as "non-emission related", and are divided into 4 code types to identify type of fault. The 4 code types are defined as follows:

Type "A" Emission related faults that illuminate MIL at first occurrence of a fail condition. PCM records the operating conditions at the time diagnostic failed into Failure Records and Freeze Frame.

Type "B" Emission related faults that illuminate MIL if a fault occurs in 2 consecutive ignition cycles. PCM records the operating conditions at the time diagnostic failed the first time into Failure Records and Freeze Frame. When diagnostic fails a second time, PCM records the operating conditions at the time diagnostic failed into Freeze frame and updates Failure Records.

Type "C" Non-emission related faults that do not illuminate MIL but the DTC will be recorded in memory. PCM records the operating conditions at the time diagnostic failed into Failure Records, no Freeze Frame information will be saved. Driver information center (if equipped) may display a message.

A current DTC clears when the diagnostic runs and passes. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other diagnostic. Intermittent failures may be caused by sensor, connector or wiring related problems. See TROUBLE SHOOTING - NO CODES - 4.3L SIERRA & SILVERADO article.



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MALFUNCTION INDICATOR LIGHT

As a bulb and system check, the MIL will illuminate when ignition switch is turned to ON position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. MIL may be used on some models to display a stored DTC. To access DTCs, see SELF-DIAGNOSTICS - 4.3L SIERRA & SILVERADO article.

SERIAL DATA

PCM has a serial data line. Serial data is a stream of electrical impulses which can be exchanged between control modules. Serial data can be interpreted using a scan tool. Access serial data by connecting a scan tool to Data Link Connector (DLC). Update intervals and information contained within data stream vary with model application.

MISCELLANEOUS CONTROLS

A/C CLUTCH

PCM regulates operation of the A/C clutch through a relay. The PCM disengages the A/C compressor when compressor load on engine may cause driveability problems (i.e., during hot restart, idle, low speed steering maneuvers and wide open throttle operation) or if A/C refrigerant pressure drops to less than or rises to greater than normal operating levels.

Refrigerant pressure is sensed through the monitoring of high and/or low pressure switch(es) or a pressure sensor which registers either high or low pressure levels. Hot restart is monitored through the Engine Coolant Temperature (ECT) sensor. For component application and related wiring, see appropriate A/C COMPRESSOR CLUTCH CONTROLS article in AIR CONDITIONING & HEATING.

A/C Pressure Switches

A/C high and low pressure switches may be used in the A/C compressor clutch or compressor clutch relay circuit. Switches are normally closed, completing the circuit which energizes the compressor clutch. When system refrigerant pressure increases beyond a certain point, high side switch opens, causing compressor clutch to disengage.

If system refrigerant level decreases (causing refrigerant pressure to drop), low side pressure switch opens, preventing compressor damage by causing compressor clutch to disengage.

CRUISE CONTROL

On models equipped with cruise control, the system is operated by the PCM. PCM receives inputs from Vehicle Speed Sensor (VSS), servo diaphragm, cruise control switch and brake release switch. Based on these inputs,

NOTE: Although not considered true engine performance-related systems, some devices may affect driveability if they malfunction.



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PCM controls position of cruise control stepper motor. PCM prevents system engagement at speeds of less than 25 MPH. PCM is not serviceable; if defective, it must be replaced. A system fault is stored as a DTC in PCM memory.

TRANSMISSION


The transmission Torque Converter Clutch (TCC) eliminates power loss of torque converter stage when vehicle is in a cruise condition, allowing driver the convenience of an automatic transmission while providing the fuel economy of a manual transmission.

When vehicle speed is great enough (about 20-45 MPH as indicated by vehicle speed sensor), PCM energizes TCC solenoid mounted in transmission, allowing torque converter to directly connect engine to the transmission. When operating conditions indicate transmission should operate as normal, TCC solenoid is de-energized, allowing transmission to return to normal automatic operation. Since power for the TCC solenoid is delivered through the brake switch, transmission also returns to normal automatic operation when brake pedal is depressed.

Electronic Transmission

PCM controls transmission and other vehicle functions. PCM monitors a number of engine/vehicle functions and uses data to control shift solenoid valves and TCC solenoid. PCM also regulates TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

1-2 & 3-4 Shift Solenoid Valves The 1-2 and 2-3 shift solenoid valves (also called "A" and "B" solenoids) are identical devices that control the movement of the 1-2 and 2-3 shift valves (the 3-4 shift valve is not directly controlled by a shift solenoid). The solenoids are normally-open exhaust valves that work in four combinations to shift the transmission into different gears. The PCM energizes each solenoid by grounding the solenoid through an internal quad driver. This sends current through the coil winding in the solenoid and moves the internal plunger out of the exhaust position. When on, the solenoid redirects fluid to move a shift valve. PCM-controlled shift solenoids eliminate the need for TV and governor pressures to control shift valve operation.

3-2 Shift Solenoid Valve The 3-2 shift solenoid valve assembly is a normally-closed, 3-port, on/off device that is used in order to improve the 3-2 downshift. The solenoid regulates the release of the 3-4 clutch and 2-4 band apply.

Transmission Pressure Control Solenoid The transmission pressure control solenoid is an electronic pressure regulator that controls pressure based on the current flow through its coil winding. The magnetic field produced by the coil moves the solenoid's internal valve which varies pressure to the pressure regulator valve. PCM controls the pressure control solenoid by commanding current between 0.1-1.1 amps. This changes the duty cycle of the



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solenoid, which can range between 5-95 percent (typically less than 60 percent). High amperage (1.1 amps) corresponds to minimum line pressure, and low amperage (0.1 amp) corresponds to maximum line pressure (if the solenoid loses power, the transmission defaults to maximum line pressure). The PCM commands the line pressure values, using inputs such as engine speed and throttle position sensor voltage. The pressure control solenoid takes the place of the throttle valve or the vacuum modulator.

Torque Converter Clutch Solenoid Valve The TCC solenoid valve is a normally-open exhaust valve that is used to control torque converter clutch apply and release. When grounded (energized) by the PCM, the TCC solenoid valve stops converter signal oil from exhausting. This causes converter signal oil pressure to increase and move the TCC solenoid valve into the apply position.

TCC Pulse-Width Modulation Solenoid Valve The PCM energizes the Torque Converter Clutch Pulse-Width Modulated (TCC PWM) solenoid valve, which is located on transmission valve body. The TCC PWM solenoid valve acts on the TCC apply valve in order to control the torque converter clutch application. TCC PWM solenoid valve is pulse-width modulated by PCM. This means that the PCM pulses the solenoid so that the hydraulic pressure against the torque converter clutch modulates. This modulated pressure allows the TCC to slip slightly, thus keeping the TCC balanced just at the point of engagement.

Transmission Fluid Pressure Manual Valve Position Switch A gear range sensing device called an automatic Transmission Fluid Pressure (TFP) manual valve position switch assembly is used by the PCM in order to sense which gear range has been selected by the vehicle operator. The TFP manual valve position switch assembly is located on valve body and consists of 5 pressure switches combined into one unit. PCM applies system voltage to the TFP manual valve position switch assembly on 3 separate wires. These 3 circuits are either grounded or open, depending on which gear range has been selected, and on which combination of the 5 switches gave pressure applied to them. When vehicle is in PARK, with ignition on and engine off, the normal state of the TFP manual valve position switch assembly will be DRIVE 2. When engine is running, the normal state of the TFP manual valve position switch assembly is in PARK/NEUTRAL. There are 2 possible combinations of the switches within the pressure switch manifold that do not represent an actual gear range. If PCM detects either of these combinations, then a DTC will set.

Transmission Fluid Temperature Sensor The automatic Transmission Fluid Temperature (TFT) sensor is part of the automatic Transmission Fluid Pressure (TFP) manual valve position switch. The TFT sensor is a resistor or thermistor, which changes value based on temperature. The sensor has a negative-temperature coefficient. This means that as the temperature increases, resistance decreases and as temperature decreases, resistance increases. PCM supplies a 5-volt reference signal to the TFT sensor and measures voltage drop in the circuit. When transmission fluid is cold, sensor resistance is high and PCM detects high signal voltage. As fluid temperature warms to a normal operating temperature, resistance becomes less and signal voltage decreases. PCM uses the TFT sensor information to control shift quality and TCC application.



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Transmission Range Switch The Transmission Range (TR) switch is part of the Park/Neutral Position (PNP) and backup light switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains four internal switches that indicate the transmission gear range selector lever position. PCM supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. PCM detects the selected gear range by deciphering the combination of the voltage signals. PCM compares the actual voltage combination of the switch signals to a TR switch combination chart stored in memory.



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Theory & Operation - 6.6L Sierra & Silverado - Diesel

INTRODUCTION



AIRFLOW SENSING

Speed Density

Engine is equipped with a Mass Airflow (MAF) sensor and an Intake Air Temperature (IAT) sensor. See Fig. 1 .

The MAF sensor is an air flow meter that measures the amount of air entering the engine. The Engine Control Module (ECM) uses the MAF sensor voltage signal to provide the correct fuel delivery for a reduction in emissions. The ECM uses the MAF sensor signal to control fuel delivery until a calibrated amount of engine air flow is attained. The MAF sensor has an ignition 1 voltage circuit, a signal circuit and a low reference circuit. The MAF sensor produces an output voltage based on the inlet air flow through the air induction system. This output voltage will display on scan tool as a grams per second (g/s) value. The ECM will calculate a predicted MAF value. ECM compares actual MAF sensor voltage signal to the predicted MAF value. This comparison will determine if signal is stuck or is too low or too high for a given operating condition.

The IAT sensor is a variable resistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The ECM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, ECM detects a high voltage on the IAT signal circuit. With lower sensor resistance, ECM detects a lower voltage on the IAT signal circuit. If ECM detects an out-of-range signal voltage, a DTC will set. To diagnose IAT sensor circuit, go to SELF-DIAGNOSTICS - 6.6L SIERRA & SILVERADO - DIESEL article.

NOTE: Unless specified otherwise, Sierra and Silverado information also applies Cab & Chassis Sierra and Cab & Chassis Silverado.


2003 ENGINE PERFORMANCE Theory & Operation - 6.6L Sierra & Silverado - Diesel



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Fig. 1: Locating MAF/IAT Sensor Courtesy of GENERAL MOTORS CORP.


The computerized engine control system consists of engine ECM, input devices (sensor and switch input signals) and output signals.

