Idle Air Control (IAC) System Diagnosis

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Schematic:

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

DIAGNOSTIC AIDS
Inspect for the following conditions:
- Inspect the wiring harness for damage.
- A restricted air intake system. Inspect for a collapsed air intake duct, a restricted air filter element, or foreign objects blocking the air intake system.
- The throttle body. Inspect 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.
Inspect for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which do not allow the IAC pintle to fully extend.
- A vacuum leak - Inspect for conditions that cause vacuum leaks, such as:
- Disconnected or damaged hoses
- Leaks at the EGR valve and at the EGR pipe to the intake manifold
- Leaks at the throttle body
- A faulty or incorrectly installed PCV valve
- Leaks at intake manifold
- The brake booster hose disconnected, etc.

IMPORTANT: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

- Poor terminal connection-Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams.
- Damaged harness-Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Diagrams.
- PCM and engine grounds for clean and secure connections

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

TEST DESCRIPTION
The numbers below refers to the step number on the Diagnostic Table:
11. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read-only Memory (EEPROM). When the PCM is replaced, the new PCM must be programmed.