For 1990-2009 cars only
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 |
|---|---|---|---|---|
Relay Coil B+ | 1 | 1 | -- | -- |
Relay Switch B+ | 1 | 1 | -- | -- |
A/C Low Pressure Switch Signal | -- | 1 | 1 | -- |
A/C Compressor Relay Control | P0645 | P0645 | P0645 | -- |
A/C Compressor Control | 1 | 1 | 2 | -- |
A/C Compressor Ground | -- | 1 | -- | -- |
A/C Low Pressure Switch Ground | -- | 1 | -- | -- |
1. A/C Compressor Inoperative 2. A/C Compressor Always Engaged | ||||
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 engine control module (ECM) to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPRSR CLUTCH relay on. 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:
| • | Engine speed is greater than 600 RPM |
| • | Engine speed is less than 4,760 RPM |
| • | A/C high side pressure is between 2957-151 kPa (22-429 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
Connector End View Reference
Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Verification
- Engine running, press the A/C request switch. The A/C compressor clutch should engage.
- 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
- Ignition OFF, disconnect the harness connector at the A/C low pressure switch.
- Test for less than 5 ohms between the ground circuit terminal B and ground.
- Ignition ON, verify the scan tool Low Pressure Switch parameter is Open.
- 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.
- Connect the harness connector at the A/C low pressure switch. Verify the scan tool Low Pressure Switch parameter is Closed.
- Ignition OFF, disconnect the A/C relay.
- Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 86 and ground.
- Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground.
- Verify that a test lamp illuminates between the relay B+ circuit terminal 30 and ground.
- Disconnect the harness connector at the A/C clutch coil.
- Test for less than 1 ohm between the ground circuit terminal 2 and ground.
- Connect the harness connector at the A/C clutch coil.
- 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.
- Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
- 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 all circuits test normal, test or replace the A/C clutch relay.
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
| ⇒ | 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 |
| ⇒ | 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 |
| ⇒ | If not the specified value, test or replace the A/C low pressure switch. |
| ⇒ | If the test lamp illuminates, test the control circuit for a short to voltage. |
| ⇒ | 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. |
| ⇒ | 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. |
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
| ⇒ | 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. |
| ⇒ | 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. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for HVAC control module or ECM replacement, setup, and programming |
