Engine Idle Speed Control Operation

The engine idle speed is controlled by the Powertrain Control Module (PCM) through the Idle Speed Control (ISC) motor. There are several reasons for idle speed control:
1. Maintain the engine idle speed at the specified RPM at all times. The engine idle speed can vary due to any of the following reasons:
^ A change in the load applied to engine (when an electrical load is applied, the automatic transaxle is shifted to R, D, 2 or L ranges, the A/C is turned ON, the headlights or stop lights are turned ON, etc.).
^ A change in the atmospheric pressure.
^ A change in the engine's condition over time.
^ Other factors causing the idle speed to change.
2. Improve the starting performance of the engine.
3. Improve the driveability of the engine during warm up.
4. Compensate for the change in the air/fuel mixture ratio when decelerating (dash-pot effect).

The ISC motor opens and closes the throttle valve according to signals from the PCM. The PCM determines the correct engine idle speed by using input from various sensors and switches in order to assess engine status and requirements. When a forward rotation signal is sent from the PCM to the ISC motor, the ISC motor plunger extends in order to open the throttle valve. When a reverse rotation signal is sent, the ISC motor plunger contracts and the throttle valve closes by the return spring force. How much the throttle valve opens or closes is controlled according to the time during which signals are transmitted from the PCM to the ISC motor. The longer the time interval, the larger the throttle valve movement.