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.

Schematic Reference

Defogger Schematics

Connector End View Reference

Window Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

1. Ignition ON, observe the scan tool rear defog switch parameter while pressing the rear window defogger switch. The reading should change between ON and OFF.

⇒	If the reading does not change between the commanded states, refer to Rear Window Defogger Switch Malfunction.

2. Command the rear window defogger ON and OFF with a scan tool. The rear window defogger and rear window defogger indicator should turn ON and OFF when changing between the commanded states.

⇒	If the rear window defogger does not turn ON and OFF when changing between the commanded states, refer to Rear Window Defogger Malfunction.

⇒	If the rear window defogger indicator does not illuminate or continuously illuminates, refer to Rear Window Defogger Indicator Malfunction.

Rear Window Defogger Switch Malfunction

1. Ignition OFF, disconnect the C2 harness connector at the HVAC control module.
2. Ignition ON, verify the scan tool rear defog switch parameter is OFF.

⇒	If not the specified value, test the signal circuit terminal J for a short to ground. If the circuit tests normal, replace the BCM.

3. Install a 3A fused jumper wire between the signal circuit terminal J and ground. Verify the scan tool rear defog switch parameter is ON.

⇒	If not the specified value, test the signal circuit terminal J for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

4. If all circuits test normal, replace the HVAC control module.

Rear Window Defogger Malfunction

1. Ignition OFF, disconnect the rear window defog relay.
2. Test for less than 1.0 ohm of resistance between the rear defog relay coil side ground circuit and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify that a test lamp does not illuminate between the relay switch controlled output circuit and ground.

⇒	If the test lamp illuminates, test the control circuit for a short to voltage.

4. Verify that a test lamp illuminates between the relay switch B+ circuit terminal and ground.

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If the test lamp does not illuminate, test the relay switch B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the relay switch B+ circuit fuse is open, test the relay switch controlled output circuit for a short to ground. If the circuit tests normal, test or replace the rear defog relay.

5. Ignition OFF, disconnect the C2 harness connector at the rear window defogger grid.
6. Test for less than 1.0 ohm of resistance between the rear window defogger grid ground circuit terminal A and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

7. Connect the C2 harness connector at the rear window defogger grid.
8. Connect a 20A fused jumper wire between the relay switch B+ circuit and the relay switch controlled output circuit. Verify the rear window defogger grid is activated.

⇒	If the rear window defogger grid does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the rear window defogger grid.

9. Connect a test lamp between the rear defog relay control circuit and ground.
10. Command the rear window defog relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

11. If all circuits test normal, test or replace the rear defog relay.

Rear Window Defogger Indicator Malfunction

1. Ignition OFF, disconnect the C2 harness connector at the HVAC control module.
2. Disconnect the rear defog relay.
3. Connect a test lamp between the control circuit terminal F and ground.
4. Ignition ON, command the rear window defogger ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

5. If all circuits test normal, replace the HVAC control module.

Rear Window Defogger Grid Test

1. Ignition ON, turn on the rear defogger.
2. Verify that a test lamp illuminates between each rear defogger grid line and ground. The test lamp should be off when testing at the ground side of the grid and get brighter as the test lamp moves closer to the voltage supply side of the grid. The test lamp should be dim when testing each grid line in the middle.

⇒	If the test lamp remains bright at the middle of a grid line, test for an open between the test point and the ground circuit.

⇒	If the test lamp remains off at the middle of a grid line, test for an open between the test point and the voltage supply side of the grid.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.