When the rear window defogger switch is activated, the HVAC control module energizes the rear defog relay by applying voltage to the rear defog relay control circuit. Ground for the relay coil is provided at G401. When the rear defog relay coil is energized, the switch contacts of the rear 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 G402.

Schematic Reference

Defogger Schematics

Connector End View Reference

Window Systems Connector End Views

Description and Operation

Rear Window Defogger Description and Operation

Electrical Information Reference

Scan Tool Reference

Rear Window Defogger Malfunction

1. Ignition OFF, disconnect the rear window defog relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal R2 and ground.

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

3. Verify that a test lamp illuminates between the relay switch B+ circuit terminal R3 and ground.

⇒	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 controlled output circuit terminal R2 for a short to ground.

4. Ignition OFF, disconnect the C1 harness connector at the rear window defogger grid.
5. Ignition OFF, all doors closed, test for less than 5.0 ohms 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.

6. Connect the C1 harness connector at the rear window defogger grid.
7. Ignition ON, connect a 30A fused jumper wire between the B+ circuit terminal R3 and the control circuit terminal R2. 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.

8. Ignition OFF, test for less than 5.0 ohms of resistance between the relay coil ground circuit terminal R5 and ground.

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

9. Connect a test lamp between the relay control circuit terminal R1 and the ground circuit terminal R5.
10. 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 HVAC control module.

⇒	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 HVAC control module.

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

Rear Window Defogger Indicator Malfunction

1. Ignition OFF, disconnect the C3 harness connector at the HVAC control module.
2. Connect a test lamp between the control circuit terminal 1 and ground.
3. Command the LED dimming control 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 OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

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

4. 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.