Idle Air Control (IAC) System Diagnosis

CIRCUIT DESCRIPTION
The Idle Air Control (IAC) valve is located in the throttle body of both the Throttle Body Fuel Injection (TBI) and the Multiport Fuel Injection (MFI) systems. The valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The IAC valve motor is a 2-phase, bi-polar permanent magnet stepper motor that is capable of highly accurate rotation, or movement, every time the polarity of a winding is changed. This change in polarity can be seen when observing a test lamp 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 is changed. The PCM does not use a physical sensor in order to determine the 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 the PCM has commanded in order to determine the IAC pintle position. The PCM uses the IAC valve in order to control the engine idle speed. The IAC does this by changing the pintle position in the idle air passage of the throttle body. This process varies the air flow around the throttle plate when the throttle is closed. In order to determine the desired position of the IAC pintle at idle or during deceleration, the PCM refers to the following inputs:
^ The engine RPM
^ The battery voltage
^ The air temperature
^ The Engine Coolant Temperature (ECT)
^ The Throttle Position (TP) sensor angle
^ The engine load
^ The vehicle speed

After an ignition cycle, when the ignition key is turned OFF, the PCM will first seat the AC pintle in the air bypass bore, and will then retract the pintle a predetermined number 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 TP sensor signal
^ A restricted air intake system
^ Objects blocking the IAC passage or the throttle bore
^ The correct, properly installed, and proper operation of the Positive Crankcase Ventilation (PCV) valve
^ Proper operation and installation of all air intake components
^ Proper installation and operation of the Mass Air Flow (MAF) sensor, if equipped
^ A tampered with or damaged throttle stop screw
^ A tampered with or damaged throttle plate, throttle shaft, throttle linkage, or cruise control linkage, if equipped
^ Excessive deposits in the IAC passage or on the IAC pintle
^ Excessive deposits in the throttle bore or on the throttle plate
^ Vacuum leaks
^ A high or unstable idle condition can be caused by a non-IAC system condition that can not be overcome by the IAC valve. Refer to Symptoms - Computers and Control Systems. Symptom Related Diagnostic Procedures
^ A low or unstable idle condition could be caused by a non-IAC system condition that cannot be overcome by the IAC valve. Refer to Symptoms - Computers and Control Systems. Symptom Related Diagnostic Procedures

If the condition 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 the 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 when the IAC counts are incrementing.