Idle Air Control (IAC) System Diagnosis

CIRCUIT DESCRIPTION
The Idle Air Control (IAC) valve is located in the throttle body of both Throttle Body Fuel Injection (TBI) and Multiport Fuel Injection (MFI) Systems. It consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The IAC valve motor is a two 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 Powertrain Control Module (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 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 to determine IAC pintle position. The PCM uses the IAC valve to control engine idle speed. It does this by changing the pintle position In the idle air passage of the throttle body. This varies the air flow around the throttle plate when the throttle is closed. 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

When the ignition key is turned OFF, after an ignition cycle, the PCM will first seat the IAC pintle in the air bypass bore and then retract it a predetermined amount of counts 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
^ Restricted air intake system
^ Objects blocking the IAC passage or throttle bore
^ The correct Positive Crankcase Ventilation (PCV) valve, properly installed and proper operation of the 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 could be caused by a non-IAC system problem that can not be overcome by the IAC valve-Refer to Rough, Unstable, or Incorrect Idle and Stalling. Rough, Unstable, or Incorrect Idle and Stalling
^ A low or unstable idle condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve-Refer to Rough, Unstable, or Incorrect Idle and Stalling. Rough, Unstable, or Incorrect Idle and Stalling
^ If the problem is determined to be 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.