ENGINE CONTROL MODULE

The control module system has a computer, Engine Control Module (ECM), to control fuel delivery, timing, and some emission control systems. The control module system monitors a number of engine and vehicle



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functions and controls the following operations through the Fuel Injection Control Module (FICM): Fuel rail pressure, fuel injector, and fuel injector pulse width. The diesel ECM, located in left front of engine compartment, in the control center of the control module system. See Fig. 2 . ECM constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. ECM performs the diagnostic function of the system. ECM can recognize operational problems, alert the driver through the MIL (Service Engine Soon), and store one or more DTCs which identify the problem areas to aid the technician in making repairs.



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Fig. 2: Locating Engine Control Module



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Courtesy of GENERAL MOTORS CORP.

FUEL INJECTOR CONTROL MODULE

The Fuel Injection Control Module (FICM) self monitors injector driver performance and sends this information to the Engine Control Module (ECM) through the CAN system. FICM is located on top front of right valve cover. See Fig. 3 . If a fault is detected in the injector driver circuits, the FICM will signal the ECM and a DTC will set. This is an internal circuit fault and cannot be caused by external failures.

Fig. 3: Locating Fuel Injector Control Module Courtesy of GENERAL MOTORS CORP.

INPUT DEVICES

NOTE: Components are grouped into 2 categories. The first category is INPUT DEVICES, consisting of components which control or produce voltage signals



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The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a class 2 message to the ECM for A/C compressor engagement. The ECM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

Accelerator Pedal Position Sensor

The Accelerator Pedal Position (APP) system along with the vehicle electronics and components is used to calculate and control the amount of acceleration and deceleration via fuel injector control. This eliminates the need for a mechanical cable attachment from the accelerator pedal to a throttle body. The APP system includes, but is not limited to the APP sensor assembly and ECM.

The APP sensor is mounted on the accelerator pedal control assembly. See Fig. 4 . The sensor is made up of 3 individual sensors within one housing. Three separate signal, low reference, and 5-volt reference circuits are used in order to interface the accelerator pedal sensor assembly with the ECM. Each sensor has a unique functionality to determine pedal position. ECM uses the APP sensor to determine the amount of acceleration or deceleration desired by the person driving the vehicle. The APP sensor 1 voltage should increase as the accelerator pedal is depressed, from less than 1.0 volt at zero pedal travel to greater than 2 volts at 100 percent pedal travel. APP sensor 2 voltage should decrease from greater than 4 volts at zero pedal travel to less than 3 volts at 100 percent pedal travel. APP sensor 3 voltage should decrease from greater than 3.8 volts at zero percent pedal travel to less than 3.3 volts at 100 percent pedal travel.

monitored by the control unit. The second category is OUTPUT SIGNALS, consisting of components controlled by the ECM.



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Fig. 4: Locating Accelerator Pedal Position Sensor Courtesy of GENERAL MOTORS CORP.


On models equipped with cruise control systems, ECM may monitor the brake switch circuit to determine when to engage and disengage cruise control.



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The Hall Effect Camshaft Position (CMP) sensor produces 3 ON-OFF pulses for each revolution of the camshaft. CMP sensor is located below the water pump. See Fig. 5 . The CMP output is pulse width encoded. The ECM uses the CMP sensor and Crankshaft Position (CKP) sensor output pulses to determine the engine speed and position. The CMP sensor is connected directly to the ECM by the 12-volt reference, low reference and CMP sensor signal. If ECM determines that the CMP sensor signal is out of range for less than 2 seconds, a DTC will set. To diagnose DTC, see SELF-DIAGNOSTICS - 6.6L SIERRA & SILVERADO - DIESEL article.



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Fig. 5: Locating Camshaft Position Sensor Courtesy of GENERAL MOTORS CORP.


The Hall Effect Crankshaft Position (CKP) sensor signal indicates the crankshaft speed and position. CKP sensor is located behind the crankshaft pulley. See Fig. 6 . There are 57 teeth on the front of the crankshaft sprocket, plus a sync gap. The CKP sensor will output an ON-OFF pulse as each window passes the sensing element. The CKP sensor is connected directly to the ECM by the 12-volt reference circuit, low reference



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circuit and CKP sensor signal circuit. If ECM detects that the CKP sensor signal is incorrect for less than 8 seconds, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.6L SIERRA & SILVERADO - DIESEL article.



The Engine Coolant Temperature (ECT) sensor is a variable resistor that measures temperature of engine



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coolant. ECT sensor is located on top front of cylinder head, near oil filler tube. See Fig. 7 . The ECM supplies 5 volts to the signal circuit. When coolant temperatures are low, resistance is high. When coolant temperatures are high, resistance is low. ECM uses this input for engine controls and enabling criteria for diagnostics. ECM will record the amount of time the engine is off. At restart, ECM will compare the temperature difference between the ECT and Intake Air Temperature (IAT). If temperature difference is not within the calculated amount after the predetermined soak time, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.6L SIERRA & SILVERADO - DIESEL article.

Fig. 7: Locating Engine Coolant Temperature Sensor Courtesy of GENERAL MOTORS CORP.




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The Intake Air Temperature (IAT) sensor is a variable resistor, sometimes called a thermistor. The IAT sensor measures the temperature of the air entering the engine. The ECM supplies 5 volts to the IAT signal circuit. When IAT sensor is cold, sensor resistance is high. When air temperature increases, sensor resistance lowers. With high sensor resistance, ECM detects a high voltage on the IAT signal circuit. With lower sensor resistance,ECM detects a lower voltage on IAT signal circuit. If ECM detects an out of range voltage in the IAT signal circuit, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.6L SIERRA & SILVERADO - DIESELarticle.

Transmission Fluid Pressure Manual Valve Position Switch

The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the ECM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the ECM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. The TFT sensor is part of the TFP manual valve position switch assembly.


The Transmission Fluid Temperature (TFT) sensor is a thermistor (temperature sensitive resistor) and is part of the Transmission Fluid Pressure (TFP) manual valve position switch. The ECM supplies and monitors a 5-volt signal to TFT sensor. This monitored 5-volt signal is then modified by resistance of TFT sensor. When transmission fluid temperatures are low, TFT sensor resistance is high and ECM sees a high monitored voltage signal. When transmission fluid temperatures are high, TFT sensor resistance is low and ECM sees a low monitored voltage.

ECM uses TFT sensor input to control Torque Converter Clutch (TCC) application and shift quality. Sensor circuit problem should set a related DTC.


The Transmission Range (TR) switch is part of the Park/Neutral Position (PNP) and backup light switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains 4 internal switches that indicate the transmission gear range selector lever position. The ECM supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. The ECM detects the selected gear range by deciphering the combination of the voltage signals. ECM compares actual voltage combination of the switch signals to a TR switch combination chart stored in memory.


The Vehicle Speed Sensor (VSS) assembly provides vehicle speed information to ECM. The VSS assembly is a Permanent Magnet (PM) generator. The PM generator produces a pulsing AC voltage as rotor teeth on the transmission output shaft pass through the sensor's magnetic field. The AC voltage level and the number of



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pulses increase as speed of the vehicle increases. Output voltage varies with speed from a minimum of 0.5 volt at 100 RPM to more than 100 volts at 8000 RPM. The ECM converts the pulsing voltage to vehicle speed. The ECM uses the vehicle speed signal to determine shift timing and TCC scheduling.

OUTPUT SIGNALS

A/C Clutch Relay

See MISCELLANEOUS ECM CONTROLS .

Cruise Control

See MISCELLANEOUS ECM CONTROLS .

Fuel Injection Pump




Self-Diagnostics


Serial Data






FUEL SYSTEMS

FUEL DELIVERY

Fuel Injection Pump

The fuel injection pump is a mechanical high pressure pump. The fuel injection pump is located below the intake manifold. Fuel is pumped to the fuel rails at a specified pressure. Fuel pressure is regulated by a valve on the inlet of the fuel pump, controlled by the ECM. Excess fuel from the fuel injection pump returns to the fuel



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tank through the fuel return pipe and a fuel cooler.

On vehicles equipped with dual fuel tanks, an electric transfer fuel pump is located on the left frame rail. This fuel pump is powered by the fuel pump relay that is controlled by the ECM. Fuel is transferred from auxiliary fuel tank to primary fuel tank in order to ensure all of the usable fuel volume is available to the fuel injection pump.

FUEL CONTROL

The ECM, using input signals, determines adjustments to the air/fuel mixture to provide the optimum ratio for proper combustion under all operating conditions. Fuel control systems can operate in the open loop or closed loop mode.


The fuel tank stores fuel supply. A mechanical fuel injection pump, located below engine intake, draws fuel through the fuel injector control module and fuel filter. The fuel is used as a coolant for the fuel injector control module. Fuel pump output is controlled by the ECM and provides fuel at the pressure needed by the fuel injectors. Fuel injectors supply fuel directly to the engine combustion chambers. A separate pipe returns unused fuel through a fuel cooler and to fuel tank.


CATALYTIC CONVERTER


EGR SYSTEM

The EGR system is used to reduce the amount of Nitrogen Oxide (NOx) emission levels caused by combustion temperatures exceeding 1500°F (816°C). It does this by introducing small amounts of exhaust gas back into the combustion chamber. The exhaust gas absorbs a portion of the thermal energy produced by the combustion process and thus decreases combustion temperature. The EGR system will only operate under specific temperature, barometric pressure and engine load conditions in order to prevent driveability concerns and to increase engine performance.

The EGR valve is vacuum operated. Vacuum for the EGR valve control system is supplied by a belt driven vacuum pump. The EGR valve vacuum control solenoid and EGR valve vacuum vent solenoid work together to control the position of the EGR valve. ECM monitors EGR vacuum sensor signal to determine the amount of vacuum that is applied to EGR valve. The ECM uses the MAF sensor to calculate the amount of exhaust gas that is consumed by the engine. When EGR valve is opened, MAF rate will decrease. If EGR vacuum sensor signal is lower than desired and MAF rate is higher than expected, it is assumed that EGR valve is not opening enough due to a vacuum problem and a DTC will set.




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The ECM is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, ECM will illuminate the MIL located on instrument cluster. When a malfunction is detected and MIL is turned on, a corresponding DTC will be stored in ECM memory. Malfunctions are designated as either "emission related" or as "non-emission related", and are divided into 4 code types to identify type of fault. The 4 code types are defined as follows:

Type "A" Emission related faults that illuminate MIL at first occurrence of a fail condition. ECM records the operating conditions at the time diagnostic failed into Failure Records and Freeze Frame.

Type "B" Emission related faults that illuminate MIL if a fault occurs in 2 consecutive ignition cycles. ECM records the operating conditions at the time diagnostic failed the first time into Failure Records and Freeze Frame. When diagnostic fails a second time, ECM records the operating conditions at the time diagnostic failed into Freeze frame and updates Failure Records.

