Rear Window Defogger Malfunction
Rear Window Defogger Malfunction
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
When the HVAC control module receives a rear window defog request from the HVAC control faceplate, the HVAC control module will supply voltage to the relay coil. 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 to flow through the switch contacts and out the relay switched output to the rear window defogger grid.
Diagnostic Aids
The rear window defogger indicator will still function when the rear defogger switch is pressed, but any of the following conditions may cause the rear window defogger grid not to function:
* The battery state of charge is out of range causing the HVAC control module to suspend the rear window defogger operation.
* The Remote Start Enable ambient temperature is greater than 5°C (40°F).
Reference Information
Schematic Reference
Defogger Schematics (Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector End Views By Name)
Description and Operation
Rear Window Defogger Description and Operation (Rear Window Defogger)
Electrical Information Reference
* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)
Circuit/System Verification
1. Engine running, observe the scan tool Rear Defogger Switch parameter while pressing the heated rear window switch. The parameter should change from Inactive to Active.
• If the value does not change, replace the A26 HVAC control.
2. Observe the operation of the rear window defogger indicator while pressing the heated rear window switch. The indicator should turn ON or OFF each time the heated rear window switch is pressed.
• If the indicator does not turn ON or OFF with each switch press, replace the A26 HVAC control.
3. Command the E18 rear window defogger grid ON and OFF with a scan tool, the E18 rear window defogger grid should turn ON or OFF while changing between the commanded states.
Circuit/System Testing
1. Ignition OFF, disconnect the K17 rear defog relay.
2. Test for less than 30 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. Engine running, verify 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 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 E18 rear window defogger grid.
6. Test for less than 30 ohm between the ground circuit terminal 1 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 E18 rear window defogger grid.
8. Engine running, connect a 40 A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the E18 rear window defogger grid is activated.
• If the E18 rear window defogger grid does not activate, and the circuit fuse is good, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the E18 rear window defogger grid.
• If the E18 rear window defogger grid does not activate, and the circuit fuse is open, test the control circuit for a short to ground. If the circuit tests normal, test or replace the E18 rear window defogger grid.
9. Connect a test lamp between the control circuit terminal 86 and the ground circuit terminal 85.
10. Command the E18 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 K33 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 K33 HVAC control module.
11. If all circuits test normal, test or replace the K17 rear defog relay.
Component Testing
Rear Window Defogger Grid Test
1. Engine running, turn on the E18 rear window defogger grid.
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 K17 rear defog relay.
2. Test for 60-180 ohm between terminals 85 and 86.
• If not within the specified range, replace the K17 rear defog 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 K17 rear defog relay.
4. Install a 25 A fused jumper wire between relay terminal 85 and 12 V. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohm between terminals 30 and 87.
• If greater than the specified range, replace the K17 rear defog relay.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Rear Window Replacement (Rear Window Replacement)
* Relay Replacement (Attached to Wire Harness) (Relay Replacement (Attached to Wire Harness))Relay Replacement (Within an Electrical Center) (Relay Replacement (Within an Electrical Center))
* Control Module References (Programming and Relearning) for HVAC control module and HVAC control replacement, setup, and programming