Powertrain Controls - ECM/PCM

DTC 26 Chart (VCM):

VCM QDM (1) Circuit:

Circuit Description:
The VCM controls most components with electronic switches to complete ground circuit when turned "ON". These switches are arranged in groups of 4 called Quad-Driver Module (QDM) which can independently control up to 4 outputs (VCM terminals); although not all outputs are used. When an output is "ON" the terminal is grounded and its voltage normally will he low. When an output is "OFF," its terminal voltage normally will be high.

QDM's are fault protected. If a relay or solenoid coil is shorted (having very low or zero resistance) or if the control side of the circuit is shorted to voltage, it would allow too much current into the QDM. The QDM senses this and the output turns "OFF," and its internal resistance increases to limit current flow and protect the QDM. The result is a high output terminal voltage when it should be low. If the circuit from B + or the component is open, or the control side of the circuit is shorted to ground, terminal voltage will be low, even when output is commanded "OFF." Either of these conditions is considered to be a QDM fault.

The QDM has a fault line to indicate the presence of a current fault to the VCMs central processor. A Tech 1 scan tool displays the status of each of these fault lines as "Low" = OK, "High" = Fault.

Diagnostic Aids:

Intermittent faults must be continuously present for 200 milliseconds to cause DTC 26 to set. QDM controlled circuits should be inspected for poor terminal contact or damaged harnesses. QDM faults can be detected when outputs are "ON" or "OFF" as follows:

- Open circuit or control circuit shorted to ground - output commanded "OFF."

- Shorted device or control circuit shorted to voltage - output commanded "ON."

Refer to "Intermittents." - Intermittent Malfunctions