IDLE SPEED CONTROL

§To comply with federal emissions standards, idle speed control systems are used §Idle speed controlled by electronic module §Earlier systems, engines idled in open loop §Idle systems depend on inputs to PCM

Throttle Kicker controlled by Vacuum Diaphragm §Electrically controlled by solenoid §Operates during engine warm up, with air conditioning on and engine overheating §Some act as a dashpot 327C

Throttle Kicker and Idle Stop Solenoid §Electrically controlled §Vacuum operated throttle kicker for additional load §Allows curb idle speed to be lower §Controlled by PCM §Must be adjusted 327C

IDLE SPEED CONTROL MOTOR (ISC) §PCM controlled §Reversible gear drive dc motor §PCM reverses polarity §Has idle tracking switch §Must be adjusted 325C

ISC Schematic

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 Location §Screws into throttle body §Must be careful to not over-tighten §Must keep passages clean §Can use Scanner to diagnose idle problems

IAC SCHEMATIC §Contains two motor windings §Can test with ohmmeter (GM are ohms) §Use IAC Test tool to check operation §Can monitor operation with Scanner 508L

IAC SCOPE PATTERNS §Typical patterns §Compare patterns to known good patterns §Some IAC’s operate on 12 volts and others on less voltage §IAC operates with a duty cycle

ADJUSTING CURB IDLE (Idle Hardstop or Minimum Idle speed) §Minimum idle rpm is factory adjusted §Idle must be adjusted correctly §Passages must be clean §Some systems, you must adjust TPS also 502L

IDLE 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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