Rear Window Defogger Malfunction (RPO C68)
Rear Window Defogger Malfunction (RPO C68)
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 each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
When the rear defogger button is pressed, voltage is supplied to the relay coil and the body control module (BCM) also illuminates the rear window defogger indicator. Battery positive voltage is supplied at all times to the relay switched input and the 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 G301.
Circuit/System Verification
1. Ignition ON, observe the scan tool Rear Defog Request parameter while pressing the rear window defogger switch. The parameter should change between Yes and No.
^ 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 window defogger ON and OFF with a scan tool. The rear window defogger should turn ON and OFF when changing between the commanded states.
Circuit/System Testing
1. Ignition OFF, disconnect the R/WDO DEFOG relay.
2. Test for less than 1.0 ohm between the ground circuit terminal B 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 D 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 C and ground.
^ If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
5. Disconnect the X2 harness connector at the rear window defogger grid.
6. Test for less than 1.0 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 X2 harness connector at the rear window defogger grid.
8. Connect a 30-amp fused jumper wire between the relay B+ circuit terminal C and the control circuit terminal D. Verify the rear window defogger grid is activated.
^ If the rear window defogger grid 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 control circuit terminal A and the ground circuit terminal B.
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 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.
11. If all circuits test normal, test or replace the R/WDO 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 R/WDO 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 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 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
* Rear Window Replacement