Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Air Control (IAC) System:

NOTE Because different models and engine applications vary in wire colors, circuit numbers, and pin numbers, the above image is a typical example. Refer to COMPUTERS AND CONTROL SYSTEMS/SCHEMATIC AND ROUTING DIAGRAMS for specific schematic applications.

PURPOSE
To control the engine idle speed and prevent stalling due to changes in engine load.


OPERATION
All engine idle speeds are controlled by the control module through the Idle Air Control (IAC) valve mounted on the throttle body. The control module sends voltage pulses to the IAC motor windings causing the IAC motor shaft and pintle to move in and out a given distance (number of steps) for each pulse, (called counts).
This movement controls airflow around the throttle plate, which in turn controls engine idle speed, either cold or hot. IAC pintle position counts can be seen using a "Scan" tool. 0 counts correspond to a fully closed passage, while 140 counts or more (depending on application) corresponds to full airflow through the throttle body bypassing the throttle blade.
The control module uses sensor and switch inputs to control the position of the IAC pintle. If the pintle is stuck open, the idle speed will remain high. If the pintle is stuck closed, the idle speed will be too low, and stalling may occur. If the pintle is stuck partially open, the idle speed will be higher than normal, and there will be no response to changes in engine load such as A/C ("ON") or transmission in ("Drive").