When the rear window defogger switch is activated, the HVAC control module energizes the rear defog relay by grounding the rear defog relay control circuit. When the rear defog relay coil is energized, the switch contacts of the rear defog relay close allowing battery voltage to flow from the BATT 2 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.

Schematic Reference

Defogger Schematics

Connector End View Reference

Window Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

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 C19 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 coil B+ circuit terminal A19 and ground.

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

4. Verify that a test lamp illuminates between the relay switch B+ circuit terminal A18 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 coil B+ circuit fuse is open, test the relay controlled output circuit for a short to ground.

5. Ignition OFF, disconnect the C2 harness connector at the rear window defogger grid.
6. Ignition OFF, 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. Ignition ON, connect a 40A fused jumper wire between the B+ circuit terminal A18 and the control circuit terminal C19. 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 B+ circuit terminal A19 and the control circuit terminal C18.
10. Command the rear window defogger ON and OFF with the rear defogger switch. 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 ground. 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 voltage 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 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.