A/C Request

WARNING: This page is about a different car, the 2014 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.

For vehicles equipped with EMTC base system with 4 blower motor speed positions, when A/C button is pressed, a message is sent from the HVAC module over the MS-CAN bus to the BCM , then from the BCM through the HS-CAN bus to the PCM.

For vehicles equipped with EMTC non touch 4.2" display (FCDIM ) system (with 7 blower motor speed positions), when A/C is requested by pressing the A/C button on the FCIM , the A/C request message is sent over the MS-CAN bus to the HVAC module that A/C has been selected. The HVAC module then sends an A/C request message over the MS-CAN bus to the BCM , then from the BCM through the HS-CAN bus to the PCM.

For vehicles equipped with 8" touchscreen, when A/C is requested by touching the FDIM , a message is sent from the APIM over the MS-CAN bus to the HVAC module. The HVAC module then sends an A/C request message over the MS-CAN bus to the BCM , then from the BCM through the HS-CAN bus to the PCM.

Voltage is provided to the A/C clutch relay coil and switch contacts from BJB . When A/C is requested and A/C line pressures allow, a ground is provided to the A/C clutch relay coil from the PCM, energizing the A/C clutch relay. When the PCM energizes the relay, voltage is supplied to the A/C compressor clutch field coil from the relay. Ground is supplied for the A/C compressor clutch field coil.

An accurate evaporator temperature is critical for compressor engagement. The PCM uses the temperature measurement to turn the A/C compressor off before the evaporator temperatures are cold enough to freeze the condensation on the evaporator core.

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 WOT condition.
The HVAC module does not detect an evaporator discharge air temperature below 2°C (36°F).