Idle Air Control (IAC) System Diagnosis

Schematic:

CIRCUIT DESCRIPTION

The Powertrain Control Module (PCM) controls the engine idle speed by adjusting the position of the motor pintle of the Idle Air Control (IAC). The AC is a bi-directional motor driven by 2 coils. The PCM pulses current to the AC coils in steps in order to extend the IAC pintle into a passage in the throttle body in order to decrease air flow. The PCM reverses the current pulses in order to retract the pintle and increase the air flow. This method allows highly accurate control of the idle speed and quick response to changes in the engine load.

DIAGNOSTIC AIDS
Check for the following conditions:
^ A faulty connection at the PCM or at the IAC valve. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal-to-wire connections. Use a corresponding mating terminal to test for proper terminal tension.
^ Inspect the wiring harness for damage.
^ A restricted air intake system. Check for a collapsed air intake duct, a restricted air filter element, or any foreign objects blocking the air intake system.
^ Check the throttle body for objects blocking the IAC passage or the throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the throttle plate Check for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which prevent the IAC pintle from fully extending.
^ Check for a condition that causes a vacuum leak, such as disconnected or damaged hoses, any leaks at the EGR valve and the EGR pipe-to-intake manifold, any leaks at the throttle body, a faulty or incorrectly installed PCV valve, any leaks at the intake manifold, a disconnected brake booster hose, etc.
^ Check for a missing oil filler cap or a missing oil level indicator.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.


TEST DESCRIPTION

Steps 1 - 7:

Steps 8 - 12:

The numbers below refer to the step numbers on the diagnostic table.
11. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read-only Memory (EEPROM). If the PCM is replaced, the new PCM must be programmed.