Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Battery positive voltage is supplied through the REAR DEFOG fuse in the underhood fuse block to the REAR DEFOG relay switched input. Ground for the coil of the REAR DEFOG relay is provided by the BCM. Battery voltage is supplied from the IGNITION 30A MaxiFuse to the supply voltage on the rear defog relay coil side. When you depress the rear defogger switch, the rear defogger switch pulls the signal circuit low as an input to the BCM. The BCM interprets this as a request for the rear defogger system. The BCM enables the rear defogger system by supplying ground to the REAR DEFOG relay coil. The relay coil ground is also spliced off to the ground circuit of the rear defogger indicator. The REAR DEFOG relay is energized and the rear defogger indicator is illuminated. With the relay energized, battery positive voltage is allowed from the relay switched input through the switch contacts and out the relay switched output to the rear defogger grid. Ground for the rear defogger grid is provided by G401. When you start the engine and press the rear defogger switch for the first time, the defogger cycle lasts for 15 minutes. Further operation results in 7.5 minute defogger cycles. The defogger cycle resets to 15 minutes when you cycle the ignition to the OFF position and then to the ON position.

Schematic Reference

Defogger Schematics

Connector End View Reference

Master Electrical Component List

Description and Operation

Rear Window Defogger Description and Operation

Electrical Information Reference

Scan Tool Reference

1. Ignition ON, observe the scan tool Rear Defog Switch parameter while pressing the Rear Defogger Switch. The reading should change between ON and OFF.

⇒	If the reading does not change between the commanded states, refer to Rear Defogger Switch Malfunction in Circuit/System Testing

2. Command the rear window defogger ON and OFF with the scan tool. The rear window defogger and 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 Defogger Malfunction in Circuit/System testing.

⇒	If the Defogger Indicator does not illuminate, replace the HVAC control module.

Rear Defogger Switch Malfunction

1. Ignition OFF, disconnect the harness connector C1 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 1 for a short to ground. If the circuit tests normal, replace the BCM.

3. Install a 3-A fused jumper wire between the signal circuit terminal 1 and ground. Verify the scan tool Rear Defog Switch parameter is ON.

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

4. If all circuits test normal, test or replace the HVAC Control Module.

Rear Defogger Malfunction

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Ignition ON, verify that a test lamp does not illuminate between the coil side control circuit terminal 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 coil side B+ terminal and ground.

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

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

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal switch side control for a short to ground.

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

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

7. Connect the harness connector at the rear window defogger grid.
8. Ignition ON, connect a 25 amp fused jumper wire between the switch side B+ terminal and the switch side control terminal. Verify the rear window defogger is activated.

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

9. Connect a test lamp between the coil side B+ terminal and the coil side control terminal.
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 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 BCM.

11. If all circuits test normal, test or replace the REAR DEFOG relay.

Relay Test

1. Ignition OFF, disconnect the RR DEFOG relay.
2. Test for 60-180 ohms of resistance between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If not the specified value, replace the relay.

4. Install a 25-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms of resistance between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for HVAC control module replacement, setup, and programming.

Liftgate Window Replacement

Relay Replacement