Type "C" Non-emission related faults that do not illuminate MIL but the DTC will be recorded in memory. ECM records the operating conditions at the time diagnostic failed into Failure Records, no Freeze Frame information will be saved. Driver information center (if equipped) may display a message.

A current DTC clears when the diagnostic runs and passes. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other diagnostic. Intermittent failures may be caused by sensor, connector or wiring related problems. See TROUBLE SHOOTING - NO CODES - 6.6L SIERRA & SILVERADO - DIESEL article.


As a bulb and system check, the MIL will illuminate when ignition switch is turned to ON position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. MIL may be used on some models to display a stored DTC. To access DTCs, see SELF-DIAGNOSTICS - 6.6L SIERRA & SILVERADO - DIESEL article.

SERIAL DATA

ECM has a serial data line. Serial data is a stream of electrical impulses which can be exchanged between control modules. Serial data can be interpreted using a scan tool. Access serial data by connecting a scan tool to DLC. Update intervals and information contained within data stream vary with model application.


NOTE: Although not considered true engine performance-related systems, some



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A/C CLUTCH

ECM regulates operation of the A/C clutch through a relay. The ECM disengages the A/C compressor when compressor load on engine may cause driveability problems (i.e., during hot restart, idle, low speed steering maneuvers and wide open throttle operation) or if A/C refrigerant pressure drops to less than or rises to greater than normal operating levels.





CRUISE CONTROL

On models equipped with cruise control, the system is operated by the ECM. ECM receives inputs from Vehicle Speed Sensor (VSS), servo diaphragm position sensor, cruise control switch and brake release switch. Based on these inputs, ECM controls position of cruise control stepper motor. ECM prevents system engagement at speeds of less than 25 MPH. ECM is not serviceable; if defective, it must be replaced. A system fault is stored as a DTC in ECM memory.

TRANSMISSION

Torque Converter Clutch (TCC)


The 2nd, 3rd and 4th gear hydraulic apply switches (located within transmission) may also be in series with solenoid power or ground circuit. Switch status may only be monitored by ECM, without sharing power or ground with TCC solenoid. For wiring reference, see appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS.

The TCC engages when vehicle is moving faster than a pre-calibrated speed, engine is at normal operating temperature, TP sensor output is not changing (indicating a steady road speed) and transmission 3rd gear or

devices may affect driveability if they malfunction.



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high gear switch (if equipped) and brake switch are closed.

When vehicle speed is great enough (about 20-45 MPH as indicated by vehicle speed sensor), ECM energizes TCC solenoid mounted in transmission, allowing torque converter to directly connect engine to the transmission. When operating conditions indicate transmission should operate as normal, TCC solenoid is de-energized, allowing transmission to return to normal automatic operation. Since power for the TCC solenoid is delivered through the brake switch, transmission also returns to normal automatic operation when brake pedal is depressed. To check function of TCC system, perform functional check of system. See MISCELLANEOUS CONTROLS in SYSTEM & COMPONENT TESTING - 6.6L SIERRA & SILVERADO - DIESEL article.


ECM controls transmission and other vehicle functions. ECM monitors a number of engine/vehicle functions and uses data to control shift solenoid valves and TCC solenoid. ECM also regulates TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

1-2 & 2-3 Shift Solenoid Valves The 1-2 and 2-3 shift solenoid valves (also called "A" and "B" solenoids) are identical devices that control the movement of the 1-2 and 2-3 shift valves (the 3-4 shift valve is not directly controlled by a shift solenoid). The solenoids are normally-open exhaust valves that work in 4 combinations to shift the transmission into different gears. ECM energizes each solenoid by grounding the solenoid through an internal quad driver. This sends current through the coil winding in the solenoid and moves the internal plunger out of the exhaust position. When on, the solenoid redirects fluid to move a shift valve. The ECM controlled shift solenoids eliminate the need for TV and governor pressures to control shift valve operation.

3-2 Shift Solenoid Valve The 3-2 shift solenoid valve assembly is a normally closed, 3-port, ON/OFF device that is used in order to improve the 3-2 downshift. The solenoid regulates the release of the 3-4 clutch and the 2-4 band apply.

Transmission Pressure Control Solenoid The transmission pressure control solenoid is an electronic pressure regulator that controls pressure based on the current flow through its coil winding. The magnetic field produced by the coil moves the solenoid's internal valve which varies pressure to the pressure regulator valve. ECM controls the pressure control solenoid by commanding current between 0.1-1.1 amps. This changes the duty cycle of the solenoid, which can range between 5-95 percent (typically less than 60 percent). High amperage (1.1 amps) corresponds to minimum line pressure, and low amperage (0.1 amp) corresponds to maximum line pressure (if the solenoid loses power, the transmission defaults to maximum line pressure). ECM commands the line pressure values, using inputs such as engine speed and throttle position sensor voltage. The pressure control solenoid takes the place of the throttle valve or the vacuum modulator.

Torque Converter Clutch Solenoid Valve The Torque Converter Clutch (TCC) solenoid valve is a normally-open exhaust valve that is used to



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control torque converter clutch apply and release. When grounded (energized) by the ECM, the TCC solenoid valve stops converter signal oil from exhausting. This causes converter signal oil pressure to increase and move the TCC solenoid valve into the apply position.

TCC Pulse-Width Modulation Solenoid Valve The Torque Converter Clutch Pulse Width Modulation (TCC PWM) solenoid valve controls the fluid acting on the converter clutch valve. The converter clutch valve controls the TCC apply and release. This solenoid is attached to the control valve body assembly within the transmission. The TCC PWM solenoid valve provides a smooth engagement of the torque converter clutch by operating during a duty cycle percent of ON time.

Transmission Fluid Pressure Manual Valve Position Switch The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the ECM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the ECM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. In order to monitor the TFP manual valve position switch operation, the ECM compares the actual voltage combination of the switches to a TFP combination table stored in its memory. The Transmission Fluid Temperature (TFT) sensor is part of the TFP manual valve position switch assembly.

Transmission Fluid Temperature Sensor The automatic Transmission Fluid Temperature (TFT) sensor is part of the automatic Transmission Fluid Pressure (TFP) manual valve position switch. The TFT sensor is a resistor or thermistor, which changes value based on temperature. The sensor has a negative-temperature coefficient. This means that as the temperature increases, resistance decreases and as temperature decreases, resistance increases. ECM supplies a 5-volt reference signal to the TFT sensor and measures voltage drop in the circuit. When transmission fluid is cold, sensor resistance is high and ECM detects high signal voltage. As fluid temperature warms to a normal operating temperature, resistance becomes less and signal voltage decreases. ECM uses the TFT sensor information to control shift quality and TCC application.

Transmission Range Switch The Transmission Range (TR) switch is part of the Park/Neutral Position (PNP) and backup light switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains 4 internal switches that indicate the transmission gear range selector lever position. ECM supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. ECM detects the selected gear range by deciphering the combination of the voltage signals. ECM compares the actual voltage combination of the switch signals to a TR switch combination chart stored in memory.



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Theory & Operation - 8.1L Avalanche, Sierra, Silverado, Suburban & Yukon XL

INTRODUCTION



AIRFLOW SENSING

Speed Density

Engine is equipped with a Mass Airflow (MAF) sensor, a Manifold Absolute Pressure (MAP) sensor and an Intake Air Temperature (IAT) sensor. The IAT/MAF sensor is a combination sensor. See Fig. 1 or Fig. 5 .



The IAT sensor is a variable resistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, go to SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.

NOTE: Unless specified otherwise, Sierra and Silverado information also applies to Cab & Chassis Sierra and Cab & Chassis Silverado.


2003 ENGINE PERFORMANCE Theory & Operation - 8.1L Avalanche, Sierra, Silverado, Suburban & Yukon XL



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Fig. 1: Locating Intake Air Temperature/MAF Sensor Courtesy of GENERAL MOTORS CORP.



The computerized engine control system consists of engine PCM, input devices (sensor and switch input



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signals) and output signals.


The powertrain has electronic controls to reduce exhaust emissions while maintaining excellent driveability and fuel economy. The Powertrain Control Module (PCM) is the control center of this system. PCM is located on left front of engine compartment. See Fig. 2 . PCM monitors numerous engine and vehicle functions and constantly looks at the information from various sensors and other inputs and controls the systems that affect vehicle performance and emissions. PCM also performs the diagnostic tests on various parts of the system. The PCM can recognize operational problems and alert the driver via the Malfunction Indicator Light (MIL). When PCM detects a malfunction, the PCM stores a DTC. The problem area is identified by the particular DTC that is set. The control module supplies a buffered voltage to various sensors and switches.



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INPUT DEVICES

NOTE: Components are grouped into 2 categories. The first category is INPUT DEVICES, consisting of components which control or produce voltage signals monitored by the control unit. The second category is OUTPUT SIGNALS,



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The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a class 2 message to the PCM for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.


The accelerator pedal assembly contains 2 individual Accelerator Pedal Position (APP) sensors within the assembly. See Fig. 3 . The APP sensors 1 and 2 potentiometer-type sensors have a 5-volt reference circuit, low reference circuit and signal circuit. The APP sensors are used to determine pedal angle. The control module provides each APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensors then provide the control module with signal voltage proportional to pedal movement. Both APP sensor signal voltages are low at rest position and increase as pedal is applied.

consisting of components controlled by the PCM.



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Fig. 3: Locating Accelerator Pedal Position Courtesy of GENERAL MOTORS CORP.

Battery Voltage

Battery voltage is monitored by PCM. If battery voltage swings low, a weak spark or improper fuel control may result. To compensate for low battery voltage, PCM may increase idle speed, advance ignition timing, increase ignition dwell or richen the air/fuel mixture. If voltage swings high, PCM may set a charging system fault code and turn on Malfunction Indicator Light (MIL). If voltage signal swings excessively low (less than 9 volts) or excessively high (16 volts), PCM shuts down for as long as condition exists. If condition is short-term, MIL flickers and vehicle may stumble. Vehicle stalls if condition persists.




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On models equipped with cruise control systems, PCM may monitor the brake switch circuit to determine when to engage and disengage cruise control.


The Camshaft Position (CMP) sensor works in conjunction with a 1X reluctor wheel on the camshaft. The PCM provides a 12 volt reference to the CMP sensor as well as a low reference and a signal circuit. As the camshaft rotates, the reluctor wheel interrupts a magnetic field produced by a magnet within the sensor. The sensors internal circuitry detects this and produces a signal which the PCM reads. CMP sensor is mounted on timing chain cover. See Fig. 4 .

