Chevrolet Silverado / GMC Sierra. Service manual - part 2964

Circuit/System Description

When the rear defogger button is pressed, voltage is supplied to the relay coil by the HVAC
control module which also illuminates the rear window defogger indicator. B+ 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 G304 or G307.

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

Rear Window Defogger Grid
Ground

1. Rear Defogger Always On
2. Rear Defogger Inoperative

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Fixed and Moveable Windows - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

1. Ignition ON, observe the scan tool Rear Defrost State parameter while pressing the rear

window defogger switch. The parameter should change between On and Off.

If the parameter does not change between the commanded states, replace the HVAC
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
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 REAR DEFOG relay.

2. Test for less than 1.0 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. 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.

4. 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 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 5 ohms between the defogger grid ground circuit terminal A and ground.

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

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Fixed and Moveable Windows - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

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 Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Fixed and Moveable Windows - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

Rear Window Replacement

Relay Replacement (Attached to Wire Harness) or Relay Replacement (Within an
Electrical Center)

Control Module References for HVAC control module replacement, setup, and
programming

POWER REAR SLIDING WINDOW 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/System Description

The power sliding rear window motor is controlled from the sliding rear window switch through
OPEN and CLOSE relays. The sliding rear window switch is supplied voltage from the body
control module (BCM) accessory voltage output circuit. When the switch is pressed in the OPEN
or CLOSE position, the sliding rear window open relay or the sliding rear window close relay
coil will be supplied accessory voltage and energized through the appropriate relay control
circuit. While the sliding rear window open relay or the sliding rear window close relay are in a

Circuit

Short to

Ground

Open/High

Resistance

Short to

Voltage

Signal

Performance

Sliding Rear Window Relay
B+

Sliding Rear Window Switch
Accessory Voltage

Sliding Rear Window Relay
Control

Sliding Rear Window Motor
Control

Sliding Rear Window Relay
Ground

1. Sliding Rear Window Switch Malfunction
2. Sliding Rear Window Motor Malfunction

2008 Chevrolet Silverado 1500

2008 ACCESSORIES & EQUIPMENT Fixed and Moveable Windows - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

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