Idle/Throttle Speed Control Unit: Description and Operation

The Idle Speed Control (ISC) is mounted on the throttle body, and is used to control engine idle speed. The ISC plunger acts as a varying throttle stop, changing the throttle valve angle. The ECM monitors idle speed, and adjusts the ISC plunger to maintain the desired idle speed required for a given operating condition. An integral part of the ISC is the throttle switch. The position of the throttle switch determines whether or not the ISC controls idle speed. When the throttle lever rests against the ISC plunger, the switch contacts are closed and the ECM moves the ISC plunger to the programmed idle speed setting. When the throttle lever breaks contact with the ISC plunger, the switch contacts open and the ECM stops sending idle speed control commands. Engine speed is again controlled by the accelerator pedal.