Control System Logic

WARNING: This page is about a different car, the 2017 Lincoln Navigator and 2017 Ford Expedition. However, it is still accessible from the selected car via links, so may be relevant.

All customer requests for the EMTC system come through the HVAC control module.

A/C Request

When the A/C is requested, the HVAC control module sends the message over the Medium Speed Controller Area Network (MS-CAN) to the Body Control Module (BCM). The BCM sends the A/C request to the PCM through the High Speed Controller Area Network (HS-CAN).

When an A/C request is received by the PCM, the PCM engages the A/C clutch relay when all of the following conditions exist:

the refrigerant pressure is not excessively high or low.
the engine coolant temperature is not excessively high.
the ambient air temperature is above approximately 0 °C (32 °F).
a Wide Open Throttle (WOT) condition is not present.
the evaporator temperature is above approximately 1 °C (33.8 °F).
the PCM does not detect temperatures from the Mass Airflow/Intake Air Temperature sensor that does not correlate with other temperature sensor readings at ignition ON. (Most Mass Airflow (MAF) sensors have an integrated Intake Air Temperature (IAT) sensor).

The PCM monitors multiple temperature sensors correlation (AAT, CHT, ECT IAT, MAF, MAP...). The PCM runs this logic after an engine off and a calibrated soak period, typically 6 to 8 hours. This soak period allows the Ambient Air Temperature (AAT) sensor and the other temperature sensors to stabilize and not differ by greater than a calibrated value, typically 18 °C (32.4 °F). If a sensor input is found to be reporting a temperature imbalance the PCM does not allow the A/C clutch to engage. For more information on PCM sensors, Refer to Engine Controls - Description & Operation (Except Diesel & Hybrid) .

The PCM monitors the discharge pressure measured by the A/C pressure transducer. The PCM interrupts A/C compressor operation in the event the A/C pressure transducer indicates high system discharge pressures. It is also used to sense low charge conditions. If the pressure is below a predetermined value for a given ambient temperature, the PCM does not allow the A/C clutch to engage.