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

Battery positive voltage is supplied through the RT IP #3 fuse in the underhood junction block, to the REAR DEFOG relay switched input. With the engine started, the body control module (BCM) supplies battery positive voltage to REAR DEFOG relay coil. When you depress the rear window defogger switch the HVAC control assembly illuminates the rear window defogger indicator and energizes the REAR DEFOG relay by grounding the control circuit of the relay. This allows battery positive voltage from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger. Ground for the rear window defogger is provided by G309.

When you start the engine and press the rear window defogger switch for the first time, the defogger cycle lasts for 10 minutes. Further defogger cycles vary based on ambient temperature and vehicle speed. The rear window defogger will be extended when the ambient temperature is cooler or if the vehicle speed is above 72 km per hour (45 mph).

If the battery voltage is very low, the rear defogger feature is disabled.

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

1. Ignition ON, command the rear window defogger ON and OFF by pressing the defog button. Observe the rear window defogger operation as the rear window defogger turns ON & OFF.
2. If the rear window defogger indicator light does not illuminate, replace the HVAC control module.

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal L2 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 B+ terminal N3 and ground.

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

4. Verify that a test lamp illuminates between the B+ terminal L3 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 RT IP #3 fuse is open, test the control circuit terminal switch side control for a short to ground.

5. 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 ground circuit terminal A 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 B+ terminal L3 and the control terminal N2. 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 B+ terminal N3 and the control terminal L2.
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 HVAC control module.

⇒	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 REAR 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 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

•

35 and 87

⇒	If not the specified value, replace the relay.

4. Install a 60-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.