The CMP sensor 1X signal is used by the PCM to determine if the cylinder at Top Dead Center (TDC) is on the firing stroke or exhaust stroke. The PCM can determine TDC for all cylinders by using the CKP sensor 24X signal alone. The engine will start without a CMP signal as long as the PCM receives the CKP sensor 24X signal. A slightly longer cranking time may be a symptom of this condition. The system attempts synchronization and looks for an increase in engine speed indicating that the engine started. If PCM does not detect an increase in engine speed, PCM assumes that the PCM incorrectly synchronized to the exhaust stroke and re-syncs to the opposite cam position. If PCM detects that a CMP to CKP mis-match has occurred a DTC will set.



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Fig. 4: Locating Camshaft Position & Knock SensorsCourtesy of GENERAL MOTORS CORP.

Cranking Signal



The Crankshaft Position (CKP) sensor signal indicates the crankshaft speed and position. CKP sensor is located on top left rear of engine, near bellhousing. See Fig. 5 . The CKP sensor is connected directly to the PCM and consists of a 12-volt reference circuit, low reference circuit and CKP sensor signal circuit. If PCM detects no signal from the CKP sensor for more than 3 seconds, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.



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Fig. 5: Locating Crankshaft Position Sensor, EGR Valve, Engine Oil Pressure Sensor & MAP Sensor Courtesy of GENERAL MOTORS CORP.


The Engine Coolant Temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. ECT sensor is located on side of left cylinder head. The PCM supplies 5 volts to the signal circuit and a ground for the ECT low reference circuit. When ECT is low, sensor resistance is high. When ECT is high, sensor resistance is low. The PCM uses this input for engine controls and enabling criteria for diagnostics. The internal clock of the PCM will record the amount of time ignition is off. At restart, PCM will compare the temperature difference between the ECT and the IAT sensor. Before failing this test, the PCM will perform a



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calculation to determine the presence of a block heater. If PCM detects that the temperature difference is not within the calibrated amount after the ignition off time, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.


The Intake Air Temperature (IAT) sensor is a variable resistor. IAT sensor is located in fresh air intake tube. IAT sensor is combined with MAF sensor assembly. See Fig. 1 . The IAT sensor has a signal and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, the sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. Failure in IAT sensor circuit should set a related DTC. To diagnose, see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.

Knock Sensor

The Knock Sensor (KS) system enables the PCM to control the ignition timing advance for the best possible performance while protecting the engine from potentially damaging levels of detonation. KS is located on each side of engine block. See Fig. 6 . The sensors in the KS system are used by the PCM as microphones to listen for abnormal engine noise that may indicate pre-ignition/detonation. A fault in the KS circuit may set a DTC. See SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.


The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. MAP sensor is mounted on top of intake manifold. See Fig. 6 . The pressure changes occur based on engine load. The MAP sensor has a 5-volt reference circuit, a low reference circuit and a signal circuit. The PCM supplies 5 volts to MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when ignition is on, with engine off. The BARO reading may also be updated whenever engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If PCM detects a MAP sensor signal voltage that is out-of-range, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.




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Fig. 6: Locating ECT & MAP Sensors & Throttle Actuator Motor Courtesy of GENERAL MOTORS CORP.




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Park/Neutral Position Switch

The Park/Neutral Position (PNP) switch is connected to transmission gear selector and signals PCM when transmission is in Park or Neutral. PCM uses this information for determining control of ignition timing, Torque Converter Clutch (TCC) and idle speed.

Throttle Actuator Control System

The Throttle Actuator Control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs the accelerator pedal position sensing, throttle positioning to meet driver and engine demands, throttle position sensing, internal diagnostics, cruise control functions and manage TAC electrical power consumption.

The TAC system components include the Accelerator Pedal Position (APP) sensors, throttle body assembly, throttle actuator control module and the PCM. See Fig. 7 .


NOTE: Also see TRANSMISSION RANGE SWITCH.



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Fig. 7: Locating Throttle Actuator Control Module Courtesy of GENERAL MOTORS CORP.


The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. The TFT sensor is part of the TFP manual valve position switch



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assembly.


The Transmission Fluid Temperature (TFT) sensor is a thermistor (temperature sensitive resistor) and is part of the Transmission Fluid Pressure (TFP) manual valve position switch. The PCM supplies and monitors a 5-volt signal to TFT sensor. This monitored 5-volt signal is then modified by resistance of TFT sensor. When transmission fluid temperatures are low, TFT sensor resistance is high and PCM sees a high monitored voltage signal. When transmission fluid temperatures are high, TFT sensor resistance is low and PCM sees a low monitored voltage.





The Vehicle Speed Sensor (VSS) assembly provides vehicle speed information to the PCM. The VSS assembly is a Permanent Magnet (PM) generator. The PM generator produces a pulsing AC voltage as rotor teeth on the transmission output shaft pass through the sensor's magnetic field. The AC voltage level and the number of pulses increase as the speed of the vehicle increases. Output voltage varies with speed from a minimum of 0.5 volt at 100 RPM to more than 100 volts at 8000 RPM. The PCM converts the pulsing voltage to vehicle speed. The PCM uses vehicle speed signal to determine shift timing and TCC scheduling.

OUTPUT SIGNALS

A/C Clutch Relay


Cruise Control


Electronic Ignition




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Exhaust Gas Recirculation

See EMISSION SYSTEMS & SUB-SYSTEMS .

Fuel Injectors






Self-Diagnostics


Serial Data




Throttle Actuator Control




FUEL SYSTEMS

FUEL DELIVERY





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Fuel Pump

The primary fuel pump is mounted in the fuel sender assembly reservoir. The primary fuel pump is an electric high pressure pump. Fuel is pumped to the fuel rail at a specified flow and pressure. Excess fuel from the fuel rail assembly returns to the fuel tank through the fuel return pipe. The fuel pump delivers a constant flow of fuel to the engine even during low fuel conditions and aggressive vehicle maneuvers. The PCM controls the electric fuel pump operation through a fuel pump relay. The fuel pump flex pipe acts to dampen the fuel pulses and noise generated by the fuel pump.

On dual tank applications, a secondary fuel pump is located in the secondary fuel tank. The secondary fuel pump is powered by a secondary fuel pump relay when the fuel level drops below a predetermined value. Fuel is transferred from the secondary fuel tank to the primary fuel tank in order to ensure all of the usable fuel volume is available to the primary fuel pump. The secondary fuel pump relay supply voltage is received from the primary fuel pump relay when primary fuel pump is energized.

Fuel Pump Relay

When ignition switch is turned to ON position, PCM turns electric fuel pump on by energizing fuel pump relay. PCM keeps relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off.

For additional information on fuel pump activation, see SYSTEM & COMPONENT TESTING - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.

FUEL CONTROL


Closed Loop Mode




Starting With ignition switch in the ON position, before engaging starter, the PCM energizes the fuel pump relay for 2seconds allowing fuel pump to build pressure. Speed density is determined by inputs from the engine



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RPM, the IAT and MAP sensors. The PCM first tests speed density, then switches to the MAF sensor. PCM also uses the ECT, TP and MAP sensors to determine the proper air/fuel ratio for starting. The PCM controls the amount of fuel delivered in the starting mode by changing the width of the fuel injector pulse.

Clear Flood If the engine floods, clear the engine by pushing accelerator pedal down to the floor and then crank engine. When TP sensor is at Wide Open Throttle (WOT), the PCM reduces the injector pulse width in order to increase the air-to-fuel ration. The PCM maintains injector rate as long as the throttle stays wide open and engine speed is below a predetermined RPM. If throttle is not held wide open, PCM returns to the starting mode.

Run Mode The run mode has 2conditions. These conditions are called open loop and closed loop. When engine is first started and engine speed is above a predetermined RPM, the system begins open loop operation. The PCM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system stays in open loop until the following conditions are met: Both HO2Ss have varying voltage output, showing that they are hot enough to operate properly. This depends upon engine temperature. The ECT sensor is above a specified temperature. A specific amount of time has elapsed after starting the engine. Specific values for the above conditions exist for each different engine. These values are stored in the Electrically Erasable Programmable Read-Only Memory (EEPROM). The system begins closed loop operation after reaching these values. In closed loop, the PCM calculates the air/fuel ratio, fuel injector ON time, based upon the signal from various sensors, but mainly form the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration When the driver pushes on the accelerator pedal, the air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. In order to prevent possible hesitation, the PCM increases the pulse width to the fuel injectors in order to provide extra fuel during acceleration. The PCM determines amount of fuel required based upon the throttle position, coolant temperature, MAP, MAF and engine speed.

Deceleration When the driver releases the accelerator pedal, the air flow into the engine is reduced. The PCM reads the corresponding changes in the TP, MAP and MAF sensors. The PCM shuts OFF fuel completely if deceleration is very rapid or for long periods, such as during a long, closed-throttle coast-down. The fuel shuts OFF in order to protect the TWC.

Battery Voltage Correction PCM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. PCM is able to perform these commands because of a built-in memory/learning function.

Fuel Cut-Off



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When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Fuel injector signals are cut off during periods of sudden, closed throttle deceleration (when fuel is not needed).

Open Loop Mode


IDLE SPEED



The Throttle Actuator Control (TAC) system uses vehicle electronics and components to calculate and control the position of the throttle plate. In order to decrease idle speed, the TAC system closes the throttle plate reducing air flow into the engine. In order to increase idle speed, the TAC system opens the throttle plate allowing more air flow into the engine. If actual idle RPM does not match the desired idle RPM within a calibrated time, a DTC will set.

The TAC system components include the Accelerator Pedal Position (APP) sensor, throttle body assembly, throttle actuator control module and PCM. See Fig. 6 and Fig. 7 .

IGNITION SYSTEMS

ELECTRONIC IGNITION SYSTEM

The Electronic Ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy and control of exhaust emissions. This ignition system consists of a separate ignition coil connected to each spark plug by a short secondary wire. The driver modules within each coil assembly are commanded ON/OFF by the PCM. The PCM primarily uses engine speed and position information from the CKP and CMP sensors to control the sequence, dwell, and timing of the spark.

The EI system consists of Crankshaft Position (CKP) sensor, Camshaft Position (CMP) sensor, ignition coils, secondary ignition components and portion of the PCM.


Ignition spark timing and ignition dwell time are controlled entirely by the PCM. The PCM monitors information from various engine sensors, computes the desired spark timing and dwell, and firing of the ignition



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coil via ignition control circuit to the coil driver.


