Powertrain Controls - ECM/PCM

Idle Air Control (IAC) System Check:

Wiring Diagram For Idle Air Control (IAC) System Check:

CODE 35 - IDLE AIR CONTROL (IAC) SYSTEM CHECK

Circuit Description:

The ECM controls idle rpm with the IAC valve. To increase idle rpm, the ECM moves the IAC valve away from it's seat, allowing more air to pass by the throttle plate. To decrease rpm, it moves the IAC valve toward it's seat, reducing air flow by the throttle plate. A "Scan" tool will read the ECM commands to the IAC valve in counts. The higher the counts, the more air allowed (higher idle). The lower the counts, the less air allowed (lower idle).


Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Continue with test, even if engine will not idle. If idle is to low, "Scan" will display 80 or more counts, or steps. If idle is high, it will display "0" counts. Occasionally, an erratic or unstable idle may occur. Engine speed may vary 200 rpm, or more, up and down. Disconnect IAC. If the condition is unchanged, the IAC is not at fault.

2. When the engine was stopped, the IAC valve retracted (more air) to a fixed "park" position for increased air flow and idle speed during the next engine start. A "Scan" will display 100 or more counts. When peforming this test, immediately note rpm on start up, because, on a warm engine, the rpm will decrease rapidly.

3. Be sure to disconnect the IAC valve prior tothis test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.

4. There is a remote possibility that one of the circuits is shorted to voltage, which would have been indicated by a steady light. Disconnect ECM and turn the ignition "ON" and probe terminals to check for this condition.



IAC VALVE RESET PROCEDURE

^ Ignition "OFF" for 10 seconds
^ Start and run engine for 5 seconds
^ Ignition "OFF" for 10 seconds


Diagnostic Aids:

A rough or unstable idle may be caused by system problem that cannot be overcome by th IAC. "Scan" counts will be above 60 counts, engine speed is too low, or "0" counts, if engine speed is too high.

^ If IAC valve pintle position counts are low or zero, check for vacuum leaks at vacuum fitting, tees and hoses, the throttle body and the intake manifold.
^ If IAC valve pintle position counts are high, look for carbon build-up in the IAC valve air inlet passage or evidence of tampering with stop screw Also check for low engine power or excessive accessory loads.
^ System Too Lean (High Air/Fuel Ratio)
Idle speed may be too low. Engine speed may vary up and down, disconnecting IAC does not help.
^ System Too Rich (Low Air/Fuel Ratio)
Idle speed too low. "Scan" counts usually above 80. System obviously rich and may exhibit black exhaust smoke.

Check:

- High fuel pressure
- Injector leaking or sticking

^ Throttle body - Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore. (Repair as required)
^ A/C Compressor or Relay Failure - See Chart C-10B. See Diagnosis By Symptoms - No Trouble Codes Stored.