Normal Operation
Under normal operation, for Fusion and Milan equipped with navigation and all MKZ vehicles, when 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 all 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 Fusion and Milan not equipped with navigation , when A/C is requested, a message is sent over the MS-CAN bus to the IC , then from the IC through the HS-CAN bus to the PCM.
Ignition voltage is provided to the A/C clutch relay switch through circuit SBB25 (RD). When the PCM energizes the relay, ignition voltage is supplied to the A/C clutch through circuit CH401 (VT/WH). Ground is supplied for the A/C clutch through circuit GD121 (BK/YE).
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 discharge air temperature below 2°C (36°F).
This pinpoint test is intended to diagnose the following:
- Fuse(s)
- Wiring, terminals or connectors
- PCM
- HVAC module
- FCIM
- ACM
- A/C evaporator discharge temperature sensor
- A/C pressure transducer
- A/C compressor clutch field coil
- A/C control relay
- A/C clutch air gap