GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Idle Speed Control (ISC)


229, 231 V6 (Except Turbo)See Figure 1

The Idle Speed Control (ISC) does just what its name implies - it controls the idle. The ISC is used to maintain low engine speeds while at the same time preventing stalling due to engine load changes. The system consists of a motor assembly mounted on the carburetor which moves the throttle lever so as to open or close the throttle blades.

The whole operation is controlled by the ECM. The ECM monitors engine load to determine the proper idle speed. To prevent stalling, it monitors the air conditioning compressor switch, the transmission, the park/neutral switch and the ISC throttle switch. The ECM processes all this information and then uses it to control the ISC motor which in turn will vary the idle speed as necessary.

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Fig. Fig. 1: The Idle Speed Control (ISC) motor is attached to the carburetor