When the rear defogger button is pressed, voltage is supplied to the relay coil and the HVAC control assembly 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 grids.

Schematic Reference

Defogger Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Rear Window Defogger Description and Operation

Electrical Information Reference

Ignition ON, command the rear window defogger ON and OFF by pressing the rear window defogger switch. The rear window defogger and rear window defogger indicator should turn ON or OFF each time the rear window defogger switch is pressed.

Single Rear Window Defog Grid Malfunction

1. Ignition OFF, disconnect the harness connector at the appropriate rear window defogger grid.
2. Test for less than 1 ohm between the ground circuit terminal listed below and ground.

•	RPO A12 rear window defogger grid ground terminal A

•	RPO A18 rear window defogger grid ground terminal B

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

3. Ignition ON, connect a test light between the control circuit terminal A and ground.
4. Command the rear window defogger ON and OFF by pressing and depressing the rear window defog switch. The test lamp should turn ON and OFF when changing between the command 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, test or replace the rear window defogger grid.

Rear Window Defogger Malfunction

1. Ignition OFF, disconnect the X3 harness connector at the HVAC control assembly.
2. Test for less than 1 ohm between the ground circuit terminal C 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 illuminates between the B+ circuit terminal A and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for an open/high resistance

4. Ignition OFF, connect the harness connector at the HVAC control assembly and disconnect the REAR DEFOG relay.
5. 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.

6. 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.

7. 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 an open/high resistance.

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 grids are activated.

⇒	If the rear window defogger grids do 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 DEFOG relay.

9. Connect a test lamp between the control circuit terminal 86 and the ground circuit terminal 85.
10. Command the rear window defogger grid ON and OFF by pressing and depressing the rear defogger switch. 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 assembly.

⇒	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 assembly.

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.