Control valve for a variable displacement compressor

An air-conditioning system has a refrigerant circuit (30). The circuit includes a condenser (31), an expansion valve (32), an evaporator (33) and a variable displacement compressor. The system has a pressure difference detector (36). The pressure difference detector (36) detects the pressure difference between two pressure monitoring points (P1, P2) located on the refrigerant circuit. A control valve (40) and a controller (60) control the displacement of the compressor in accordance with the pressure difference detected by the pressure difference detector (36). This permits the displacement of the compressor to be quickly changed.