CATALYTIC CONVERTER


EXHAUST GAS RECIRCULATION

The Exhaust Gas Recirculation (EGR) system is used to reduce the amount of Nitrogen Oxide (NOx) emission levels caused by combustion temperatures exceeding 1500°F (816°C). It does this by introducing small amounts of exhaust gas back into the combustion chamber. The exhaust gas absorbs a portion of the thermal energy produced by the combustion process and thus decreases combustion temperature. EGR system will only operate under specific temperature, barometric pressure and engine load conditions in order to prevent driveability concerns and to increase engine performance. EGR is a linear type and is located on top rear of engine. See Fig. 5 .


The Evaporative Emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON (open), allowing engine vacuum to be applied to EVAP canister. With EVAP vent solenoid OFF (open), fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

EVAP Canister

The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process. EVAP canister is located on top of fuel tank.

EVAP Canister Purge Solenoid

The EVAP canister purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. EVAP canister purge solenoid is located crossmember, near fuel tank. See Fig. 8 . This normally closed solenoid is Pulse Width Modulated (PWM) by the control module to precisely control the flow of fuel vapor to engine. The solenoid will also be opened during some portions of EVAP testing, allowing engine vacuum to enter the EVAP system.



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Fig. 8: Locating EVAP Purge Solenoid Courtesy of GENERAL MOTORS CORP.

EVAP Canister Vent Solenoid

The EVAP canister vent solenoid controls fresh air flow into the EVAP canister. EVAP canister vent solenoid is mounted on frame rail, near fuel tank. See Fig. 9 . The solenoid is normally open. The control module will command the solenoid closed during some EVAP tests, allowing the system to be tested for leaks.



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Fig. 9: Locating EVAP Canister Vent Solenoid Courtesy of GENERAL MOTORS CORP.


The Fuel Tank Pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. A high FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high fuel tank pressure.




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The Positive Crankcase Ventilation (PCV) system provides effective evacuation of crankcase vapors. Fresh air from the air filter housing is supplied to the crankcase, where it is mixed with blow-by gases and passed through the PCV valve and into the intake manifold. This mixture is then passed into the combustion chamber and burned.










A current DTC clears when the diagnostic runs and passes. A history DTC clears after 40 consecutive warm-up



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cycles, if no failures are reported by this or any other diagnostic. Intermittent failures may be caused by sensor, connector or wiring related problems. See TROUBLE SHOOTING - NO CODES - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.


As a bulb and system check, the MIL will illuminate when ignition switch is turned to ON position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. MIL may be used on some models to display a stored DTC. To access DTCs, see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.

SERIAL DATA

PCM has a serial data line. Serial data is a stream of electrical impulses which can be exchanged between control modules. Serial data can be interpreted using a special scan tool. Access serial data by connecting a scan tool to DLC. Update intervals and information contained within data stream vary with model application.


A/C CLUTCH






CRUISE CONTROL




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On models equipped with cruise control, the system is operated by the PCM. PCM receives inputs from Vehicle Speed Sensor (VSS), servo diaphragm position sensor, cruise control switch and brake release switch. Based on these inputs, PCM controls position of cruise control stepper motor. PCM prevents system engagement at speeds of less than 25 MPH. PCM is not serviceable; if defective, it must be replaced. A system fault is stored as a DTC in PCM memory.

TRANSMISSION



The 2nd, 3rd and 4th gear hydraulic apply switches (located within transmission) may also be in series with solenoid power or ground circuit. Switch status may only be monitored by PCM, without sharing power or ground with TCC solenoid. For wiring reference, see appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS.

The TCC engages when vehicle is moving faster than a pre-calibrated speed, engine is at normal operating temperature, TP sensor output is not changing (indicating a steady road speed) and transmission 3rd gear or high gear switch (if equipped) and brake switch are closed.




1-2 Shift Solenoid Valve The 1-2 shift solenoid valve is a normally open exhaust valve that is attached to the valve body. The PCM controls the solenoid by grounding the solenoid through an internal quad driver. The 1-2 shift solenoid valve is ON in 1st and 4th gear. When commanded ON, the 1-2 shift solenoid valve redirects fluid to act on the 1-2 shift valve. There are 2 PCM related DTCs associated with the 1-2 shift solenoid valve: P0751 and P0753. The PCM monitors the 1-2 shift solenoid circuit for an open or short to ground condition. If PCM detects an open or short to ground condition, then DTC P0753 sets. If PCM detects an incorrect gear ratio, then DTC P0751 sets. When DTC P0753 or P0751 sets, the PCM commands maximum line pressure, freezes shift adapts from being updated, and inhibits 3-2 downshifts. To diagnose DTC(s), see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON



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XL article.

2-3 Shift Solenoid Valve The 2-3 shift solenoid valve is a normally open exhaust valve that is attached to the valve body. The PCM controls the solenoid by grounding the solenoid through an internal quad driver. The 2-3 shift solenoid valve is ON in 3rd and 4th gear. When commanded ON, the 2-3 shift solenoid valve redirects fluid to act on the 2-3 shift valve. There are 2 PCM related DTCs associated with the 2-3 shift solenoid : P0756 and P0758. The PCM monitors the 2-3 shift solenoid circuit for an open or short to ground condition. If PCM detects an open or short to ground condition, then DTC P0758 sets. If PCM detects an incorrect gear ratio, then DTC P0756 sets. When DTC P0758 or P0756 sets, the PCM commands maximum line pressure, freezes shift adapts from being updated, and inhibits 3-2 downshifts. To diagnose DTC(s), see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.

Transmission Pressure Control Solenoid The transmission pressure control solenoid is an electronic pressure regulator that controls pressure based on the current flow through its coil winding. The magnetic field produced by the coil moves the solenoid's internal valve which varies pressure to the pressure regulator valve. PCM controls the pressure control solenoid by commanding current between 0.1-1.1 amps. This changes the duty cycle of the solenoid, which can range between 5-95 percent (typically less than 60 percent). High amperage (1.1 amps) corresponds to minimum line pressure, and low amperage (0.1 amp) corresponds to maximum line pressure (if the solenoid loses power, the transmission defaults to maximum line pressure). PCM commands the line pressure values, using inputs such as engine speed and throttle position sensor voltage. The pressure control solenoid takes the place of the throttle valve or the vacuum modulator.

Torque Converter Clutch Solenoid Valve The Torque Converter Clutch (TCC) solenoid valve is a normally-open exhaust valve that is used to control torque converter clutch apply and release. When grounded (energized) by the PCM, the TCC solenoid valve stops converter signal oil from exhausting. This causes converter signal oil pressure to increase and move the TCC solenoid valve into the apply position.


Transmission Fluid Pressure Manual Valve Position Switch The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used



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by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. In order to monitor the TFP manual valve position switch operation, the PCM compares the actual voltage combination of the switches to a TFP combination table stored in its memory. The Transmission Fluid Temperature (TFT) sensor is part of the TFP manual valve position switch assembly.





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Theory & Operation - 4.8L, 5.3L & 6.0L "C", "G", "H", "K", "N", "S" & "T" Series - Flex Fuel & Gasoline

MODEL IDENTIFICATION

Vehicle model is identified by the fifth character of Vehicle Identification Number (VIN). The VIN is stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION table.

MODEL IDENTIFICATION

INTRODUCTION



AIRFLOW SENSING

Speed Density


The MAF sensor is an airflow meter that measures the amount of air entering the engine. The PCM uses the MAF sensor signal to provide the correct fuel delivery for all engine speeds and loads. A small quantity of air

Series (1)

Model

"C" 2WD Avalanche, Escalade, Sierra, Silverado, Suburban, Tahoe, Yukon & Yukon XL"G" RWD Chevy Express & Savana"H" AWD Chevy Express & Savana"K" AWD/4WD Avalanche, Escalade, Escalade ESV, Escalade EXT, Sierra, Silverado, Suburban,

Tahoe, Yukon & Yukon XL"N" Hummer H2"S" Envoy XL & TrailBlazer EXT (2WD)"T" Envoy XL & TrailBlazer EXT (AWD/4WD)(1) Vehicle series is fifth character of VIN.

NOTE: Unless specified otherwise, Sierra and Silverado information also applies to Cab & Chassis Sierra and Cab & Chassis Silverado. Savana information also applies to Savana Special and Savana Camper Special.


2003 ENGINE PERFORMANCE Theory & Operation - 4.8L, 5.3L & 6.0L "C", "G", "H", "K", "N", "S" & "T" Series - Flex Fuel & Gasoline



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entering the engine indicates a deceleration or idle condition. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit and a signal circuit.


The IAT sensor is a variable resistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.



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Fig. 1: Locating ECT Sensor, EVAP Canister Purge Solenoid, IAC Valve, MAP Sensor & TP Sensor Courtesy of GENERAL MOTORS CORP.






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The PCM has a "learning" ability which allows it to make minor corrections for fuel system variations. PCM is located on left front of engine compartment, behind radiator. See Fig. 2 . If battery power is interrupted, a vehicle performance change may be noticed. PCM module corrects itself, and normal performance returns if vehicle is allowed to "relearn" optimum control conditions. "Relearning" occurs when vehicle is driven at normal operating temperature under part throttle, moderate acceleration and idle conditions.


THROTTLE ACTUATOR CONTROL



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The Throttle Actuator Control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs accelerator pedal position sensing, throttle positioning to meet driver and engine demands, throttle position sensing, internal diagnostics, cruise control functions and manage TAC electrical power consumption.

The TAC system components include the Accelerator Pedal Position (APP) sensor, throttle body assembly, throttle actuator control module and PCM. See Fig. 3 .


INPUT DEVICES



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NOTE: Components are grouped into 2 categories. The first category is INPUT DEVICES, consisting of components which control or produce voltage signals monitored by the control unit. The second category is OUTPUT SIGNALS, consisting of components controlled by the PCM.



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Battery Voltage

Battery voltage is monitored by PCM. If battery voltage swings low, a weak spark or improper fuel control may



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result. To compensate for low battery voltage, PCM may increase idle speed, advance ignition timing, increase ignition dwell or richen the air/fuel mixture. If voltage swings high, PCM may set a charging system fault code and turn on Malfunction Indicator Light (MIL). If voltage signal swings excessively low (less than 9 volts) or excessively high (16 volts), PCM shuts down for as long as condition exists. If condition is short-term, MIL flickers and vehicle may stumble. Vehicle stalls if condition persists.




The Camshaft Position (CMP) sensor is a hall-effect type sensor. The sensor produces one signal for each revolution of the camshaft in order to control the sequential fuel injection. The CMP sensor is designed to detect changes in a magnetic field. The PCM supplies the CMP sensor with a 12-volt reference circuit, a low reference circuit and a signal circuit. The CMP sensor produces a magnetic field whenever ignition is on. The CMP sensor is mounted near a reluctor wheel that is attached to the distributor shaft. See Fig. 5 . When distributor shaft rotates, and reluctor wheel tooth passes by the CMP sensor, there is a change in the magnetic field. The CMP sensor converts each change in the magnetic field into a PULSE. If PCM does not detect the CMP signal while the engine is running, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.



