Controls and Compressor Operation

All customer commands for the EMTC climate control system come through the HVAC control head.

A/C Request 

When the A/C button is pressed, the HVAC control module uses a dedicated wire to send the A/C request message to the BCM. The BCM sends the A/C request through the HS-CAN to the PCM.

When an A/C request is received by the PCM, the PCM engages the A/C clutch relay when:

• 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 WOT condition is not present.
• the evaporator discharge air temperature is above approximately 1 °C (33.8 °F).

Compressor Clutch 

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.