Powertrain Controls - ECM/PCM

DTC 26 - Quad-Driver Module (QDSM) Circuit:

"Service Engine Soon" Light Wiring Circuit:

Circuit Description:

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

The QDSM is 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 QDSM. The QDSM senses this and the output turns "OFF" or its internal resistance increases to limit current flow and protect the QDSM. The result is 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 QDSM fault.

The QDSM has a separate fault line to indicate the presence of a current fault to the ECM's central processor. A scan tool displays the status of each of these fault lines as "Low" OK, "High" Fault.

^ QDM A fault line:
- "High" for 20 seconds or more.
- Battery voltage is greater than 10.5 volts.