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Fig. 5: Locating Camshaft Position & Engine Oil Pressure Sensors Courtesy of GENERAL MOTORS CORP.

Cranking Signal



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The PCM uses the Crankshaft Position (CKP) sensor to detect crankshaft speed and position. The CKP sensor is located on side of engine block, behind harmonic balancer. See Fig. 6 . The CKP sensor connects to the PCM through the 12-volt reference circuit, the low reference circuit and CKP sensor 1 signal circuit. If PCM detects that the CKP sensor signal is incorrect for 3 seconds, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.




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The Engine Coolant Temperature (ECT) sensor is a variable resistor, that measures the temperature of engine coolant. ECT sensor is located on side of left cylinder head. See Fig. 1 . The PCM supplies 5 volts to the ECT sensor signal circuit and ground for the ECT sensor low reference circuit. When ECT is cold, sensor resistance is high. When ECT increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on ECT sensor signal circuit. With lower sensor resistance, PCM detects a lower voltage on the ECT sensor signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.


Failure in Fuel Tank Pressure (FTP) sensor circuit will set a related DTC. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.


The Intake Air Temperature (IAT) sensor is a variable resistor. IAT sensor is located in fresh air intake tube. See Fig. 7 . The IAT sensor has a signal and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, the sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. Failure in IAT sensor circuit should set a related DTC. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.



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Fig. 7: Locating Intake Temperature & Mass Airflow Sensors Courtesy of GENERAL MOTORS CORP.

Knock Sensor

The Knock Sensor (KS) produces an AC voltage at all engine speeds and loads. KS is located on top rear of engine. See Fig. 8 . The PCM then adjusts the spark timing based on the amplitude and frequency of the KS signal. PCM uses the KS signal to calculate the average voltage. Then PCM assigns a voltage value. PCM checks the knock sensor and related wiring by comparing the actual knock signal to the assigned voltage range. A normal KS signal should stay within the assigned voltage range. A fault in the KS circuit may set a DTC. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article. When a



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related DTC is not present and the KS system is the suspected cause of a driveability problem, check of KS system. See SYMPTOMS in SYSTEM & COMPONENT TESTING - 4.8L, 5.3L & 6.0L "C", "G", "H", "K" & "N" SERIES- FLEX FUEL & GASOLINE article.

Fig. 8: Locating Knock Sensors Courtesy of GENERAL MOTORS CORP.


The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. MAP sensor is mounted on top of intake manifold. See Fig. 1 . The pressure changes occur based on engine load. The MAP sensor has a 5-volt reference circuit, a low reference circuit and a signal circuit. The PCM supplies 5 volts to MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. The



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MAP sensor provides a signal to PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when ignition is on, with engine off. The BARO reading may also be updated whenever engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If PCM detects a MAP sensor signal voltage that is out-of-range, a DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.

If MAP sensor fails, the PCM substitutes a fixed MAP value, and uses the TP sensor to control fuel delivery. A fault in the MAP circuit should set a related diagnostic trouble code. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.



Throttle Actuator Control System

The Throttle Actuator Control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs the accelerator pedal position sensing, throttle positioning to meet driver and engine demands, throttle position sensing, internal diagnostics, cruise control functions and manage TAC electrical power consumption.

The TAC system components include the Accelerator Pedal Position (APP) sensors, throttle body assembly, throttle actuator control module and the PCM.


The Throttle Position (TP) sensor is used by the PCM to determine the throttle plate angle for various engine management systems. TP sensor is mounted on throttle body assembly. See Fig. 1 . The TP sensor is a potentiometer type sensor with a 5-volt reference circuit, a low reference circuit and a sensor signal circuit. PCM provides the TP sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to Wide Open Throttle (WOT) position




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provides the PCM with a signal voltage from less than one volt to greater than 4 volts through the TP sensor signal circuit. When TP sensor signal voltage is not within the predicted range, a DTC sets. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.


The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. The TFT sensor is part of the TFP manual valve position switch assembly.







The Vehicle Speed Sensor (VSS) is a Permanent Magnet (PM) generator. VSS is mounted in transmission tailshaft. The VSS sends a pulsing signal to the PCM. The PCM then converts this signal into miles per hour by monitoring the time interval between pulses. PCM uses this sensor input in controlling Torque Converter Clutch (TCC) engagement, shift speed, etc.



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OUTPUT SIGNALS

A/C Clutch Relay


Cruise Control


Electronic Ignition


EVAP Canister

See EVAP CANISTER under EMISSION SYSTEMS & SUB-SYSTEMS.


See EVAP CANISTER PURGE SOLENOID under EMISSION SYSTEMS & SUB-SYSTEMS.


See EVAP CANISTER VENT SOLENOID under EMISSION SYSTEMS & SUB-SYSTEMS.

Fuel Injectors





See FUEL TANK PRESSURE SENSOR under EMISSION SYSTEMS & SUB-SYSTEMS.



Self-Diagnostics


Serial Data



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FUEL SYSTEMS

FUEL DELIVERY



Fuel Pump


When ignition switch is turned to ON position, PCM turns on electric fuel pump by energizing fuel pump relay. PCM keeps pump on if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off.

Fuel Pump Relay

When ignition switch is turned to ON position, PCM turns electric fuel pump on by energizing fuel pump relay. PCM keeps relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off within 2-20 seconds after key on.

FUEL CONTROL




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Closed Loop Mode




Starting With ignition switch in the ON position, before engaging starter, the PCM energizes the fuel pump relay for 2seconds allowing fuel pump to build pressure. Speed density is determined by inputs from the engine RPM, the IAT and MAP sensors. The PCM first tests speed density, then switches to the MAF sensor. PCM also uses the ECT, TP and MAP sensors to determine the proper air/fuel ratio for starting. The PCM controls the amount of fuel delivered in the starting mode by changing the width of the fuel injector pulse.


Run Mode The run mode has 2conditions. These conditions are called open loop and closed loop. When engine is first started and engine speed is above a predetermined RPM, the system begins open loop operation. The PCM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system stays in open loop until the following conditions are met: Both HO2Ss have varying voltage output, showing that they are hot enough to operate properly. This depends upon engine temperature. The ECT sensor is above a specified temperature. A specific amount of time has elapsed after starting the engine. Specific values for the above conditions exist for each different engine. These values are stored in the Electrically Erasable Programmable Read-Only Memory (EEPROM). The system begins closed loop operation after reaching these values. In closed loop, the PCM calculates the air/fuel ratio, fuel injector ON time, based upon the signal from various sensors, but mainly form the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration When the driver pushes on the accelerator pedal, the air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. In order to prevent possible hesitation, the PCM increases the pulse width



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to the fuel injectors in order to provide extra fuel during acceleration. The PCM determines amount of fuel required based upon the throttle position, coolant temperature, MAP, MAF and engine speed.



Fuel Cut-Off When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Fuel injector signals are cut off during periods of sudden, closed throttle deceleration (when fuel is not needed).

Open Loop Mode




IDLE SPEED



The engine idle speed is controlled by the Idle Air Control (IAC) valve. IAC valve is mounted on throttle body. See Fig. 1 . IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation or of movement, called steps. The stepper motor



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has 2 separate windings that are called coils. Each coil is supplied current by 2 circuits from the PCM. When PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition is turned off. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If engine idle speed is out of range for a calibrated period of time, an idle speed related DTC will set. To diagnose, perform related diagnostic test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.

IGNITION SYSTEMS


The Electronic Ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy and control of exhaust emissions. This ignition system consists of a separate ignition coil connected to each spark plug by a short secondary wire. The driver modules within each coil assembly are commanded ON/OFF by the PCM. The PCM primarily uses engine speed and position information from the CKP and CMP sensors to control the sequence, dwell, and timing of the spark.





CATALYTIC CONVERTER



The Evaporative Emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON (open), allowing engine vacuum to be applied to EVAP canister. With EVAP vent solenoid OFF (open), fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the



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canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

EVAP Canister

The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process. EVAP canister is located near fuel tank. See Fig. 9 .

Fig. 9: Locating EVAP System Canister & EVAP Canister Vent Solenoid Courtesy of GENERAL MOTORS CORP.


The EVAP canister purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. EVAP canister purge solenoid is located on top of intake manifold. See Fig. 1 . This normally closed solenoid is



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Pulse Width Modulated (PWM) by the control module to precisely control the flow of fuel vapor to engine. The solenoid will also be opened during some portions of EVAP testing, allowing engine vacuum to enter the EVAP system.















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A current DTC clears when the diagnostic runs and passes. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other diagnostic. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 4.8L, 5.3L & 6.0L "C", "G", "H", "K" & "N" SERIES- FLEX FUEL & GASOLINE article.


As a bulb and system check, the MIL will illuminate when ignition switch is turned to ON position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. MIL may be used on some models to display a stored DTC. To access DTCs, see SELF-DIAGNOSTICS - 4.8L, 5.3L & 6.0L "C", "K" & "N" SERIES - FLEX FUEL & GASOLINE article.

SERIAL DATA



A/C CLUTCH

PCM regulates operation of the A/C clutch through a relay. The PCM disengages the A/C compressor when compressor load on engine may cause driveability problems (i.e., during hot restart, idle, low speed steering




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maneuvers and wide open throttle operation) or if A/C refrigerant pressure drops to less than or rises to greater than normal operating levels.





CRUISE CONTROL


TRANSMISSION





When vehicle speed is great enough (about 20-45 MPH as indicated by vehicle speed sensor), PCM energizes TCC solenoid mounted in transmission, allowing torque converter to directly connect engine to the transmission. When operating conditions indicate transmission should operate as normal, TCC solenoid is de-energized, allowing transmission to return to normal automatic operation. Since power for the TCC solenoid is



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delivered through the brake switch, transmission also returns to normal automatic operation when brake pedal is depressed.



1-2 & 2-3 Shift Solenoid Valves The 1-2 and 2-3 shift solenoid valves (also called "A" and "B" solenoids) are identical devices that control the movement of the 1-2 and 2-3 shift valves (the 3-4 shift valve is not directly controlled by a shift solenoid). The solenoids are normally-open exhaust valves that work in four combinations to shift the transmission into different gears. PCM energizes each solenoid by grounding the solenoid through an internal quad driver. This sends current through the coil winding in the solenoid and moves the internal plunger out of the exhaust position. When on, the solenoid redirects fluid to move a shift valve. The PCM controlled shift solenoids eliminate the need for TV and governor pressures to control shift valve operation.




