Normal Operation

Under normal operation, for vehicles equipped  with navigation, when the A/C button is pressed, a message is sent from the Front Controls Interface Module (FCIM) over the Medium Speed Controller Area Network (MS-CAN) bus to the HVAC module. The HVAC module then sends an A/C request message over the MS-CAN  bus to the Instrument Cluster (IC), then from the IC  through the High Speed Controller Area Network (HS-CAN) bus to the PCM.

For vehicles equipped  with navigation, when MAX A/C is selected on the touch screen, the Audio Control Module (ACM) sends a message over the MS-CAN  bus to the HVAC module that MAX A/C has been selected.

For vehicles not equipped  with navigation, 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
• PCM
• FCIM 
• ACM 
• Electric A/C compressor
• A/C evaporator discharge temperature sensor
• A/C pressure transducer