Defogger Inoperative - Rear Window

DEFOGGER INOPERATIVE - REAR WINDOW




DIAGNOSTIC FAULT INFORMATION

IMPORTANT: Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Testing and Inspection

CIRCUIT/SYSTEM DESCRIPTION
The body control module (BCM) controls the rear window defog relay located in the underhood fuse block. The rear window defog switch located on the HVAC control module is hard wired to the BCM. The rear window defog switch provides the request signal to the BCM. With the ignition switch in the RUN position, the BCM supplies voltage to the rear window defog switch via the rear defog switch signal circuit. When the rear window defog switch is pressed, the switch contacts close and ground is supplied. The BCM interprets the flow of voltage as a request to turn the rear defogger ON. The BCM energizes the rear window defog relay by applying voltage to the relay coil via the rear defog relay control circuit. When the rear window defog relay coil is energized, the switch contacts of the rear window defog relay close allowing battery voltage to flow from the REAR DEFOG fuse through the closed switch contacts and out the relay switched output to the rear window defogger grid. Ground for the rear window defogger grid is provided at G301. The BCM also illuminates the rear defog indicator LED located on the HVAC control module with the supplied voltage from the rear defog relay control circuit.

CIRCUIT/SYSTEM TESTING
1. Ignition ON, install a scan tool and display the Rear Defog Switch Input parameter in the HVAC data list.
2. Turn the rear window defogger switch ON and OFF while observing the scan tool. The scan tool should display on and off as the defogger switch is cycled.
- If the scan tool parameter does not change from ON to OFF as the switch is cycled, repair the rear defog switch signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module.
3. Ignition OFF, disconnect the rear window defog relay.
4. Ignition ON, turn on the rear window defogger.
5. Verify that a test lamp illuminates between the rear defog relay control circuit and ground at the rear window defog relay terminals.
- If the test lamp does not illuminate, repair a short to ground or an open/high resistance in the rear defog relay control circuit. If the circuit tests normal, replace the BCM.
6. Verify that a test lamp illuminates between the rear defog relay control circuit and the rear defog relay coil side ground circuit at the rear window defog relay terminals.
- If the test lamp does not illuminate, repair an open/high resistance in the rear defog relay coil side ground circuit.
7. Verify that a test lamp illuminates between the rear defog relay voltage supply circuit and ground at the rear defog relay terminals.
- If the test lamp does not illuminate, repair a short to ground or an open/high resistance in the rear defog relay voltage supply circuit.
8. Disconnect the rear window defogger grid harness connector.
9. Verify that a test lamp illuminates between battery voltage and the rear window defogger grid ground circuit.
- If the test lamp does not illuminate, repair an open/high resistance in the rear window defogger grid ground circuit.
10. Connect the rear window defogger grid harness connector.
11. Connect a 20 amp fused jumper between the rear defog relay voltage supply circuit and the rear window defogger grid voltage supply circuit at the rear defog relay terminals. Verify the defogger grid is activated.
- If the rear window defogger grid is not activated, repair a short to ground or an open/high resistance in the rear window defogger grid voltage supply circuit. If the circuit tests normal, test or replace the rear window defogger grid.
12. If all circuits test normal, test or replace the rear window defog relay.

REPAIR INSTRUCTIONS

IMPORTANT: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

- Rear Window Replacement
- Underhood Electrical Center or Junction Block Replacement
- Relay Replacement (Within an Electrical Center) Relay Replacement (Attached to Wire Harness)
- Control Module References for HVAC and BCM replacement, setup, and programming.