Idle Air Control System Diagnosis: Description

Engine idle speed is controlled by Idle Air Control (IAC) valve. IAC valve is on throttle body. IAC valve pintle moves in and out of an idle air passage bore to control air flow around throttle plate. IAC 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 Powertrain Control Module (PCM). When PCM changes polarity of a coil, stepper motor moves one step. PCM uses a predetermined number of counts to determine IAC pintle position. Observe IAC counts with scan tool. IAC counts will increment up or down as PCM attempts to change IAC valve pintle position. An IAC Reset will occur when ignition key is turned OFF. First, PCM will seat IAC pintle in idle air passage bore. Second, PCM will retract pintle a predetermined number of counts to allow for efficient engine start-up. If engine idle speed is out of range for a calibrated period of time, an idle speed Diagnostic Trouble Code (DTC) may set.