Idle Air Control (IAC) System Diagnosis

CIRCUIT DESCRIPTION
The Idle Air Control (IAC) valve is located in the throttle body of both the SFI and the MFI Systems. The IAC consists of a movable pintle that is driven by a gear that is attached to an electric motor called a stepper motor. The IAC valve motor is a two-phase bi-polar permanent magnet stepper motor. The motor is capable of highly accurate rotation or movement, every time the polarity of a winding changes. You can see this change in polarity when you observe a test lamp that is connected between ground or B+ and an IAC Valve circuit while the PCM is attempting to change engine RPM. The test lamp will flash ON or OFF each time the polarity changes. The PCM does not use a physical sensor to determine IAC pintle position, but uses a predicted number of counts. One count represents one change in polarity, which equals one step of the stepper motor. The PCM counts the steps it has commanded in order to determine IAC pintle position. The PCM uses the IAC valve in order to control the engine idle speed. It does this by changing the pintle position in the idle air passage of the throttle body. This position varies the air flow around the throttle plate when the throttle is closed. In order to determine the desired position of the IAC pintle during idle or during deceleration, the PCM refers to the following inputs:
^ The engine RPM
^ The battery voltage
^ The air temperature
^ The engine coolant temperature
^ The throttle position sensor angle
^ The engine load
^ The vehicle speed

When you turn OFF the ignition after an ignition cycle, the PCM will seat the IAC pintle in the air bypass bore, and then retract the pintle a predetermined amount of counts in order to allow the proper amount of air to bypass the throttle plate for engine start-up. This procedure is known as an IAC Reset.

DIAGNOSTIC AIDS
Inspect for the following conditions:
^ A skewed high TP sensor
^ A restricted air intake system
^ A blocked IAC passage or a blocked throttle bore
^ A PCV valve that is incorrect, improperly installed, and improperly operating
^ Air intake components that are improperly installed and improperly operating
^ A MAF sensor that is improperly installed or improperly operating, if the vehicle has this equipment
^ A throttle stop screw that has evidence of tampering or damage
^ Evidence of tampering or damage to the following components:
- The throttle plate
- The throttle shaft
- The throttle linkage
- The cruise control linkage, if the vehicle has this equipment
^ Excessive deposits in the following components:
- The AC passage
- The LAC pintle
- The throttle bore
- The throttle plate
^ Vacuum leaks
^ Excessive load on the engine, from the following components:
- The transmission
- The power steering
- The alternator
^ A high or unstable idle condition can be caused by a non-IAC system problem that the IAC valve cannot overcome. Refer to Symptoms -Computers and Control Systems. Symptom Related Diagnostic Procedures
^ A low or unstable idle condition can be caused by a non-IAC system problem that the IAC valve cannot be overcome. Refer to Symptoms - Computers and Control Systems. Symptom Related Diagnostic Procedures
^ If the problem is intermittent, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-6:

Steps 7-10:

Steps 11-17:

The numbers below refer to the step numbers on the diagnostic table.
5. This test will determine the ability of the PCM and IAC valve circuits to control the IAC valve.
7. This test will determine the ability of the PCM to provide the IAC valve circuits with a ground. On a normally operating system, the test lamp should not flash while the IAC counts are incrementing.