Vehicle temperature control system

A heating and air conditioning control system havng a compressor clutch cycle control feature for controlling the in-car temperature of a vehicle passenger compartment is disclosed. The system comprises an automatic temperature control (ATC) sensor (32) which provides a modulated vacuum control signal for a vacuum motor (38). The vacuum motor has an output which adjusts a biasing element in an electrical switch (82), resulting in on/off duty cycling of an electrically energizable clutch (28) which transmits power from the vehicle engine to the air conditioner compressor (26). The percentage of time that the compressor is on versus the time off is a function of the control signal which is, in turn, a function of the ambient, in-car, and manually selected set point temperatures sensed by the ATC sensor. At ambient temperatures below a predetermined minimum, the ATC sensor produces a control signal which shuts off the compressor and places the system in a heating mode, thereby preventing needless cooling of ambient air prior to heating. During the cooling mode, the refrigeration unit compressor is energized only as needed to maintain the passenger compartment at the set-point temperature, thus minimizing power usage.