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

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

Stationary Windows 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 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 HVAC Rear Defog Switch 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 Defogger Indicator does not illuminate or continuously illuminates, refer to Rear Defog Indicator Malfunction.

HVAC Rear Defog Switch Malfunction

1. Ignition OFF, disconnect the harness connector C2 at the HVAC Control Assembly.
2. Test for less than 1.0 ohm of resistance between the ground circuit terminal K 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 J for a short to ground. If the circuit tests normal, replace the BCM

4. Install a 3-A fused jumper wire between the signal circuit terminal J and ground. Verify the scan tool Manual HVAC Rear Defog Sw 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 Defog 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 G 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. Test for less than 1.0 ohm of resistance between the relay coil ground circuit V10 and ground.

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

3. Ignition ON, verify that a test lamp does not illuminate between the relay controlled output circuit U10 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 V12 and ground.

⇒	If the test lamp does not illuminate, test the relay switch B+ circuit V12 for a short to ground or an open/high resistance. If the circuit tests normal and the RR DEFOG fuse is open, test the relay controlled output circuit U10 for a short to ground.

5. Ignition OFF, disconnect the harness connector at the Rear Window Defogger Grid.
6. Test for less than 1.0 ohm of resistance between the Rear Window Defogger Grid 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 30 amp fused jumper wire between the relay switch B+ circuit V12 and the relay controlled output circuit U10. Verify the Rear Window Defogger Grid is activated.

⇒	If the Rear Window Defogger Grid does not activate, test the relay controlled output circuit U10 for an open/high resistance. If the circuit tests normal, test or repair the Rear Window Defogger Grid.

9. Connect a test lamp between the rear defogger control circuit U12 and the relay coil ground circuit V10.
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 U12 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 U12 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 Defog Switch

1. Ignition OFF, disconnect the harness connector C2 at the HVAC control assembly
2. Test for infinite resistance between the signal terminal J and the ground circuit terminal K with the switch in the open position.

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

3. Test for less the 2.0 ohms between the signal terminal J and the ground circuit terminal K 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 harness C2 at the HVAC Control Assembly.
2. Install a 5 amp fused jumper wire between circuit terminal G and 12 volts. Install a jumper wire between the ground circuit terminal K and ground. Verify the Defog Indicator illuminates.

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

Relay Test

1. Ignition OFF, disconnect the REAR 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.