With the ignition in the RUN position, and the rear window defogger switch depressed, a class 2 serial data message is sent from the HVAC control module to the body control module (BCM) requesting rear defog operation. The BCM responds by providing ground to the rear defog control signal circuit energizing the RR defog relay. The RR defog relay then provides voltage to the rear defog element supply voltage circuit operating the rear defog system. The HVAC control module illuminates the rear window defogger indicator when a class 2 serial data message is received indicating that the BCM is requesting rear defog operation. The RR defog relay receives battery positive voltage to both the coil and switch input sides from the RR DEFOG fuse in the underhood fuse block. Ground for the rear window defogger is provided at G401 or G400. When you start the engine and press the rear window 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 back to the ON position. If battery voltage is low, the BCM will disable the rear window defogger.

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

Ignition ON, command the rear window defogger ON and OFF by pressing the defogger switch. Observe the rear window defogger operation as the rear window defogger turns ON and OFF.

1. Ignition OFF, disconnect the RR 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 RR 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 is open, test the control circuit terminal switch side control for a short to ground.

5. Disconnect the harness connector C1 at the rear window defogger grid.
6. Test for less than 1 ohm of resistance between the rear window defogger ground circuit terminal C2 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. Connect a 40-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, test or replace the rear window defogger grid.

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 BCM.

⇒	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 RR 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 40-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.
5. If greater than the specified range, replace the relay

Rear Window Defogger Grid Test

1. Ignition ON, turn on the rear window defogger.
2. Verify that a test lamp illuminates between each rear window 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 the grid line, test for an open between the test point and the ground side of the grid.

⇒	If the test lamp remains off at the middle of the 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.