Idle Speed Control (ISC) (CARBURETED Models)

The ISC is a motor which opens or closes the throttle (in idle position) according to commands from ECM. The ISC maintains low idle speeds while preventing stalling due to engine load. The base idle speed is programmed into ECM memory and is not adjustable.

When engine is cold, ISC holds throttle valve open for a 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 (nose switch "off").

NOTE: Not all carbureted engines are equipped with ISC system. Some may use an Idle Stop Solenoid (ISS) or an Idle Load Compensator (ILC). To control engine idle speed when A/C is on, ISS controls throttle angle using a solenoid which is energized by an ECM controlled relay. The ILC system controls idle speed during long deceleration modes using a vacuum motor which is regulated by an ECM-controlled vacuum solenoid. To check function of ISC, ISS and ILC system, see appropriate component CHART C-2 for that system.