Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Air Control (IAC) Valve:

DESCRIPTION
All engine idle speeds are controlled by the PCM through the Idle Air Control (IAC) valve. The PCM sends voltage pulses to the IAC motor windings causing the IAC motor shaft and pintle to move "IN" or "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.

OPERATION
The IAC valve pintle position counts can be seen using a Tech 1 scan tool. Zero (0) counts correspond to a fully closed passage, while 140 counts or more (depending on the application) corresponds to full flow.

Actual or "controlled" idle speed is obtained by the PCM positioning the IAC valve pintle. Resulting idle speed is generated from the total idle air flow (IAC/passage + crankcase ventilation + throttle valve + vacuum leaks).
Controlled idle speed is always specified for normal operating conditions. Normal operating conditions are: engine coolant temperature in normal operating range, the air conditioning "OFF," automatic transmission in drive, verify proper PRNDL position adjustment. A high or low engine coolant temperature, or A/C clutch engagement may signal the PCM to change the IAC counts.

The idle stop screw is set at the factory. This setting allows enough air flow by the throttle valve to cause the IAC valve pintle to he positioned at a calibrated number of steps (counts) from the seat during normal controlled idle operation. The IAC counts will be higher than normal on an engine with less than 500 miles, or an engine operating at high altitude or an engine with an accessory load such as the alternator. A/C or power steering activated.