Battery positive voltage is supplied through the RR DEFOG fuse, in the underhood fuse block, to the REAR DEFOG relay switched input. Ground for the REAR DEFOG relay is provided by G101. The BCM supplies 12 volts on the rear window defogger switch signal circuit to the HVAC control assembly. When you depress the rear window defogger 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 rear window defogger indicator. The REAR DEFOG relay is energized and the rear window 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. Ground for the rear window defogger grid is provided by G403.

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 rear defogger feature will not time out if vehicle speed is above 80 km/h (50 mph). 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

Window Systems Connector End Views

Description and Operation

Rear Window Defogger Description and Operation

Electrical Information Reference

Scan Tool Reference

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

⇒	If the reading does not change between the commanded states, refer to HVAC Control Assembly Malfunction.

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.

⇒	If the Defog Indicator does not illuminate or continuously illuminates, refer to Rear Defogger Indicator Malfunction.

HVAC Defog Switch Malfunction

1. Ignition OFF, disconnect the harness connector C2 at the HVAC control module.
2. Ignition OFF, test for less than 1.0 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 Manual HVAC Rear Defog Sw 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 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 Indicator Malfunction

1. Ignition OFF, disconnect the harness connector C2 at the HVAC Control Assembly.
2. Disconnect the REAR DEFOG relay.
3. Connect a test lamp between the control circuit terminal 13 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.

Rear Defogger Malfunction

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Ignition OFF, test for less than 1.0 ohm between the relay coil ground circuit and ground.

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

3. Ignition ON, verify that a test lamp does not illuminate between the relay controlled output circuit and ground.

⇒	If the test lamp illuminates, test the relay controlled output circuit for a short to voltage.

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

5. Disconnect the harness connector at the Rear Window Defogger Grid.
6. Ignition OFF, test for less than 1.0 ohm between the Rear Window Defogger Grid ground circuit terminal 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 relay switch B+ circuit and the relay controlled output circuit. Verify the Rear Window Defogger Grid is activated.

⇒	If the Rear Window Defogger Grid does not activate, test the relay controlled output 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 rear defogger control circuit and the relay coil ground circuit.
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 rear defogger control circuit for a short to voltage. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the rear defogger 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.

Defog Switch

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

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

3. Test for less than 2.0 ohms between the signal terminal 3 and the ground circuit terminal 9 with the switch in the closed position.

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

Defogger Indicator

1. Ignition OFF, disconnect the harness at the HVAC Control Assembly.
2. Install a 5A fused jumper wire between circuit terminal 13 and 12 volts. Install a jumper wire between the ground circuit terminal 9 and ground. Verify the Defogger Indicator illuminates.

⇒	If the Defogger Indicator does not illuminate, replace the HVAC Control Assembly.

Relay Test

1. Ignition OFF, disconnect the RR DEFOG relay.
2. Test for 60-180 ohms 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 25A 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.