Automotive AC – Compressor Clutch

The AC compressor clutch is driven by an engine drive belt on most internal combustion engines. The compressor pulley rotates at all times when the engine is running, but the front clutch plate only rotates when power is sent to the stationary coil mounted to the front of the compressor body. When power is received at the coil, a magnetic field is produced; it pulls the front clutch plate into the face of the pulley, which causes the front plate to begin rotating. The front clutch plate is mechanically connected to the internal pumping components of the compressor, which causes the refrigerant to be pressurized and pumped throughout the AC system.

The front of the compressor is comprised of a front clutch plate, a pulley, and a stationary coil. When the coil is energized, it creates a magnetic field that pulls the front clutch plate into contact with the pulley, which causes the internal parts of the compressor to begin pumping

The method to deliver power to the compressor clutch coil that is used on most vehicles is through a relay that is controlled by the engine control module. The engine control module looks for an input from the HVAC control head that is routed through switches or sensors that notify the engine control module that the AC compressor can be energized.

Having a wiring diagram for a circuit that is not functioning is a valuable tool. The main components in the circuit above include the AC compressor clutch coil, the AC compressor control relay, the ECM, the pressure switches, the heater, and AC control assembly and the fuses. Using a wiring diagram, digital multimeters, and some other testing tools can help troubleshoot electrical problems in the AC

A more modern method of controlling the AC compressor operation involves not only turning the compressor on and off but also controlling the amount of work that the compressor performs. These compressors are called variable-displacement style and have been used for many years in a mechanical form. More modern designs are now being used that control the output of the compressor by using a solenoid that controls the compressor piston stroke.

The solenoid receives signals from the engine control module based upon the requested AC needs. These compressors are able to work at full capacity when the heat load is high but can be throttled back when the cab begins to get cool. The advantage of these styles of compressors is that the clutch does not have to be cycled on and off to control evaporator freeze up.

The rear center portion of a variable-displacement compressor is where the regulating valve is located. This valve causes the compressor to change the angle on the internal plate, which changes the displacement ratio of the

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