Chapter 9

Cranking Systems

Objectives (1 of 2)

• Identify the components in a truck cranking system.

• Explain the operating principles of magnetic switches, solenoids, and starter motors.

• Describe the operating principles of lightweight, planetary gear reduction starter motors.

• Test and troubleshoot a cranking circuit using voltage drop testing.

Objectives (2 of 2)

• Disassemble a heavy-duty truck starter motor.

• Test an armature for shorts using a growler.

• Test an armature for grounds and opens.

• Use a test light to check out field coils.

• Outline the procedure required to rebuild a Delco-Remy 42MT starter motor.

Cranking Systems (1 of 3)

Cranking Systems (2 of 3)

• The cranking system in any vehicle is designed to turn the engine over until it can operate under its own power.

• A cranking system can be divided into two sub-circuits:– Control circuit– Starter circuit

Cranking Systems (3 of 3)

• In a typical cranking system, the components are:– Battery– Key switch (or starter button)– Battery cables– Magnetic switch– Starter motor

Battery Terminals

• Cleaning terminals– If any evidence of corrosion is observed on

visual inspection, a battery terminal should be removed from the post and cleaned.

• Testing terminals– The only way to determine whether a battery

terminal is doing its job is to test it by measuring voltage drop.

Cables

• Cables conduct the heavy current load from the battery to the starter and from the starter back to the battery.

• All cables must be in good condition.

• Cables must be heavy enough to carry the required current load.

• Cables can be corroded by battery acid.

• Corrosion will cause a voltage drop and decrease circuit amperage, reducing power available to the starter.

Cranking Circuit Components

• See Table 9-1 on page 230 of the textbook.

Starter Motor Operation (1 of 2)

Starter Motor Operation (2 of 2)

Starter Solenoids

Override Clutches

Reduction-gearing Starter Motors (1 of 4)

• Reduction gearing starters are lightweight starters capable of cranking engines up to 16 liters.

• Lightweight starters are able to crank large bore, high-compression engines because they use a planetary gearset.

• This arrangement provides for a reduction ratio of about 3.5:1.

Reduction-gearing Starter Motors (2 of 4)

Reduction-gearing Starter Motors (3 of 4)

Reduction-gearing Starter Motors (4 of 4)

Cranking Circuit Testing

• The cranking circuit requires testing when:– The engine will not crank– The engine cranks slowly– The starter motor will not turn

Preliminary Checks

• Cranking output obtained from the starter motor is affected:– By the condition and charge of the battery– The wiring circuit – The engine cranking requirements

Troubleshooting

• See Table 9-2 on pages 240-241 in the textbook.

Shop Talk

• Starter motors can only operate for short periods without rest.

• The high current needed to operate the starter creates considerable heat, and continuous operation will cause overheating.

• A starter motor should never operate for more than 30 seconds at a time and should rest for 2 minutes between cranking. This permits the heat to dissipate without damage to the unit.

Starter Relay Testing

• The starter relay bypass test is a simple method of determining if the relay is operational.

• Connect a jumper cable around the starter relay. This bypasses the relay.

• Then, crank the engine.

• If the engine cranks with the jumper installed, the starter relay is defective and should be replaced.

• If the motor still will not crank, check the control circuit.

Shop Talk

• In some starters, the solenoid also performs the function of a relay.

• The control circuit is wired to the windings of the solenoid.

• Battery current is routed directly to the starter motor through the drive solenoid.

Cranking Current Testing (1 of 2)

• The cranking current test measures the amount of current, in amperes, that the starter circuit draws to crank the engine.– 1. Connect the leads of a volt-amp tester

(Figure 9-20).– 2. Set the carbon pile to its maximum resistance

(open).– 3.Crank the engine and observe the voltmeter

reading.– 4. Stop cranking and adjust the carbon pile until the

voltmeter reading matches the reading taken in step 3.

– 5. Note the ammeter reading.

Shop Talk

• If the analyzer uses an inductive pickup, ensure that the arrow on the inductive pickup is pointing in the right direction as specified on the ammeter.

• Then, crank the engine for 15 seconds and observe the ammeter reading.

Cranking Current Testing (2 of 2)

• See Table 9-3 on page 242 of the textbook.

Shop Talk

• The starter should not be operated if the voltage at the battery is below 9.6 volts.

• Some leasing companies now use a voltage-sensing module to prevent starter operation if voltage is below 9.6 volts to prevent drivers from destroying starters.

Starter Circuit Testing

Warning

• When performing this test, do not operate the solenoid for extended periods of time as severe overheating will occur.

Adjusting Pinion Clearance

Shop Talk

• When testing starter circuits, use the OEM- recommended method of preventing the engine from starting.

• Whenever possible, use autoranging DMMs to perform voltage-drop tests.

Summary (1 of 3)

• The vehicle cranking circuit functions to crank the engine until it can operate under its own power.

• A cranking circuit is managed by a control circuit that uses low current to switch and energize a high-current starter motor circuit.

• A starter motor converts the electrical energy of the vehicle batteries into mechanical energy for cranking the engine.

• A powerful electro-magnetic field in the starter motor field coils is used to rotate the armature assembly in a typical starter motor.

Summary (2 of 3)

• Most starter motors are series wound, so there is only one path for current flow through the assembly.– This means that all of the current flowed through the

field coils also flows through the armature producing peak torque at close to stall speeds.

• A new generation of lightweight, high-torque starter motors uses planetary gear reduction to multiply cranking torque and greatly reduce motor weight.

• Voltage-drop testing should be used to troubleshoot a cranking circuit malfunction because it does so dynamically.

Summary (3 of 3)

• A heavy-duty truck starter motor found to be defective should be removed from the engine and either rebuilt or replaced.

• A growler is used to test an armature for shorts: A steel strip such as a hacksaw blade is placed on top of the armature during testing and if it vibrates, a short is indicated.

• A test light is used to test an armature for grounds and opens. • Field coils should be checked using a test light. • After reassembling a starter motor, the pinion clearance should

be checked with thickness gauges and adjusted if necessary.