Chart C-2 - Idle Speed Error

Chart C-2 - Idle Speed Error:

Wiring Diagram For Chart C-2 - Idle Speed Error:

CHART C-2 - IDLE SPEED ERROR

Circuit Description:

The ECM sends voltage pulses to the proper IAC motor winding, which causes the motor shaft and valve to move "in" or "out" a given distance for each pulse (called counts) received. This movement controls air flow around the throttle plate, which, in turn, controls engine idle speed.



Test Description: Step numbers refer to step numbers on diagnostic chart.

1. Continue with test, even if engine will not idle. If idle is too low, "Scan" will display 80, or more, counts or steps. 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. There is a system problem. Proceed to paragraph three below.

2. When the engine was stopped, the IAC valve retracted (more air) to a fixed "Park" position, for increased air now and idle speed during the next engine start. A "Scan" will display 100 or more counts.

3. Be sure to disconnect the IAC valve prior to this 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.

Diagnostic Aids:

A slow unstable idle may be caused by a system problem that cannot be overcome by the IAC. "Scan" counts will be above 60 counts, if too low, and 0 counts, if too high.


If idle is too high, stop engine. Ignition "ON". Ground diagnostic terminal. Wait a few seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is above 800, +/- 50 rpm, locate and correct vacuum leak.

^ System Too Lean (High Air/Fuel Ratio) - Idle speed may be too high, or too low. Engine speed may vary up and down, disconnecting IAC does not help. May set Code 44. "Scan" and/or Voltmeter will read an oxygen sensor output less than 300mv (0.3v). Check for low regulated fuel pressure or water in fuel. A lean exhaust with an oxygen sensor output fixed above 800mv (0.8v) will be a contaminated sensor, usually silicon. This may, also, set a Code 45.
^ System Too Rich (Low Air/Fuel Ratio) - Idle speed too low. "Scan" counts usually above 80. System obviously rich and may exhibit black smoke exhaust. "Scan" tool and/or Voltmeter will read an oxygen sensor signal fixed above 800mv (0.8v).

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.
^ A/C Compressor or Relay Failure - See A/c diagnosis if circuit is shorted to ground. If the relay is faulty, idle problem may exist.
^ Refer to Rough, Unstable, Incorrect Idle or Stalling. See Diagnosis By Symptoms - No Trouble Codes