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Rear Window Defogger Inoperative



Rear Window Defogger Inoperative

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of the diagnostic category.

Diagnostic Fault Information






Circuit/System Description

Battery positive voltage is supplied through the REAR DEFOG fuse in the underhood fuse block to the REAR DEFOG relay switched input. Ground for the coil of the REAR DEFOG relay is provided by the body control module (BCM). Battery voltage is supplied from the IGNITION 30A MaxiFuse to the supply voltage on the rear defog relay coil side. When you depress the rear defogger switch, the rear defogger switch pulls the signal circuit low as an input to the BCM. The BCM interprets this as a request for the rear defogger system. The BCM enables the rear defogger system by supplying ground to the REAR DEFOG relay coil. The relay coil ground is also spliced off to the ground circuit of the rear defogger indicator. The REAR DEFOG relay is energized and the rear 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 defogger grid. Ground for the rear defogger grid is provided by G401. When you start the engine and press the rear defogger switch for the first time, the defogger cycle lasts for 15 minutes. Further operation results in 7.5 minute defogger cycles. The defogger cycle resets to 15 minutes when you cycle the ignition to the OFF position and then to the ON position.

Circuit/System Verification

1. Ignition ON, observe the scan tool BCM Rear Defog Switch parameter while pressing the rear window defogger switch. The reading should change between ON and OFF.

• If the reading does not change between the commanded states, refer to Rear Defogger Switch Malfunction.

2. Command the rear window defogger ON and OFF with a scan tool. The rear window defogger and rear window 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, replace the HVAC control module.

Circuit/System Testing

Rear Defogger Switch Malfunction
1. Ignition OFF, disconnect the harness connector C1 at the HVAC Module.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the ground circuit terminal 12 and ground.

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

3. Ignition ON, verify the scan tool BCM Rear Defog Switch parameter if OFF.

• If not the specified value, test the signal circuit terminal 1 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 1 and ground. Verify the scan tool BCM Rear Defog Switch 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 module.

Rear Defogger Malfunction
1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Ignition ON, verify a test lamp does not illuminate between the relay controlled output circuit terminal and ground.

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

3. Verify a test lamp illuminates between the relay coil B+ circuit terminal and ground.

• If the test lamp does not illuminate, test the relay coil B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test or replace the relay REAR DEFOG relay.

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

5. Disconnect the harness connector C2 at the rear window defogger grid.
6. Ignition OFF, test for less than 1.0 ohm of resistance 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 C2 at the rear window defogger grid.
8. Ignition ON, connect a 25 A 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 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 relay coil B+ circuit terminal and the relay coil control circuit terminal.
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 voltage. If the circuit tests normal, replace the BCM.
• If the test lamp is always OFF, test the relay coil 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.

Component Testing

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 RR 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.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

Control Module References for HVAC control module replacement, setup, and programming. Programming and Relearning

Relay Replacement (Attached to Wire Harness) Relay Replacement (Within an Electrical Center)

Liftgate Window Replacement
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