Refrigerant Pressure Sensor / Switch: Description and Operation

The clutch cycling pressure switch is mounted on a Schrader-type valve fitting on the top of the suction accumulator/drier assembly (illustration). A valve depressor, located inside the threaded end of the pressure switch, presses in on the Schrader valve stem as the switch is mounted, and allows the suction pressure inside the accumulator/drier canister to act on the switch. The electrical switch contacts are normally open when the suction pressure is approximately 151 kPa (22 psi). the contacts close when the suction pressure rises to approximately 410 kPa (45 psi) or above.

Temperatures below 9°C(48°F) will also open the clutch cycling pressure switch contacts due to the pressure/temperature relationship of the refrigerant in the system. The electrical switch contacts control the electrical circuit to the compressor magnetic clutch coil. When the switch contacts are closed, the magnetic clutch coil is energized and the A/C clutch is engaged to drive the compressor. When the switch contacts are open, the compressor magnetic clutch coil is de-energized, the A/C clutch is disengaged, and the compressor does not operate. The clutch cycling pressure switch, when functioning properly, will control the evaporator core pressure at a point where the plate/fin surface temperature will be maintained slightly above freezing. This prevents evaporator icing and the blockage of airflow.