Pinpoint Test P: The Air Conditioning (A/C) is Always On: Notes

Refer to SYSTEM WIRING DIAGRAMS article, Automatic Climate Control System for schematic and connector information.

When A/C is requested, a message is sent from the HVAC module over the MS-CAN bus to the IC, then from the IC through the HS-CAN  bus to the PCM.

When an A/C request is received from the HVAC module, an A/C demand message is sent from the PCM over the HS-CAN  bus to the Air Conditioning Compressor Module (ACCM) if all of the following conditions are met:

• The PCM does not detect excessively high or low refrigerant pressure from the A/C pressure transducer.
• The PCM does not detect excessively high engine coolant temperature.
• The PCM does not detect an ambient air temperature below -1°C (30°F).
• The PCM has not detected a Wide Open Throttle (WOT) condition.
• The HVAC module does not detect an evaporator discharge air temperature below 2°C (36°F).

When the ACCM  receives the A/C demand message, it will engage. Instead of cycling ON and OFF like a traditional A/C compressor, the PCM monitors the evaporator discharge temperature from the HVAC module to raise or lower the electric A/C compressor speed as required.

This pinpoint test is intended to diagnose the following: 

• HVAC module
• Electric A/C compressor
• A/C evaporator discharge temperature sensor
• A/C pressure transducer