Circuit Description
WARNING: This page is about a different car, the 1997 GMC Pickup and 1997 Chevrolet Pickup. However, it is still accessible from the selected car via links, so may be relevant.
VCM controls idle speed with an Idle Air Control (IAC) valve to a calculated desired RPM based on sensor inputs and actual engine RPM. VCM uses 4 circuits to move an IAC valve, allowing varying amounts of airflow into intake manifold.
To increase idle RPM, VCM moves IAC valve out, allowing more air to by-pass throttle plate. To decrease RPM, VCM moves IAC valve in, reducing amount of air by-passing throttle plate. Scan tool reads VCM commands to IAC valve in counts. The higher the counts, the more air that is allowed to by-pass throttle plate (higher idle). The lower the counts, less air is allowed to by-pass throttle plates (lower idle).
DTC will test when the following conditions are present:
- No TP sensor DTCs are set.
- No ECT sensor DTCs are set.
- Engine coolant temperature is greater than 162°F (72°C).
- Intake air temperature is greater than -13°F (-25°C).
- BARO is greater than 70 kPa (less than 10,300 ft.).
- VSS is less than 2 MPH.
- No VSS DTCs are set.
- No MAP sensor DTCs are set.
- System voltage is greater than 10 volts but less than 18 volts.
- Engine run time is greater than 30 seconds.
- Throttle position is less than one percent.
- All conditions are met for greater than 3 seconds.