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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2009 🢒 LaCrosse 🢒 HVAC 🢒 HVAC - Manual 🢒 Air Conditioning Compressor 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

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 module grounds the A/C request signal circuit. This input will request the powertrain control module (PCM) to ground the A/C compressor clutch relay control circuit, which will switch the A/C COMP 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 PCM / ECM to turn on the compressor clutch:

    • Battery voltage is between 9-18 volts
    • Engine coolant temperature (ECT) is less than 123°C (253°F)
    • Engine speed is less than 4,760 RPM
    • Engine speed is more than 600 RPM
    • A/C high side pressure is between 2929-269 kPa (425-39 psi)
    • Throttle position is less than 100 percent
    • Evaporator temperature is greater than 0°C (32°F)
    • PCM / ECM does not detect excessive torque load
    • PCM / ECM does not detect insufficient idle quality

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

    •  Air Temperature Description and Operation
    •  Air Delivery 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
  1. Ignition ON, Engine running.
  2. Press the A/C request switch. The A/C compressor clutch should engage.
  3. Place the mode switch in the defrost position. The A/C compressor clutch should engage.

Circuit/System Testing
  1. Ignition OFF, disconnect the A/C COMP relay.
  2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal G8 and ground.
    3. ⇒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 E8 and ground.
    4. ⇒If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ignition circuit fuse is open, test or replace the A/C COMP relay.
  4. Verify that a test lamp illuminates between the B+ circuit terminal E7 and ground.
    5. ⇒If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal for a short to ground.
  5. Disconnect the harness connector at the A/C compressor clutch.
  6. Test for less than 1.0 ohm between the A/C compressor clutch ground circuit terminal B and ground.
    7. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  7. Connect the harness connector at the A/C compressor clutch.
  8. Connect a 10A fused jumper wire between the B+ circuit terminal E7 and the control circuit terminal G8. Verify the A/C compressor clutch is activated.
    9. ⇒If the A/C compressor clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the A/C compressor clutch.
  9. Connect a test lamp between the ignition circuit terminal E8 and the control circuit terminal G7.
  10. Command the A/C relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
    11. ⇒If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the PCM / 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 PCM.
  11. If all circuits test normal, test or replace the A/C COMP relay.

Component Testing

Relay Test

  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.
  3. Test for infinite resistance between the following terminals:
    4. • 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.
    5. ⇒If greater than the specified range, replace the relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for PCM / ECM replacement, setup, and programming