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For 1990-2009 cars only
Makes 🢒 HUMMER 🢒 2008 🢒 H3 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 Rear Window Defogger Malfunction

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • 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

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Relay Switch B+

1

1

--

--

Rear Defog Switch Signal

2

1

1

--

Rear Defog Relay Control

B0285

B0286

B0286

--

Rear Defogger Control

1

1

2

--

Rear Defog Indicator Control

B0285

B0286

B0286

--

HVAC Control Module Ground

--

1

1

--

Relay Coil Ground

--

B0286

1

--

Rear Window Defogger Grid Ground

--

1

1

--

1. Rear Defogger Inoperative

2. Rear Defogger Always ON

Circuit/System Description

With the ignition in the RUN position, and the rear window defogger switch depressed, a signal is sent from the HVAC control module to the body control module (BCM) requesting rear defog operation. The BCM responds by providing voltage to the rear defog indicator and for the RR DEFOG relay. The RR DEFOG relay then provides voltage to the rear defog element supply voltage circuit operating the rear defog system.

Reference Information

Schematic Reference

Defogger Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Rear Window Defogger Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification
  1. Ignition ON, observe the scan tool Rear Defog Switch parameter while pressing the rear window defogger switch. The parameter should change between Active and Inactive.
    2. ⇒If the parameter does not change between the commanded states, refer to Rear Window Defogger Switch Malfunction.
  2. Command the Rear Defog Relay 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.
    3. ⇒If the rear window defogger does not turn ON and OFF when changing between the commanded states, refer to Rear Window Defogger Malfunction.
    ⇒If the rear window defogger indicator is always ON or always OFF, refer to Rear Window Defogger Indicator Malfunction.

Circuit/System Testing

Rear Window Defogger Switch Malfunction

  1. Ignition OFF, disconnect the X2 harness connector at the HVAC control module.
  2. Test for less than 1 ohm between the ground circuit terminal L and ground.
    3. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  3. Ignition ON, verify the scan tool Rear Defog Switch parameter is Inactive.
    4. ⇒If not the specified value, test the rear defog signal circuit terminal J for a short to ground. If the circuit tests normal, replace the BCM.
  4. Install a 3A fused jumper wire between the signal circuit terminal J and ground. Verify the scan tool Rear Defog Switch parameter is Active.
    5. ⇒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 control module.

Rear Window Defogger Malfunction

  1. Ignition OFF, disconnect the RR DEFOG relay.
  2. Test for less than 1 ohm between the ground circuit terminal 85 and ground.
    3. ⇒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.
    4. ⇒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.
    5. ⇒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 defogger grid.
  6. Test for less than 1 ohm between the ground circuit terminal A and ground.
    7. ⇒ If greater than the specified range, test the ground circuit for an open/high resistance.
  7. Connect the harness connector at the defogger grid.
  8. Ignition ON, connect a 30A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the defogger grid is activated.
    9. ⇒If the 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 defogger grid.
  9. Connect a test lamp between the control circuit terminal 86 and ground.
  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.
    11. ⇒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 RR DEFOG relay.

Rear Window Defogger Indicator Malfunction

  1. Ignition OFF, disconnect the X2 harness connector at the HVAC control module.
  2. Disconnect the RR DEFOG relay.
  3. Connect a test lamp between the control circuit terminal 86 and ground.
  4. Ignition ON, 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.
    5. ⇒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.
  5. If all circuits test normal, test or replace the HVAC control module.

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.
    3. ⇒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 between terminals 85 and 86.
    3. ⇒If not within the specified range, replace the relay.
  3. Test for infinite resistance between the following terminals:
    4. • 30 and 86
    • 30 and 87
    • 30 and 95
    • 85 and 87
    ⇒If not the specified value, replace the relay.
  4. Install a 5A fused jumper wire between the relay terminal 85 and B+. Install a jumper wire between the relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87.
    5. ⇒If greater than the specified range, replace the relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Relay Replacement
    •  Endgate Window Replacement
    •  Control Module References for HVAC control module and BCM replacement, setup and programming