For 1990-2009 cars

### 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 Switch B+ | 2 | 2 | -- | --
A/C Relay Coil Ignition | 2 | 2 | -- | --
A/C Request Signal | 2 | 2 | 2 | --
A/C Relay Control | P1545 | P1545 | P1545 | --
A/C Clutch Control | 2 | 2 | 1 | --
Evaporator Temperature Sensor Ground | -- | 2 | -- | --
A/C Compressor Clutch Ground | -- | 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 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:

• | 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)

• | ECM does not detect excessive torque load

• | 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 Delivery Description and Operation

• | Air Temperature 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

Ignition ON, command the A/C relay ON and OFF with a scan tool. Observe the HVAC compressor operation, the A/C compressor clutch should engage and disengage when changing between the commanded states.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the evaporator temperature sensor.
2. Test for less than 5 ohms between the ground circuit terminal 3 and ground.

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

3. Ignition ON, test for greater than 76 volts between the signal circuit terminal 1 and ground.

⇒ | If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

4. Ignition OFF, connect the harness connector at the evaporator temperature sensor.
5. Engine running, observe the scan tool BCM A/C Request parameter while cycling the A/C switch on the HVAC control module. The parameter should change between Yes and No.

⇒ | If the parameter does not change between the specified values, test the signal circuit terminal A43 for a short to ground or an open high/resistance. If the circuit tests normal, test or replace the evaporator temperature sensor. If the A/C compressor remains inoperative, replace the BCM.

6. Ignition OFF, disconnect the A/C relay.
7. 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.

8. 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 an open/high resistance. If the circuit tests normal and the ignition circuit fuse is open, test or replace the relay.

9. 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. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal for a short to ground.

10. Disconnect the harness connector at the A/C compressor clutch.
11. Test for less than 1.0 ohm between the ground circuit terminal B and ground

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

12. Connect the harness connector at the A/C compressor clutch.
13. Connect a 10A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the A/C compressor clutch is activated.

⇒ | 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.

14. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
15. 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.

⇒ | 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.

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

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Air Conditioning Compressor Replacement

• | Control Module References for ECM replacement, setup, and programming