Circuit Description

The powertrain control module (PCM)/engine control module (ECM) controls engine idle speed with Idle Air Control (IAC) valve. To increase idle speed, PCM/ECM pulls IAC pintle away from its seat, allowing more air to pass by throttle body. To decrease idle speed, it extends IAC valve pintle toward its seat, reducing bypass air flow. A scan tool will read PCM/ECM commands to IAC valve in counts. The higher counts indicate more air bypass (higher idle). The lower counts indicate less air is allowed to bypass (lower idle).