When the A/C is in the MAX A/C, A/C, Bi-Level, Defog, or Defrost position, the HVAC control assembly applies voltage to the A/C request signal circuit. This input will request the body control module (BCM) send a serial data message to the engine control module (ECM) to engage the A/C compressor. The EMC will then 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.

The following conditions must be met in order for the ECM/PCM to engage the A/C compressor clutch:

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Engine running, mode switch in the MAX A/C position, verify the scan tool ECM A/C Request Signal parameter is Yes.

⇒	If not the specified value, refer to A/C Request Malfunction.

2. Command the A/C Relay ON and OFF with the scan tool. The A/C compressor clutch should engage and disengage when changing between the commanded states.

⇒	If the A/C compressor clutch does not engage or does not disengage, refer to A/C Compressor Malfunction.

A/C Request Malfunction

1. Ignition OFF, disconnect the X3 harness connector at the HVAC control assembly.
2. Ignition ON, verify a test lamp illuminates between the ignition circuit terminal B and ground.

⇒	If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance.

3. Ignition OFF, connect the X3 harness connector at the HVAC control assembly. Disconnect the X4 harness connector at the HVAC control assembly.
4. Ignition ON, verify a test lamp illuminates between the signal circuit terminal 2 and ground with the mode switch in each of the following positions:

•

MAX A/C

•

A/C

•

Bi-Level

•

Defog

•

Defrost

⇒	If the test lamp does not illuminate in each of the positions, test the signal circuit for a short ground or an open/high resistance. If the circuit tests normal, test or replace the HVAC control assembly.

5. Ignition OFF, connect the X4 harness connector at the HVAC control assembly. Disconnect the harness connector at the A/C low pressure switch.
6. Ignition ON, test for B+ between the signal circuit terminal B and ground.

⇒	If less than the specified value, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

7. Install a 3A fused jumper between the signal circuit terminal B and ground. Verify the scan tool ECM A/C Request Signal parameter is Yes.

⇒	If not the specified value, test the signal circuit for a short to voltage. If the circuit tests normal, replace the BCM.

8. Install a 3A fused jumper between the signal circuit terminal B and the signal circuit terminal A. Verify the scan tool ECM A/C Request Signal parameter is Yes.

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

9. If all circuits test normal, test or replace the A/C low pressure switch.

A/C Compressor Malfunction

1. Ignition OFF, disconnect the A/C compressor relay.
2. 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.

3. Verify that a test lamp illuminates between the B+ 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.

4. 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.

5. Ignition OFF, disconnect the harness connector at the A/C compressor clutch.
6. Test for less than 1 ohm between the ground circuit terminal A ground.

⇒	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. Ignition ON, connect a 10A 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 a short to ground or 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 85 and the control circuit terminal 86.
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.

⇒	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.

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

1. Ignition OFF, disconnect the A/C relay.
2. Test for 60-180 ohms between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

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 

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.