IDLE SPEED CONTROLIDLE SPEED CONTROL

IDLE SPEED CONTROLIDLE SPEED CONTROL

To comply with federal emissions standards, idle speed control systems are used

Idle speed controlled by electronic moduleEarlier systems, engines idled in open loopIdle systems depend on inputs to PCM

Throttle Kicker controlled by Throttle Kicker controlled by Vacuum DiaphragmVacuum Diaphragm

Electrically controlled by solenoidOperates during engine warm up, with air

conditioning on and engine overheatingSome act as a dashpot

327C

Throttle Kicker and Idle Stop Throttle Kicker and Idle Stop SolenoidSolenoid

Electrically controlledVacuum operated

throttle kicker for additional load

Allows curb idle speed to be lower

Controlled by PCMMust be adjusted

327C

IDLE SPEED CONTROL IDLE SPEED CONTROL MOTOR (ISC)MOTOR (ISC)

PCM controlledReversible gear drive

dc motorPCM reverses

polarityHas idle tracking

switchMust be adjusted325C

ISC SchematicISC Schematic

IDLE AIR CONTROL (IAC)IDLE AIR CONTROL (IAC)

Used with Fuel Injection systems

PCM controlled Stepper motor

Diverts air around throttle plate

Do not over extend when replacing

Not adjustable

Throttle Body IAC LocationThrottle Body IAC Location

Screws into throttle body

Must be careful to not over-tighten

Must keep passages clean

Can use Scanner to diagnose idle problems

IAC SCHEMATICIAC SCHEMATIC

Contains two motor windingsCan test with ohmmeter (GM are 40 - 80 ohms)Use IAC Test tool to check operationCan monitor operation with Scanner

508L

IAC SCOPE PATTERNSIAC SCOPE PATTERNS

Typical patternsCompare patterns to

known good patternsSome IAC’s operate

on 12 volts and others on less voltage

IAC operates with a duty cycle

ADJUSTING CURB IDLEADJUSTING CURB IDLE(Idle Hardstop or Minimum (Idle Hardstop or Minimum Idle speed)Idle speed)Minimum idle rpm is

factory adjusted Idle must be adjusted

correctlyPassages must be

cleanSome systems, you

must adjust TPS also

502L

IDLE SPEED ADJUSTMENTIDLE SPEED ADJUSTMENT

Must shut off idle air to adjust minimum idle speed

Use idle plug on TBI units

Other systems, you just turn a screw to control air flow (idle rpm)

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