Normal Operation
Under normal operation, when A/C is requested, in the Dual Automatic Temperature Control (DATC) system, a message is sent over the Medium Speed Controller Area Network (MS-CAN) bus from the HVAC module - DATC to the Instrument Cluster (IC), then from the IC through the High Speed Controller Area Network (HS-CAN) bus to the PCM.
Under normal operation, when A/C is requested, in the Electronic Manual Temperature Control (EMTC) system, voltage is sent to the IC from the HVAC module - EMTC , then a message is sent from the IC through the HS-CAN bus to the PCM.
Voltage is provided to the A/C clutch relay switch contacts. When the PCM energizes the relay, voltage is supplied to the A/C compressor clutch field coil. The A/C compressor clutch field coil is grounded.
When an A/C request is received by the PCM, the A/C compressor clutch will only be engaged through the A/C clutch relay 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 temperature below 2°C (36°F).
The evaporator temperature sensor is used for A/C compressor cycling. An accurate evaporator temperature is critical for compressor engagement. The temperature measurement is used by the HVAC to turn off the A/C compressor when the evaporator temperatures are cold enough to freeze the condensation.
This prevents ice blockage of airflow over the evaporator core.
This pinpoint test is intended to diagnose the following:
- Fuse
- Wiring, terminals or connectors
- PCM
- HVAC module - DATC
- HVAC module - EMTC
- A/C evaporator temperature sensor
- A/C pressure transducer
- A/C compressor
- A/C control relay
- Battery Junction Box (BJB)