The HVAC control assembly is supplied 12 volts to the A/C switch from the Powertrain Control Module (PCM) through circuit 762. When the A/C switch is closed the 12 volt signal is grounded in the HVAC control assembly and pulled low within the PCM indicating an A/C request. The A/C refrigerant pressure sensors 5 volt reference (circuit 474) and ground (circuit 407) are supplied by the PCM. The A/C refrigerant pressure sensor signal circuit 380 supplies the PCM with 0 to 5 volts depending on the A/C refrigerant pressure. Ignition voltage is supplied to the coil and switch sides of the A/C compressor clutch relay. The PCM energizes the A/C compressor clutch relay by grounding the relay coil using the relay control circuit 459. When the relay is energized ignition voltage is supplied to the A/C compressor clutch through the A/C compressor clutch supply circuit 59.