Circuit Description
The VCM controls the idle speed with an Idle Air Control (IAC) valve
to a calculated desired RPM based on the sensor inputs and the actual engine
RPM. The VCM uses 4 circuits in order to move an Idle Air Control (IAC) valve,
allowing a varying amount of air flow into the intake manifold.
In order to increase the idle RPM, the VCM moves the IAC valve out,
allowing more air to bypass the throttle plate. In order to decrease the RPM,
the VCM moves the IAC valve in, reducing the amount of air bypassing the throttle
plate. A scan tool reads the VCM commands to the IAC valve in counts. The
higher the counts, the more air is allowed to bypass the throttle plate (higher
idle). The lower the counts, the less air that is allowed to bypass the throttle
plates (lower idle). This DTC is a type B DTC.
Conditions For Setting DTC
Noninstrusive Test
| • | ECT is greater than 50° C |
| • | IAT is greater than -25° C |
| • | BARO is graeter than 70 kPa (10,300 ft.) |
| • | Vehicle Speed is less than 2 mph |
| • | System voltage is between 10 volts and 16 volts |
| • | Engine run time is greater than 30 seconds. |
| • | Above met for a time greater than 3 seconds to enable diagnostic. |
If the nonintrusive test fails, the intrusive test is ran.
Intrusive Test
| • | Calculated Air Flow is between 17.5 grams per second and
50 grams per second |
| • | Vehicle Speed is between 25 MPH and 85 MPH |
| • | Change in TP Sensor is less than 1% |
| • | Change in engine speed is less than 50 RPM |
| • | IAC motor commanded to 10% for 100 msec |
Action Taken When the DTC Sets
The VCM will turn ON the MIL after 2 consecutive driving cycles with
the fault active.
Conditions for Clearing the MIL/DTC
The VCM turns OFF the MIL after 3 consecutive driving
trips without a fault condition present. A history DTC will clear if no fault
conditions have been detected for 40 warm-up cycles (the coolant temperature
has risen 22°C (40°F) from the start-up coolant temperature and the
engine coolant temperature exceeds 71°C (160°F) during that same
ignition cycle) or the scan tool clearing feature has been used.
Diagnostic Aids
The following items could cause this DTC:
| • | An IAC valve which is stopped and cannot respond to the VCM |
| • | A throttle stop screw which has been tampered with |
| • | A damaged throttle or linkage |
A slow, unstable, or fast idle may be caused by a non - IAC valve system
problem that cannot be overcome by the IAC valve. Out of range control IAC
scan tool counts will read above 60 if the idle is too low and zero if the
idle is too high. Check for the following:
| • | Fuel system too lean and rich |
| • | Throttle body - inspect the bore for foreign material. |
| • | Refer to
(Link no longer avaliable) . |
| • | IAC reset is done with the scan tool misc. functions mode. |
