Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Air Control System:

The ECM controls idle speed by means of an Idle Air Control (IAC) Valve. The IAC is situated to allow a measured amount of air to bypass the throttle valve according to ECM calculations. The ECM sends voltage signals to the IAC motor windings that command the valve to move "IN" (less air, lower idle speed) or "OUT" (more air, higher idle speed) according to conditions existing at any given time. ECM voltage signals (pulses) are referred to as "counts."

Each time the engine is started and shut off, the ECM resets the IAC valve by sending enough counts to seat the valve. The fully seated valve is ECM reference zero. When the engine is started again, a given number of counts are sent to open the valve and normal ECM control of idle speed begins. In this way the ECM knows the IAC valve position for a given idle speed.

To calculate IAC valve position, the ECM uses the following inputs:

Battery voltage
Coolant temperature
Throttle position
Engine speed
A/C switch
Power steering pressure switch
Vehicle speed sensor