Sierra 1500 2WD V8-5.3L (2008)
^
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.
^
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 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 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
HVAC control module.
11. If all circuits test normal, test or replace the REAR DEFOG relay.
Component Testing
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.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. See: Powertrain Management/Computers and Control
Systems/Testing and Inspection/Diagnostic Trouble Code Tests and Associated Procedures/Verification Tests and Procedures
*
Rear Window Replacement
*
Relay Replacement (Attached to Wire Harness) Relay Replacement (Within an Electrical Center)
