Powertrain Controls - ECM/PCM

DTC 28 Chart Quad Drive Module #3 Circuit:

Quad Driver Module Wiring Diagram:

Circuit Description:

The ECM is used to control several components such as those illustrated in the wiring diagram. The ECM controls these devices through the use of a Quad-Driver Module (QDM). When the ECM is commanding a component "ON," the voltage of the output circuit will be "low" (near 0 volts). When the ECM is commanding the output circuit to a component "OFF," the voltage potential of the circuit will be "high." (Near battery voltage.) The primary function of the QDM is to supply the ground for the component being controlled.

Each QDM has a fault line which is monitored by the ECM. The fault line signal status can be displayed on the scanner. The ECM will compare the voltage at the QDM only when an A/C request signal is detected at the ECM. If the QDM fault detection circuit senses a voltage other than what is expected, the fault line status will change and a DTC 28 will set if applicable.

A QDM #3 fault will store a DTC 28 in the ECM memory but will not turn "ON" the MIL (Service Engine Soon) light.

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

1.DTC 28 will set if the ECM detects an improper voltage potential for 26 seconds.

These tests will begin to determine if the QDM connected devices can be controlled by the ECM. If a relay appears to operate but the device does not activate, refer to "Electric Cooling Fan, CHART C-12 " or "A/C Clutch Circuit Diagnosis, CHART C-10" for further diagnosis.

2. This check can detect a partially shorted coil which would cause excessive current flow. Excessive current flow to a QDM will be detected as a fault and set this DTC. If excessive current flow is detected, a circuit check will be performed to isolate the device from the wiring

3. The remaining checks will identify a circuit problem that has caused an excessive current flow or inoperative relay. If a QDM circuit check is done on a relay, it is important to identify and test the relay coil terminals of the harness connector to avoid improper diagnosis.

Diagnostic Aids:

Engine should be idling while monitoring QDM status. Using a scanner, monitor the QDM status while moving related harness connectors, including ECM harness. If the failure is induced, a fault will appear on the scanner. This can help locate the intermittent. Check for bent pins at ECM and ECM connector terminals. If DTC reoccurs with no apparent connector problem, replace ECM.