Powertrain Controls - ECM/PCM

DTC 27 - Quad-Driver Module (QDM 1) Circuit Diagnosis:

Quad-Driver Module (QDM 1) Wiring Circuit:

Circuit Description:
The PCM 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 (QDMs) which can independently control up to 4 outputs (PCM 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, except for the TCC, as noted below, which depends on the brake and 3rd gear TCC switch.

QDMs 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" or its internal resistance increases to limit current flow and protect the QDM. 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 QDM fault.

QDM 1 has a fault line to indicate the presence of a current fault to the PCM's central processor. A scan tool displays the status of the fault line as "Low" OK, "High" fault. Because of the brake and 3rd gear switch in the TCC circuit, DTC 27 is set under different conditions as follows:

DTC 27 Will Set When:
^ QDM B fault line:
- Is "high" for 20 seconds or more.
- TCC is commanded "ON."
- Battery voltage is greater than 10.5 volts.

Action Taken (PCM will default to): The Malfunction Indicator Lamp (MIL) will illuminate, and the cooling fan will enable.

DTC Chart Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. This checks to see if DTC 27 was set as a result of the EVAP canister purge solenoid valve circuit.
2. This checks TCC operation.
3. This checks the integrity of the EGR electrical circuit up to the solenoid.

Diagnostic Aids: If DTC 27 is intermittent, check all connections and wiring on QDM 1. If circuits are OK, refer to appropriate sub-section in "Component Systems" to further diagnose system(s) on QDM 1. Testing and Inspection