Chevrolet Silverado / GMC Sierra. Manual — part 2351

Circuit/System Testing

1. Ignition OFF, disconnect the A/C relay.

2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal

86 and ground.

If the test lamp illuminates, test the control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground.

If the test lamp does not illuminate, test the ignition circuit for a short to ground or a
open/high resistance. If the circuit tests normal and ignition circuit fuse is open, test all
components connected to the ignition circuit and replace as necessary.

4. Verify that a test lamp illuminates between the relay B+ circuit terminal 30 and ground.

If the test lamp does not illuminate, test the B+ circuit for a short to ground or a
open/high resistance. If the circuit tests normal and B+ circuit fuse is open, replace the
A/C clutch relay.

5. Disconnect the harness connector at the A/C clutch coil.

6. Test for less than 1 ohm between the ground circuit terminal 2 and ground.

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

7. Connect the harness connector at the A/C clutch coil.

8. Connect a 10-amp fused jumper wire between the B+ circuit terminal 30 and control circuit

terminal 87. Verify the A/C clutch is activated.

If the A/C clutch does not activate, test the control circuit for an open/high resistance.
If the circuit tests normal, test or replace the A/C clutch.

9. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal

86.

10. Command the A/C relay output function 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 ground. If the
circuit tests normal, replace the ECM.

If the test lamp is always OFF, test the control circuit for a short to voltage, or an
open/high resistance. If the circuit tests normal, replace the ECM.

11. If all circuits test normal, test or replace the A/C clutch relay.

Component Testing

1. Ignition OFF, disconnect the A/C COMP relay.

2. Test for 60-180 ohms between terminals 85 and 86.

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

If the resistance is 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.

Repair Procedures

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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

Control Module References for ECM replacement, setup, and programming

DTC P0646 OR P0647

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.

DTC Descriptors

DTC P0646

Air Conditioning (A/C) Clutch Relay Control Circuit Low Voltage

DTC P0647

Air Conditioning (A/C) Clutch Relay Control Circuit High Voltage

Diagnostic Fault Information

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

Circuit/System Description

The engine control module (ECM)/powertrain control module (PCM) controls the A/C
compressor clutch based on an A/C request serial data message from the HVAC control module.
The ECM/PCM monitors the A/C refrigerant pressure sensor and if pressure is within a specified
range, will engage the A/C compressor by grounding the A/C compressor clutch relay control
circuit.

B+ is applied to the A/C relay in the RUN/START position and with the A/C compressor clutch
relay control circuit grounded by the ECM/PCM, current flows through the coil side of the A/C
relay. With current flowing through the coil side of the relay, a magnetic field is created which
pulls the switch side of the relay closed, sending current to the A/C compressor clutch, allowing
the A/C compressor to be engaged.

Conditions for Running the DTC

An A/C request is received from the HVAC module.

Conditions for Setting the DTC

A short to voltage, short to ground, or open/high resistance is seen on the A/C compressor clutch
relay control circuit.

Action Taken When the DTC Sets

DTC P0646 and DTC P0647 are Type C DTCs

Conditions for Clearing the DTC

DTC P0646 and DTC P0647 are Type C DTCs

Reference Information

Schematic Reference

HVAC Schematics

Circuit

Short to

Ground

Open/High

Resistance

Short to

Voltage

Signal

Performance

A/C Relay Coil B+

P0646

P0647

P0647

-

A/C Compressor Clutch Relay
Control

P0646

P0647

P0647

-

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado