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

1. Engine running, press the A/C request switch. The A/C compressor clutch should engage.
2. Place the mode switch in the defrost position. The A/C compressor clutch should engage.

⇒ | If the clutch does not engage refer to Air Conditioning (A/C) System Performance Test.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the A/C low pressure switch.
2. Test for less than 5 ohms between the ground circuit terminal B and ground.

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

3. Ignition ON, verify the scan tool Low Pressure Switch parameter is Open.

⇒ | If not the specified value, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the HVAC control module.

4. Connect a 3-amp fused jumper wire between the signal circuit terminal A and ground. Verify the scan tool Low Pressure Switch parameter is Closed.

⇒ | If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module.

5. Connect the harness connector at the A/C low pressure switch. Verify the scan tool Low Pressure Switch parameter is Closed.

⇒ | If not the specified value, test or replace the A/C low pressure switch.

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

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 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.

9. 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.

10. Disconnect the harness connector at the A/C clutch coil.
11. 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.

12. Connect the harness connector at the A/C clutch coil.
13. 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.

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.

⇒ | 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 clutch relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Compressor Clutch Assembly Replacement

• | Heater and Air Conditioning Control Replacement