Defogger Always On - Rear Window

DEFOGGER ALWAYS ON - 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 OFF, disconnect the BCM C2 harness connector.
2. Turn off the rear window defogger.
3. Verify that a test lamp does not illuminate between battery voltage and the rear defog switch signal circuit at the BCM C2 harness connector.
- If the test lamp illuminates, repair the rear defog switch signal circuit for a short to ground. If the circuit tests normal, replace the HVAC control module.
4. Disconnect the rear window defog relay.
5. Ignition ON, verify that a test lamp does not illuminate between the rear defog relay control circuit and ground at the rear window defog relay terminals.
- If the test lamp illuminates, repair the rear defog relay control circuit for a short to voltage. If the circuit tests normal, replace the BCM.
6. Verify that a test lamp does not illuminate between the rear window defogger grid voltage supply circuit and ground at the rear window defog relay switched output terminal.
- If the test lamp illuminates, repair the rear window defogger grid voltage supply circuit for a short to voltage.
7. 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

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