Rear Window Defogger Malfunction SUV

With the ignition in the RUN position and the rear window defogger switch depressed, the HVAC control module responds by providing voltage to the rear defog control signal circuit energizing the defog relay. The defog relay then provides voltage to the rear defog element supply voltage circuit operating the rear defog system.

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 Defrost Switch parameter while pressing the rear window defogger switch. The parameter should change between Inactive and Active.

⇒	If the parameter does not change between the commanded states, replace the HVAC control module.

2. Observe the operation of the rear window defogger indicator while pressing the rear window defogger switch. The indicator should turn ON or OFF each time the rear window defogger switch is pressed.

⇒	If the indicator does not turn ON or OFF with each switch press, replace the HVAC control module.

3. Command the Rear Defog Request ON and OFF with a scan tool. The rear window defogger should turn ON and OFF when changing between the commanded states.

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Test for less than 1 ohm between the ground circuit terminal 85 and ground.

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

3. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.

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

4. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.

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

5. Ignition OFF, disconnect the X2 harness connector at the rear window defogger.
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.
8. Ignition ON, connect a 30A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the rear window defogger grid is activated.

⇒	If the rear window defogger grid does not activate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the rear window defogger.

9. Connect a test lamp between the control circuit terminal 86 and ground.
10. Command the Rear Defog Request 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 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 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 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.

Rear Window Defogger Malfunction SUT

With the ignition in the RUN position, and the rear window defogger switch depressed, the HVAC control module responds by providing voltage to the rear defog control signal circuit energizing the defog relay. The defog relay then provides voltage to the rear defog element supply voltage circuit operating the rear defog system. On the SUT vehicle, the relay switched output splices into 2 separate circuits that input into the midgate control module. The midgate control module then supplies 2 power output circuits spliced into one to the rear 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 Defrost Switch parameter while pressing the rear window defogger switch. The parameter should change between Inactive and Active.

⇒	If the parameter does not change between the commanded states, replace the HVAC control module.

2. Observe the operation of the rear window defogger indicator while pressing the rear window defogger switch. The indicator should turn ON or OFF each time the rear window defogger switch is pressed.

⇒	If the indicator does not turn ON or OFF with each switch press, replace the HVAC control module.

3. Command the Rear Defog Request ON and OFF with a scan tool. The rear window defogger should turn ON and OFF when changing between the commanded states.

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Test for less than 1 ohm between the ground circuit terminal 85 and ground.

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

3. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.

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

4. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.

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

5. Connect a test lamp between the control circuit terminal 86 and ground.
6. 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 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.

7. Ignition OFF, connect the REAR DEFOG relay and disconnect the harness connector at the midgate control module.
8. Connect a test lamp between the control circuit terminal listed below and ground:

•	Control circuit terminal 13

•	Control circuit terminal 15

9. Ignition ON, command the rear window defogger ON and OFF with a scan tool. The test lamp should turn ON or OFF while changing between commanded states.

⇒	If the test lamp is always ON, test or replace the REAR DEFOG relay.

⇒	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, test or replace the REAR DEFOG relay.

10. Ignition OFF, connect the harness connector at the midgate control module and disconnect the harness connector at the rear window defogger.
11. 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.

12. Connect a test lamp between the control circuit terminal 4 and ground.
13. Ignition ON, command the rear window defogger ON and OFF with a scan tool. The test lamp should turn ON or OFF while changing between commanded states.

⇒	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 midgate control module.

⇒	If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the midgate control module.

14. If all circuits test normal, test or replace the rear window defogger grid.

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 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 between terminals 30 and 87.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.