Skyhawk L4-121 2.0L (1986)
Fig. 034 Chart C-2C Idle Air Control
The ECM controls engine idle speed by moving the IAC valve, to control air flow around the throttle plate. It does this by sending voltage pulses
(called "counts" or "steps") to the proper motor winding. The motor shaft and valve move a given distance for each pulse received.
a. To increase idle speed - ECM sends enough counts to retract the IAC valve and allow more air to flow, until idle speed reaches the proper
RPM. This increases the ECM counts.
b. To decrease idle speed - ECM sends enough counts to extend the IAC valve and reduce air flow. This will reduce the ECM counts.
The ECM "learns" the correct valve position to maintain proper idle RPM. If the "learned" IAC valve position is incorrect, the ECM will command a
reset. The reset will occur after the next engine start and vehicle speed is greater than 35 to 45 MPH.
Diagnostic Aids:
a. Park/Neutral Switch (Automatic Transmission Only) - If the ECM thinks the car is always in neutral, idle speed will not be correct in drive
range, Chart C-1A.
