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 R/DEFOG relay. The R/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 serial data message is received indicating that the BCM is requesting rear defog operation. The R/DEFOG relay receives battery positive voltage to both the coil and switch input sides from the BATT MAIN 3 fuse in the underhood fuse block.

Schematic Reference

Defogger Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Rear Window Defogger Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, observe the scan tool Rear Defog Relay parameter while pressing and depressing the rear window defogger switch. The parameter should change between ON and OFF.

⇒	If the parameter does not change, replace the HVAC control module.

2. Observe the operation of the rear window defogger indicator while commanding the rear defogger ON and OFF by pressing and depressing the rear window defogger switch. The indicator should turn ON or OFF when changing between commanded states.

⇒	If the indicator is always ON or always OFF, replace the HVAC control module.

3. Command the Rear Defog Relay ON and OFF with a scan tool. The rear window defogger should turn ON and OFF when changing between the commanded states.

1. Ignition OFF, disconnect the R/DEFOG relay.
2. Ignition ON, verify a test lamp does not illuminate between the control circuit terminal J1 and ground.

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

3. Verify a test lamp illuminates between the B+ circuit terminal J3 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 J1 for a short to ground.

4. Verify a test lamp illuminates between the B+ circuit terminal K3 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

5. Ignition OFF, disconnect the X2 harness connector at the rear window defogger grid.
6. Test for less than 1 ohm between the ground circuit terminal A and ground.

⇒	If greater than the specified range, 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 40A fused jumper wire between the B+ circuit terminal K3 and the control circuit terminal J1. 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 J3 and the control circuit terminal K1.
10. Command the Rear 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 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 R/DEFOG 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.

Relay Test

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

⇒	If 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 5A fused jumper wire between the relay terminal 85 and B+. Install a jumper wire between the relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.