The temperature control knob controls a rheostat within the HVAC control assembly. The temperature control signal varies from 0 to 12 volts depending on the control knob position. The temperature door motor is supplied ignition voltage through the supply circuit 41 and is used to power the internal circuitry and operate the motor. The temperature control signal is supplied to the temperature door motor through circuit 1316 and is used to determine the temperature door motor position.