C-2C ~ Idle Air Control System (IAC)

Chart C-2C Wiring Diagram:

Chart C-2C - Idle Air Control (IAC) Valve Check:

CIRCUIT DESCRIPTION
The ECM controls engine idle speed with the IAC valve. To increase idle speed, the ECM retracts the IAC valve pintle away from its seat, allowing more air to pass by the throttle bore. To decrease idle speed, it extends the IAC valve pintle towards its seat, reducing bypass air flow. A "Scan" tool will read the ECM commands to the IAC valve in counts. The higher the counts indicate more air bypass (higher idle). The lower the counts indicate less air is allowed to bypass (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 too low, "Scan" will display "80" or more. If idle is too low, "Scan" will display "0". Occasionally, an erratic or unstable idle may occur. Engine speed mat fluctuate 200 or more rpm. Disconnect IAC. If the symptom is unchanged, the IAC is not at fault.

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

3. Be sure to disconnect 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 id is shorted to voltage, which would have been indicated by a steady light. Disconnect the ECM and turn ignition ON and probe terminals to check this condition.

DIAGNOSTIC AIDS:
A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of control range IAC "Scan" tool counts will be above 80 if idle is too low, and "0" counts, if idle is too high.
If idle is too high, stop engine. Ignition ON, ground diagnostic terminal. Wait 30 seconds for IAC to seat, then disconnect IAC. Unground diagnostic terminal and start engine.
If idle speed is above 450 rpm in drive, locate and correct vacuum leak.
If rpm is less than 450, see Powertrain Management/Computers and Control Systems/Testing and Inspection/Procedures.

The following checks should be made to repair a non-IAC system problem:
- Vacuum Leak (High Idle)
If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800 rpm, locate and correct vacuum leak including PCV system. Also check for binding of throttle blade or linkage.
- System Too Lean (High Air/Fuel Ratio)
The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting IAC valve does not help. Code 44 may set. "Scan" O2 voltage will be less than 300 mv (.3 volt). Check for low regulated fuel pressure, water in fuel, or restricted injector.
- System Too Rich (Low Air/Fuel Ratio
The idle speed will be too low. "Scan" tool IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust. "Scan" tool O2 voltage will be fixed above 800 mv (.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 sensors "Scan" voltage will be slow to respond.
- Throttle Body
Remove IAC valve and inspect bore for foreign material.
- IAC Valve Electrical Connections
IAC valve valve connections should be carefully checked for proper contact.
- PCV Valve
An incorrect or faulty PCV valve may result in an incorrect idle speed.
- If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully re-check connections, valve terminal resistance or replace IAC.