Circuit Description
The VCM controls idle RPM with the IAC valve. To increase idle RPM, the VCM retracts the IAC pintle from the seat, allowing more air to bypass the throttle plate. To decrease RPM, it extends the IAC pintle valve in towards the seat, reducing air flow through the IAC valve port in the throttle body. A scan tool will read the VCM 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).
IAC Valve Reset Procedure Without A Scan Tool
- Turn ON the ignition, with the engine OFF for 5 seconds.
- Turn OFF the ignition for 10 seconds.
- Start the engine and check for proper operation.
IAC Valve Reset Procedure With Scan Tool
- Turn ON the ignition, with the engine OFF.
- Install a scan tool.
- Select F4 Misc. Tests.
- Select F3 IAC System.
- Select F1 IAC Reset.
- Use up arrow to reset IAC.
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. The following checks should be made to repair a non-IAC system problem.
Important: If for some reason the air intake to the engine is restricted, the VCM will attempt to compensate by backing out the IAC pintle to maintain desired idle. If the engine has another source of air to the intake, the VCM will compensate by extending the IAC pintle, scan IAC counts may reach zero as the VCM tries to maintain desired idle.
| • | 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 crankcase ventilation system. Also, check for binding of throttle blade or linkage. |
| • | System 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 DTC P0171. The scan tool will read Heated Oxygen Sensor (HO2S) output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel. |
| • | System rich (low air/fuel ratio) - Idle speed too low. Scan IAC counts are usually above 80. System obviously rich and may exhibit black smoke exhaust. May set DTC P0172. The scan tool will read Heated Oxygen Sensor (HO2S) signal fixed above 800 mV (0.8 volt). |
| Check for high fuel pressure, leaking or sticking injector. Silicone contaminated HO2S scan voltage will be slow to respond. |
| • | Throttle body - Remove IAC valve and inspect bore for foreign material. |
| • | IAC Valve Electrical Connections - IAC valve connections should be carefully checked for proper contact. |
| • | Crankcase Ventilation Valve - An incorrect or faulty crankcase ventilation valve may result in an incorrect idle speed. |
| • | A/C Compressor - Refer to Air Conditioning (A/C) Compressor Clutch Control Diagnosis if circuit is shorted to ground. If the relay is faulty, an idle problem may exist. |
| • | If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance or replace IAC. |
The Diagnostic Tables incorporate diagnosis procedures using a scan tool where possible.
The scan tool saves time in diagnosis. The scan tool also prevents the replacement of good parts. In order to use the scan tool successfully for diagnosis, the technician must understand the system being diagnosed as well as understanding the tool's limitations.
Refer to Vehicle Control Module .
In order to read the DTCs, use a scan tool. The technician cannot flash the DTCs by grounding the DLC.
In order to clear the DTCs from the memory, use the scan tool.
If a diagnostic table indicates that a VCM connection or the VCM is the cause of a problem and the technician replaces the VCM but the problem is not eliminated, one of the following items may be the reason.
- After replacing the VCM, the EEPROM was not reprogrammed correctly. Refer to EEPROM Programming for V6 applications. Refer to EEPROM Programming for V8 applications.
- A problem may exist with the VCM terminal connections: In order to check the connections properly, remove the terminals from the connections.
- The problem is an intermittent. Make a careful physical inspection of all portions of the system involved. Refer to Important Preliminary Checks for V6 applications. Refer to Important Preliminary Checks for V8 applications.
- A shorted solenoid, relay coil or harness may exist: The VCM turns the solenoids and relays on and off by the VCM using an internal electronic switches called drivers. Each driver is a part of a group of 4 called quad-drivers. Failure of 1 driver can damage any other driver in the set.
- The replacement VCM may be faulty: After replacing the VCM, recheck the system for proper operation. If the diagnostic table again indicates that the VCM is the problem, substitute a known good VCM.
The VCM is not correct for the application: The incorrect components may cause a malfunction and may or may not set a DTC.
A shorted solenoid, relay coil, or harness will not damage the quad drivers in the VCM. A shorted solenoid, relay coil, or harness will cause the circuit and the controlled component to be inoperative.
When the circuit fault is not present or the technician has repaired the fault, the quad driver will again operate in a normal manner due to its fault protected design.
If the technician has repaired a fault in the a circuit controlled by a quad-driver, reinstall the original VCM. Check the circuit for proper operation.
A VCM replacement is not necessary if the repaired circuit or component operates correctly.
A J 39200 tester provides a fast, accurate means of checking for a shorted coil or a short to battery voltage.
