GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    â€¢ Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    â€¢ Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    â€¢  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

A/C Relay Coil Ignition

2

2

--

--

A/C Relay Switch B+

2

2

--

--

A/C Relay Control

P0645, P0646

P0645, P0647

P0645, P0647

--

A/C Clutch Control

2

2

1

--

A/C Compressor Clutch Ground

--

2

2

--

1. A/C Clutch Always Engaged

2. A/C Clutch Inoperative

Circuit/System Description

When the A/C switch is pressed, the HVAC control assembly sends a serial data message to the engine control module (ECM) for an A/C request. This input will request the ECM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CLTCH relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.

Diagnostic Aids

The following conditions must be met in order for the ECM to turn on the compressor clutch:

    â€¢ Battery voltage is between 9-16 volts.
    â€¢ Engine coolant temperature (ECT) is less than 123°C (253°F).
    â€¢ Engine speed is less than 4760 RPM.
    â€¢ Engine speed is more than 600 RPM.
    â€¢ A/C high side pressure is between 2929-269 kPa (425-39 psi).
    â€¢ Throttle position is less than 100 percent.
    â€¢ Evaporator temperature is greater than 0°C (32°F).
    â€¢ ECM does not detect excessive torque load.
    â€¢ ECM does not detect insufficient idle quality.

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

    â€¢  Air Temperature Description and Operation
    â€¢  Air Delivery Description and Operation

Electrical Information Reference

    â€¢  Circuit Testing
    â€¢  Connector Repairs
    â€¢  Testing for Intermittent Conditions and Poor Connections
    â€¢  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Engine running, command the A/C compressor On and OFF using a scan tool. The A/C compressor clutch should engage and disengage as commanded.

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.
  3. ⇒If the test lamp illuminates, test the control circuit for a short to voltage.
  4. Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground.
  5. ⇒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.
  6. Verify that a test lamp illuminates between the relay B+ circuit terminal 30 and ground.
  7. ⇒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.
  8. Disconnect the harness connector at the A/C clutch coil.
  9. Test for less than 1 ohm between the ground circuit terminal 2 and ground.
  10. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  11. Connect the harness connector at the A/C clutch coil.
  12. 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.
  13. ⇒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.
  14. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
  15. 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.
  16. ⇒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.
  17. 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.
  3. ⇒If the resistance is not within the specified range, replace the relay.
  4. Test for infinite resistance between the following terminals:
  5. • 30 and 86
    • 30 and 87
    • 30 and 85
    • 85 and 87
    ⇒If not the specified value, replace the relay.
  6. 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.
  7. ⇒If greater than the specified range, replace the relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    â€¢  Compressor Clutch Assembly Replacement
    â€¢  Air Conditioning Compressor Replacement
    â€¢  Control Module References for BCM, ECM, and HVAC control module replacement, setup, and programming