Air Conditioning: Notes

WARNING: This page is about a different car, the 2011 Lincoln MKT. However, it is still accessible from the selected car via links, so may be relevant.

The A/C system components include the following:

A/C compressor
A/C clutch assembly
A/C condenser/transmission oil combo cooler
A/C evaporator core
Receiver/drier
Connecting refrigerant lines
Thermostatic Expansion Valve (TXV)

The refrigerant system incorporates an A/C compressor controlled by the PCM through an A/C clutch relay. The HVAC module sends an A/C request signal over the Medium Speed Controller Area Network (MS-CAN) to the Instrument Cluster (IC) which relays the request over the High Speed Controller Area Network (HS-CAN) to the PCM. An evaporator discharge air temperature sensor is used to disengage the A/C compressor clutch when the evaporator core temperature falls below 5.5°C (42°F).

The PCM commands the A/C clutch relay to turn on the A/C compressor clutch if all of the following conditions are met:

The HVAC module is set to a mode which provides an A/C request to the PCM via the IC.
The evaporator discharge air temperature sensor is reading a temperature above 5.5°C (42°F).
The A/C pressure transducer is not reading a pressure above 2,841 kPa (412 psi) in the high side of the refrigerant system.
The A/C pressure transducer is not reading a pressure that is too low (indicating a low charge condition) in the high-pressure side of the refrigerant system.
The A/C compressor relay is switched to the CLOSED position by the PCM.
The engine coolant temperature is not excessively high.
The PCM has not detected a Wide Open Throttle (WOT) condition.
NOTE: To prevent erroneous readings that can result from underhood heat sources, the PCM uses data from several sources to calculate ambient temperature. The vehicle must be driven for a sufficient time, speed and distance after startup before the calculated ambient temperature is updated in the PCM. If a vehicle is moved from a below-freezing environment to a warm environment (such as being moved from outside into a garage, or when delivered from a cold climate to a warm climate), the minimal amount of driving may not be sufficient for the calculated ambient temperature to be updated. In this event, the PCM will believe the ambient temperature is too low for A/C operation and, to prevent an increased risk of compressor slugging or damage, will not engage the A/C compressor in any mode. To manually reset the PCM calculated ambient temperature to the current temperature, run the vehicle until the thermostat opens and then clear the PCM Keep Alive Memory (KAM). For the KAM resetting procedure, refer to ENGINE CONTROLS - INTRODUCTION - GASOLINE MODELS article. When the PCM KAM is reset, the ambient temperature will reset to the current higher value (a value closer to actual surrounding conditions) and the A/C compressor will operate normally.

An A/C pressure relief valve protects the refrigerant system against excessively high refrigerant pressures as a redundant backup to the A/C pressure transducer.

Refrigerant flow into the evaporator core is metered by a TXV.

The HVAC module will stop requesting A/C when the ambient temperature drops below -1.7°C (29°F), and will begin requesting A/C when the ambient temperature rises above 0°C (32°F).