Normal Operation

WARNING: This page is about a different car, the 2010 Mercury Mariner and 2010 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

Under normal operation, when A/C is requested the HVAC module then sends an A/C request message over the Medium Speed Controller Area Network (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.

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.

The evaporator discharge air temperature sensor is used for A/C compressor cycling. An accurate evaporator discharge air temperature is critical for compressor engagement. The temperature measurement is used by the PCM to turn off the A/C compressor when the evaporator temperatures are cold enough to freeze the condensation.

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 A/C pressure transducer does not indicate excessively low or high refrigerant pressure. Refer to SPECIFICATIONS .
The PCM does not detect excessively high engine coolant temperature.
The PCM does not detect an ambient air temperature below 0°C (32°F).
The PCM has not detected a Wide Open Throttle (WOT) condition.
The HVAC module does not detect an evaporator discharge air temperature below 0°C (32°F).

This pinpoint test is intended to diagnose the following:

Fuse(s)
Wiring, terminals or connectors
Evaporator discharge temperature sensor
A/C pressure transducer
PCM
HVAC module
A/C compressor clutch field coil
A/C clutch relay
A/C clutch air gap
Battery Junction Box (BJB)