TCC Pulse-Width Modulation Solenoid Valve The Torque Converter Clutch Pulse Width Modulation (TCC PWM) solenoid valve controls the fluid



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acting on the converter clutch valve. The converter clutch valve controls the TCC apply and release. This solenoid is attached to the control valve body assembly within the transmission. The TCC PWM solenoid valve provides a smooth engagement of the torque converter clutch by operating during a duty cycle percent of ON time.

Transmission Fluid Pressure Manual Valve Position Switch The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. In order to monitor the TFP manual valve position switch operation, the PCM compares the actual voltage combination of the switches to a TFP combination table stored in its memory. The Transmission Fluid Temperature (TFT) sensor is part of the TFP manual valve position switch assembly.





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Theory & Operation - 6.0L Chevy Express, Savana, Sierra & Silverado - Bi-Fuel & CNG

INTRODUCTION



AIRFLOW SENSING

Speed Density



The MAP sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits: 5-volt reference, low reference circuit and MAP sensor signal circuit. The PCM supplies 5 volts to the MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. MAP sensor provides a signal to PCM on MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off, or at a Wide-Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when the ignition is on, with engine off. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range.

The IAT sensor is a variable resistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, go to SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.

NOTE: Unless indicated otherwise, Savana includes Savana Special.


2003 ENGINE PERFORMANCE Theory & Operation - 6.0L Chevy Express, Savana, Sierra & Silverado - Bi-Fuel & CNG



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Fig. 1: Locating ECT Sensor, EVAP Canister Purge Solenoid, IAC Valve, MAP Sensor & TP Sensor Courtesy of GENERAL MOTORS CORP.



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Fig. 2: Locating IAT/MAF Sensor Courtesy of GENERAL MOTORS CORP.






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The Powertrain Control Module (PCM) controls fuel delivery and determines which fuel system operates the engine. PCM is located on left front of engine compartment, near radiator. See Fig. 3 . PCM monitors various engine and vehicle functions to provide the correct amount of CNG or gasoline fuel under all operating conditions. This provides excellent driveability and fuel economy while maintaining reduced emission levels. When operating on gasoline, PCM will turn on the Fuel Indicator Light (FIL). The FIL is located within the headlight switch assembly, near the fuel gauge select switch. The FIL is also turned on for a 2 second bulb check whenever ignition is turned on. The fuel gauge select switch allows the vehicle operator the ability to request fuel gauge display of either fuel tank.

Fig. 3: Locating FICM/PCM Courtesy of GENERAL MOTORS CORP.

FUEL INJECTOR CONTROL MODULE

The Fuel Injector Control Module (FICM) is a non-flashable control module that is not capable of serial data communication. On Savana, Sierra and Silverado, FICM is located behind the right side wheelhouse panel on. On Chevy Express, FICM is located next to the PCM. See Fig. 3 . The FICM performs the following tasks:

NOTE: PCM is used on all engine operation.

NOTE: FICM is used to operate the CNG injectors, when engine is running on CNG. PCM is still used to control the other components.



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Operates the CNG injectors based upon the PCM injector signals. Supplies reference, ground and signal circuits to the Fuel Rail Temperature (FRT), Fuel Tank Pressure (FTP) and Fuel Tank Temperature (FTT) sensors. Communicates the FRT, FTP and FTT sensor values to the PCM using a single dedicated PWM circuit. Communicates FICM diagnostic information to the PCM using a dedicated PWM circuit.

The PCM controls fuel delivery and determines fuel system operation on bi-fuel (RPO KL6) equipped vehicles. The PCM monitors various engine and vehicle functions to provide the correct amount of CNG or gasoline fuel under all operating conditions. The fuel injector control circuits, the AF enable circuit and the 3 PWM communication circuits connect the PCM to the FICM. PCM is not capable of operating the high current CNG fuel injectors. PCM injector pulse width signals are received by the FICM and duplicate pulse width signals are generated by the FICM in order to operate the CNG injectors. KL6 equipped vehicles utilize a FICM that contains internal relays. When operating on gasoline, the internal relays allow the PCM injector pulse width to go directly to the gasoline injectors.

INPUT DEVICES





NOTE: Components are grouped into 2 categories. The first category is INPUT DEVICES, consisting of components which control or produce voltage signals monitored by the control unit. The second category is OUTPUT SIGNALS, consisting of components controlled by the PCM.



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Battery Voltage

Battery voltage is monitored by PCM. If battery voltage swings low, a weak spark or improper fuel control may



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result. To compensate for low battery voltage, PCM may increase idle speed, advance ignition timing, increase ignition dwell or richen the air/fuel mixture. If voltage swings high, PCM may set a charging system fault code and turn on Malfunction Indicator Light (MIL). If voltage signal swings excessively low (less than 9 volts) or excessively high (16 volts), PCM shuts down for as long as condition exists. If condition is short-term, MIL flickers and vehicle may stumble. Vehicle stalls if condition persists.




The Camshaft Position (CMP) sensor is a hall-effect type sensor. The sensor produces one signal for each revolution of the camshaft in order to control the sequential fuel injection. The CMP sensor is designed to detect changes in a magnetic field. The PCM supplies the CMP sensor with a 12-volt reference circuit, a low reference circuit and a signal circuit. The CMP sensor produces a magnetic field whenever ignition is on. The CMP sensor is mounted near a reluctor wheel that is attached to the distributor shaft. See Fig. 5 . When distributor shaft rotates, and reluctor wheel tooth passes by the CMP sensor, there is a change in the magnetic field. The CMP sensor converts each change in the magnetic field into a PULSE. If PCM does not detect the CMP signal while the engine is running, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.



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Fig. 5: Locating Camshaft Position & Engine Oil Pressure Sensors Courtesy of GENERAL MOTORS CORP.

Cranking Signal



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The PCM uses the Crankshaft Position (CKP) sensor to detect crankshaft speed and position. The CKP sensor is located on side of engine block, behind harmonic balancer. See Fig. 6 . The CKP sensor connects to the PCM through the 12-volt reference circuit, the low reference circuit and CKP sensor 1 signal circuit. If PCM detects that the CKP sensor signal is incorrect for 3 seconds, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.




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The Engine Coolant Temperature (ECT) sensor is a variable resistor, that measures the temperature of engine coolant. ECT is located on side of left cylinder head. See Fig. 1 . The PCM supplies 5 volts to the ECT signal circuit and ground for the ECT low reference circuit. When ECT is cold, sensor resistance is high. When ECT increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on ECT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the ECT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.

Fuel Rail Pressure Sensor (CNG)

The Fuel Rail Pressure (FRP) sensor is a pressure transducer. The FRP sensor is mounted on fuel rail. The FICM supplies about 5 volts on the FRP sensor reference circuit. The FICM also supplies a signal and ground circuit to the FRP sensor. When fuel pressure is high, signal voltage is high. The FRP sensor data is transmitted to the PCM through a PWM signal circuit. The FRP signal is used by the PCM for fuel trim.

Fuel Rail Temperature Sensor (CNG)

The Fuel Rail Temperature (FRT) sensor is a thermistor. The FRT thermistor has high resistance when cold and low resistance when hot. The FRT sensor is mounted on fuel rail. The FICM supplies about 5 volts to the FRT sensor signal circuit. When FRT sensor is cold, resistance is high and signal voltage is high. As the FRT sensor warms and resistance drops, less signal voltage is monitored by the FICM. The FRT sensor data is transmitted to the PCM through a PWM signal circuit. The FRT signal is used by the PCM in order to adjust fuel trim.

Fuel Tank Pressure Sensor (CNG)

The Fuel Tank Pressure (FTP) sensor is a pressure transducer. FTP sensor is threaded in rear tank High Pressure Regulator (HPR). HPR is located in-line, near CNG fuel filter. PCM supplies about 5 volts on the FTP reference circuit. PCM also supplies a signal and ground circuit to the FTP sensor. When fuel tanks are full (high pressure), a high voltage signal will be monitored by the PCM. The FTP sensor is threaded into one of the rear fuel tank HPL solenoids. The volume of CNG will vary with temperature and pressure. Accurate CNG fuel level cannot be determined by pressure only. In order to compensate for the different volume factors a Fuel Tank Temperature (FTT) sensor is mounted within the HPL. PCM monitors FTT sensor and performs a calculation on the FTP voltage vs. the in-tank temperature. The PCM will then display the temperature corrected fuel level. The CNG FTP sensor should not be confused with the gasoline Fuel Tank Pressure (FTP) sensor that is utilized for EVAP emissions monitoring.

Fuel Tank Temperature Sensor (CNG)

The Fuel Tank Temperature (FTT) is a thermistor mounted inside the HPL and is not serviceable separately from the High Pressure Lock-Off (HPL) solenoid. See Fig. 8 or Fig. 9 . The FTT thermistor has high resistance when cold and low resistance when hot. PCM supplies about 5 volts to the FTT signal circuit. When FTT resistance is high (cold sensor), the FTT signal voltage is high. As the FTT warms and resistance drops, less signal voltage is monitored by the PCM. The volume of CNG will vary with temperature and pressure. Accurate CNG fuel level cannot be determined by pressure only. In order to compensate for the different volume factors, an FTT sensor is mounted within the HPL. PCM monitors the FTT sensor and performs a calculation on the



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FTP voltage vs. the in-tank temperature. The PCM will then display the temperature corrected fuel level.

Intake Air Temperature/Mass Airflow Sensor

The Intake Air Temperature (IAT) sensor is a variable resistor and is integral to the Mass Airflow (MAF) sensor. The IAT/MAF sensor located in fresh air intake tube. See Fig. 2 . The IAT/MAF sensor has a signal and a low reference circuit. The IAT/MAF sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT/MAF signal circuit and a ground for the IAT/MAF low reference circuit. When IAT/MAF sensor is cold, the sensor resistance is high. When air temperature increases, the sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT/MAF signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT/MAF signal circuit. Failure in IAT/MAF sensor circuit should set a related DTC. To diagnose, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.

Knock Sensor

The Knock Sensor (KS) produces an AC voltage at all engine speeds and loads. KS is located on top rear of engine. See Fig. 7 . The PCM then adjusts the spark timing based on the amplitude and frequency of the KS signal. PCM uses the KS signal to calculate the average voltage. Then PCM assigns a voltage value. PCM checks the knock sensor and related wiring by comparing the actual knock signal to the assigned voltage range. A normal KS signal should stay within the assigned voltage range. A fault in the KS circuit may set a DTC. See SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.



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Fig. 7: Locating Knock Sensors Courtesy of GENERAL MOTORS CORP.


