Battery positive voltage is supplied through the REAR DEFOG fuse, in the underhood fuse block, to the REAR DEFOG relay switched input. The BCM supplies 12 volts on the rear window defogger switch signal circuit to the HVAC control module. When you depress the defog switch, the rear window defogger switch pulls the signal circuit low. The BCM interprets as a request for the rear window defogger system. The BCM enables the rear window defogger system by supplying voltage to the REAR DEFOG relay coil. The relay coil supply voltage is also spliced off internally in the BCM to the supply voltage circuit of the defogger indicator. The REAR DEFOG relay is energized and the 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 window defogger grid.

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 Cmd parameter while pressing the rear window defogger switch. The parameter should change between On and Off.

⇒	If the value does not change between the commanded states, refer to Rear Window Defogger Switch Malfunction.

2. Command the Rear Defog Relay ON and OFF with a scan tool. The rear window defogger and rear window 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.

⇒	If the rear window defogger indicator is always ON or always OFF, refer to Rear Defogger Indicator Malfunction.

Rear Window Defogger Switch Malfunction

1. Ignition OFF, disconnect the X2 harness connector at the HVAC control assembly.
2. Test for less than 1 ohm between the ground circuit terminal 9 and ground.

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

3. Ignition ON, verify the scan tool Rear Defog Relay Cmd parameter is Off.

⇒	If not the specified value, test the signal circuit terminal 3 for a short to ground. If the circuit tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the signal circuit terminal 3 and ground. Verify the scan tool Rear Defog Relay Cmd 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.

5. If all circuits test normal, test or replace the HVAC control assembly.

Rear Defogger Malfunction

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Test for less than 1 ohm between the ground circuit terminal 86 and ground.

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

3. Ignition ON, verify a test lamp does not illuminate between the control circuit terminal 30 and ground.

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

4. Verify a test lamp illuminates between the B+ circuit terminal 87 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 30 for a short to ground.

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 87 and the control circuit terminal 30. 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 control circuit terminal 85 and the ground circuit terminal 86.
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 voltage. If the circuit tests normal, replace the BCM.

⇒	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.

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

Rear Defogger Indicator Malfunction

1. Ignition OFF, disconnect the X2 harness connector at the HVAC control assembly.
2. Disconnect the REAR DEFOG relay.
3. Connect a test lamp between the control circuit terminal 85 and ground.
4. Ignition ON, 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 voltage. If the circuit tests normal, replace the BCM.

⇒	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.

5. If all circuits test normal, test or replace the HVAC control assembly.

Defog Switch

1. Ignition OFF, disconnect the X2 harness connector at the HVAC control assembly.
2. Test for infinite resistance between the signal terminal 3 and the ground terminal 9 with the switch in the open position.

⇒	If not the specified value, replace the HVAC control assembly.

3. Test for less the 2 ohms between the signal terminal 3 and the ground terminal 9 with the switch in the closed position.

⇒	If greater than the specified range, replace the HVAC control assembly.

Defog Indicator

1. Ignition OFF, disconnect the X2 harness connector at the HVAC control assembly.
2. Install a 5A fused jumper wire between control terminal 13 and 12 volts. Install a jumper wire between the ground terminal 9 and ground. Verify the defog Indicator illuminates.

⇒	If the defog indicator does not illuminate, replace the HVAC control assembly.

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 REAR 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 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 between terminals 30 and 87.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.