Circuit Description

PCM controls engine idle with Idle Air Control (IAC) valve. To increase idle speed, PCM pulse width modulates the OPEN solenoid inside IAC valve. A cylinder turns within IAC valve and allows air to by-pass throttle plate. PCM regulates (fast) idle speed using the IAC valve by increasing the opening of the air by-pass port. PCM regulates (slow) idle speed using the IAC valve by decreasing the opening of the air bypass port. DTC will set when actual idle speed varies greatly from desired idle speed. When DTC sets, MIL will illuminate after first ignition cycle in which diagnostic runs with active fault. PCM will record operating conditions at time of failure and store information in freeze frame buffer.