Idle Air Control System: Description

The engine idle speed is controlled by the Idle Air Control (IAC) valve. The IAC valve is on the throttle body. The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The valve consists of a movable pintle, driven by a gear attached to a 2 phase bi-polar permanent magnet electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is fed by 2 circuits from the Engine Control Module (ECM). When the ECM changes polarity of a coil, the stepper motor moves one step. The ECM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts using a scan tool. The IAC counts will increment up or down as the ECM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the ECM will seat the IAC pintle in the idle air passage bore. Second, the ECM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed Diagnostic Trouble Code (DTC) may set.