PCM just turns the compressor on and it pumps until too much head pressure is produced and then shuts off. Never shuts off in my truck since I have the dual fans running at max at all times and that keeps the head pressure down and the cab ice cold. If it runs low on freon the switch will break the circuit to preserve the compressor. Found that out when one of the aluminum fittings broke. Had the fitting replaced with steel and no more worries.