When you depress the rear defogger button, voltage is supplied to the rear defog relay coil and the HVAC control module also illuminates the rear window defogger indicator. Battery positive voltage is supplied at all times to the rear defog relay switched input and the rear defog relay coil is always grounded. 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 defogger grid is provided by G403.

When you start the engine and press the rear 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).

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 operation of the defogger grid while pressing the rear defogger switch. The defogger grid should ACTIVATE and DEACTIVATE when changing between commanded states.
2. Ignition ON, observe the operation of the defogger indicator while pressing the rear defogger switch. The defogger indicator should turn ON and OFF when changing between the commanded states.

⇒	If the defogger indicator does not illuminate or continuously illuminates, replace the HVAC control module.

1. Ignition OFF, disconnect the rear defog relay.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the relay coil ground circuit terminal 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 relay controlled output circuit terminal and ground.

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

4. Verify that a test lamp illuminates between relay switch B+ circuit terminal 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 BATT 3 fuse is open, test the relay controlled output circuit terminal for a short to ground.

5. Ignition OFF, disconnect the harness connector at the rear window defogger grid.
6. Ignition OFF, test for less than 1.0 ohm of resistance between the 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 40A fused jumper wire between the relay switch B+ circuit terminal and the relay controlled output circuit terminal. Verify the rear window defogger grid is activated.

⇒	If the defogger grid does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the defogger grid.

9. Connect a test lamp between the relay coil control circuit terminal and the relay coil ground circuit terminal.
10. Command the rear window defogger grid 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 HVAC control module.

⇒	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 HVAC control module.

11. If all circuits test normal, test or replace the rear defog relay.

Relay Test

1. Ignition OFF, disconnect the ear 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 30-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.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.