Idle Speed Control (ISC)
The ISC is an electrically driven actuator which changes throttle valve angle (in idle position), according to commands from ECM. The ISC maintains low idle speeds while preventing stalls due to engine load changes.
The ECM monitors engine load to determine proper idle speed. To prevent stalling, the ECM monitors A/C "ON" switch, Park-Neutral switch and ISC throttle switch. With this information, the ECM will control idle speed by operating the ISC motor.
When engine is cold, ECM holds throttle valve open for longer period of time to provide faster warm-up. This function is by-passed when throttle is opened enough to bring TPS off its idle circuit. The ISC is located on side of carburetor.
NOTE:
Not all engines are equipped with ISC system. Some may use an Idle Speed Solenoid (ISS) or an Idle Load Compensator (ILC) to control engine idle speed without ECM commands.