The Manifold Absolute Pressure (MAP) sensor responds to pressure changes in the intake manifold. MAP sensor is mounted on top of intake manifold. See Fig. 1 . The pressure changes occur based on engine load. The MAP sensor has a 5-volt reference circuit, a low reference circuit and a signal circuit. The PCM supplies 5 volts to MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric



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Pressure (BARO). This occurs when ignition is on, with engine off. The BARO reading may also be updated whenever engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range. If PCM detects a MAP sensor signal voltage that is out-of-range, a DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.




Park/Neutral Position Switch

The Park/Neutral Position (PNP) switch is connected to transmission gear selector and signals PCM when transmission is in Park or Neutral. PCM uses this information for determining control of ignition timing, Torque Converter Clutch (TCC) and idle speed. To check function of PNP switch, perform functional check of switch. See SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.


The Throttle Position (TP) sensor is used by the PCM to determine the throttle plate angle for various engine management systems. TP sensor is mounted on throttle body assembly. See Fig. 1 . The TP sensor is a potentiometer type sensor with a 5-volt reference circuit, a low reference circuit and a sensor signal circuit. PCM provides the TP sensor with 5 volts on the 5-volt reference circuit and a ground on the low reference circuit. Rotation of the TP sensor rotor from the closed throttle position to Wide Open Throttle (WOT) position provides the PCM with a signal voltage from less than one volt to greater than 4 volts through the TP sensor signal circuit. When TP sensor signal voltage is not within the predicted range, a DTC sets. To diagnose, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.


The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2




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normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. The TFT sensor is part of the TFP manual valve position switch assembly.







The Vehicle Speed Sensor (VSS) is a Permanent Magnet (PM) generator. VSS is mounted in transmission tailshaft. The VSS sends a pulsing signal to the PCM. The PCM then converts this signal into miles per hour by monitoring the time interval between pulses. PCM uses this sensor input in controlling Torque Converter Clutch (TCC) engagement, shift speed, etc.

OUTPUT SIGNALS

A/C Clutch Relay


Cruise Control



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Electronic Ignition


Fuel Injectors






Self-Diagnostics


Serial Data








FUEL SYSTEMS

FUEL DELIVERY

Fuel Pressure Regulator (Gasoline)

Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator maintains a pressure of 55-62 psi (385-425 kPa) under all operating conditions. Pressure regulator compensates for engine load by increasing fuel pressure when low



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manifold vacuum is experienced.

Fuel Pump (Gasoline)


When ignition switch is turned to ON position, PCM turns on electric fuel pump by energizing fuel pump relay. PCM keeps pump on if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off.

Fuel Pump Relay (Gasoline)

When ignition switch is turned to ON position, PCM turns electric fuel pump on by energizing fuel pump relay. PCM keeps relay energized if engine is running or cranking (PCM is receiving reference pulses from ignition module). If there are no reference pulses, PCM turns pump off within 2-20 seconds after key on.

For additional information on fuel pump activation, see SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.

High Pressure Regulator Solenoid Valve (CNG)

The High Pressure Regulator (HPR) solenoid valve is supplied with fuel from the CNG fuel tank at up to 3600 psi (24,821 kPa) through high pressure stainless steel lines. See Fig. 8 or Fig. 9 . The HPR reduces fuel pressure to about 180 psi (1241 kPa). The pressure drop within the regulator causes fuel temperature to drop. The HPR is connected to the engine cooling system in order to prevent HPR freeze-up. The outlet of the HPR is the injector pressure stage. Fuel flows out of the HPR and through the in-line filter and into the Low Pressure Lock-Off (LPL) solenoid. The HPR contains an Over-Pressure Relief Device (PRD) which will not allow pressure above 275 psi (1896 kPa) on the output stage of the HPR. The HPR has an internal, serviceable 40 micron filter. This filter must be serviced periodically.

Low Pressure Lock-Off Solenoid (CNG)

The Low Pressure Lock-Off (LPL) solenoid is a normally closed solenoid valve. The solenoid provides a fuel shut-off to the fuel rails. The LPL solenoid is mounted on right hand fuel rail. See Fig. 8 or Fig. 9 . The LPL is only commanded ON when engine RPM indicates that engine is cranking or running on CNG.

FUEL CONTROL


Closed Loop Mode



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Fuel System Operation (Bi-Fuel & CNG)

The primary fuel source of a bi-fuel vehicle is Compressed Natural Gas (CNG). The engine will only switch over to gasoline operation if one of the following conditions is present: Fuel tank pressure sensor indicates CNG fuel pressure is less than 130 psi (896 kPa), every 100th start (engine will start and run briefly on gasoline), after this, a switchover to CNG will occur during the first decel condition, a system fault is detected by PCM or FICM or engine cranks for 5 seconds and fails to start on CNG.

Fuel System Operating Modes (Gasoline)


Starting With ignition switch in the ON position, before engaging starter, the PCM energizes the fuel pump relay for 2seconds allowing fuel pump to build pressure. Speed density is determined by inputs from the engine RPM, the IAT and MAP sensors. The PCM first tests speed density, then switches to the MAF sensor. PCM also uses the ECT, TP and MAP sensors to determine the proper air/fuel ratio for starting. The PCM controls the amount of fuel delivered in the starting mode by changing the width of the fuel injector pulse.


Run Mode The run mode has 2conditions. These conditions are called open loop and closed loop. When engine is first started and engine speed is above a predetermined RPM, the system begins open loop operation. The PCM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP and TP sensors. The system stays in open loop until the following conditions are met: Both HO2Ss have varying voltage output, showing that they are hot enough to operate properly. This depends upon engine temperature. The ECT sensor is above a specified temperature. A specific amount of time has elapsed after starting the engine. Specific values for the above conditions exist for each different engine. These values are stored in the Electrically Erasable Programmable Read-Only Memory (EEPROM). The system begins closed loop operation after reaching these values. In closed loop, the PCM calculates the air/fuel ratio, fuel injector ON time, based upon the signal from various sensors, but



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mainly form the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration When the driver pushes on the accelerator pedal, the air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. In order to prevent possible hesitation, the PCM increases the pulse width to the fuel injectors in order to provide extra fuel during acceleration. The PCM determines amount of fuel required based upon the throttle position, coolant temperature, MAP, MAF and engine speed.



Fuel Cut-Off When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the PCM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM or excessive vehicle speed to prevent internal damage to engine. Fuel injector signals are cut off during periods of sudden, closed throttle deceleration (when fuel is not needed).

Open Loop Mode






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Fig. 8: Identifying CNG System Components (Chevy Express & Savana) Courtesy of GENERAL MOTORS CORP.



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Fig. 9: Identifying CNG System Components (Sierra & Silverado) Courtesy of GENERAL MOTORS CORP.

IDLE SPEED



The engine idle speed is controlled by the Idle Air Control (IAC) valve. IAC valve is mounted on throttle body. See Fig. 1 . IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper



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motor. The stepper motor is capable of highly accurate rotation or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is supplied current by 2 circuits from the PCM. When PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition is turned off. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If engine idle speed is out of range for a calibrated period of time, an idle speed related DTC will set. To diagnose, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.


The Throttle Actuator Control (TAC) system delivers improved throttle response and greater reliability and eliminates the need for mechanical cable. The TAC system performs accelerator pedal position sensing, throttle positioning to meet driver and engine demands, throttle position sensing, internal diagnostics, cruise control functions and manage TAC electrical power consumption.

The TAC system components include the Accelerator Pedal Position (APP) sensor, throttle body assembly, throttle actuator control module and PCM. See Fig. 10 .



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IGNITION SYSTEMS


The Electronic Ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy and control of exhaust emissions. This ignition system consists of a separate ignition coil connected to each spark plug by a short secondary wire. The driver modules within each coil assembly are commanded ON/OFF by the PCM. The PCM primarily uses engine speed and position



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information from the CKP and CMP sensors to control the sequence, dwell, and timing of the spark.





CATALYTIC CONVERTER



The Evaporative Emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON (open), allowing engine vacuum to be applied to EVAP canister. With EVAP vent solenoid OFF (open), fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

EVAP Canister

The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process. EVAP canister is located near fuel tank. See Fig. 11 .



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Fig. 11: Locating EVAP System Canister & EVAP Canister Vent Solenoid Courtesy of GENERAL MOTORS CORP.


The EVAP canister purge solenoid controls the flow of vapors from the EVAP system to the intake manifold. EVAP canister purge solenoid is located on top of intake manifold. See Fig. 1 . This normally closed solenoid is Pulse Width Modulated (PWM) by the control module to precisely control the flow of fuel vapor to engine. The solenoid will also be opened during some portions of EVAP testing, allowing engine vacuum to enter the EVAP system.






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A current DTC clears when the diagnostic runs and passes. A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other diagnostic. Intermittent failures may be caused by sensor, connector or wiring related problems. See TROUBLE SHOOTING - NO CODES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.


As a bulb and system check, the MIL will illuminate when ignition switch is turned to ON position and engine is not running. When engine is started, MIL should go out. If MIL does not go out, a malfunction has been detected in the computerized engine control system or MIL circuit is faulty. MIL may be used on some models to display a stored DTC. To access DTCs, see SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or SELF-DIAGNOSTICS - 6.0L SIERRA & SILVERADO - BI-FUEL & CNG article.

SERIAL DATA



A/C CLUTCH




A/C high and low pressure switches may be used in the A/C compressor clutch or compressor clutch relay




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circuit. Switches are normally closed, completing the circuit which energizes the compressor clutch. When system refrigerant pressure increases beyond a certain point, high side switch opens, causing compressor clutch to disengage.


CRUISE CONTROL


TRANSMISSION








1-2 & 2-3 Shift Solenoid Valves



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The 1-2 and 2-3 shift solenoid valves (also called "A" and "B" solenoids) are identical devices that control the movement of the 1-2 and 2-3 shift valves (the 3-4 shift valve is not directly controlled by a shift solenoid). The solenoids are normally-open exhaust valves that work in four combinations to shift the transmission into different gears. PCM energizes each solenoid by grounding the solenoid through an internal quad driver. This sends current through the coil winding in the solenoid and moves the internal plunger out of the exhaust position. When on, the solenoid redirects fluid to move a shift valve. The PCM controlled shift solenoids eliminate the need for TV and governor pressures to control shift valve operation.





Transmission Fluid Pressure (TFP) Manual Valve Position Switch The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position



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switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. In order to monitor the TFP manual valve position switch operation, the PCM compares the actual voltage combination of the switches to a TFP combination table stored in its memory. The Transmission Fluid Temperature (TFT) sensor is part of the TFP manual valve position switch assembly.





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Silverado 0a Theory & Operation

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