Notes

Code 28 Diagnostic Chart





Quad Driver Module #3 Circuit Diagram






CIRCUIT DESCRIPTION
The ECM controls several components through the use of a "Quad-Driver" module integrated into the ECM. Most devices, such as solenoids or relays, receive battery power through a fused circuit to the ignition, and turn "ON", when the circuit is completed to ground. The "Quad-Driver" consists of several electronic switching circuits that provide the ground connection to complete these circuits and turn the devices "ON".

Each "Quad-Driver" has a fault line that is monitored by the ECM. When the QDM switch for a particular circuit is "OPEN", the voltage at the "sense line" will be high (near battery voltage), and when the device is turned "ON" (circuit completed to ground), the voltage at the "sense line" will drop to zero or near zero volts. When a device is commanded "OFF", the ECM expects the voltage in that "sense line" to be high, and when a device is commanded "ON", the ECM expects the voltage in that "sense line" to go low. The QDM fault detection circuit monitors the voltage in the sense line for each device controlled by the QDM. The ECM will monitor the voltage at the coolant fan and A/C clutch sense lines only when an A/C request signal is present. If the QDM fault detection circuit "sees" a voltage other than what it expects for a device in a particular "ON" or "OFF" state, the fault line status will change, and a Code 28 will set. The fault line signal status can be displayed on a "Scan" tool.

Code 28 will set if the ECM detects the wrong voltage potential in the QDM #3 fault line for 25 seconds. This code does not turn on the "Malfunction Indicator Light" (MIL, also known as the "Service Engine Soon" or SES light.)

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.


This step begins checks to see if the QDM can control its output devices. If relay appears to turn "ON", but the clutch or fan for that relay does not operate, check voltage at relay terminal "E" (relay terminal "A" for compressor clutch relay). Should have battery voltage. If not, check power supply circuit for open or short to ground, or burned fuse.Also check the device itself.
This check can detect a partially shorted solenoid coil which would cause excessive current flow. Excessive current flow to the QDM will be detected as a fault, and set this code. If excessive current flow is detected, a circuit check must be performed to isolate the device from the wiring.
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 circuit, it is important to identify and check the relay coil terminals of the harness connector for good connections, to avoid improper diagnosis.
DIAGNOSTIC AIDS
Engine should be idling when monitoring QDM status. Using "Scan" tool, monitor the QDM status while moving the wiring harness and related connectors, including the ECM harness and connector. If the failure is induced, a fault will appear on the display. This will indicate a poor connection or an intermittent "open" or "short" in the wiring harness or connector being "wiggled". Also check for bent pins at the ECM connector and ECM terminals. If code recurs, and there are no apparent wiring or connector problems, replace ECM.
For additional information and test procedures on A/C compressor clutch and engine cooling fans and controls, See: Related Tests and Information\C Charts\Chart C-10 A/C Clutch Control Circuit Diagnosis and See: Related Tests and Information\C Charts\Chart C-12 Coolant Fan Control Circuit