Refrigerant Pressure Sensor / Switch: Description and Operation

Fig 8 Suction Accumulator/Drier Dome Type:

DESCRIPTION
The clutch cycling pressure switch is mounted on a Schrader valve-type fitting on the top of the suction accumulator/drier assembly. A valve depressor, located inside the threaded end of the pressure switch, presses the Schrader valve stem down as the switch is mounted and allows the suction pressure inside the accumulator/drier canister to act on the switch.

OPERATION
The electrical switch contacts will open when the suction pressure drops to 22 to 28 psi on R-12 systems and 22-25 psi on R-134a systems. The contacts will close when the suction pressure increases to 40-47 psi on R-12 systems and 39-47.5 psi on R-134a systems.

Ambient temperatures below approximately 45-50° F during cold weather seasons will prevent the pressure switch contacts form closing. This is due to the pressure/temperature relationship of the refrigerant and the requirement of the system pressure to reach the pressure required psi to close the switch contacts. The switch contacts control the electrical circuit to the compressor magnetic clutch coil. When the switch contacts close, the signal to energize the A/C clutch is sent to the Constant Control Relay Module (CCRM). The CCRM then supplies the voltage to energize the magnetic clutch for compressor operation. When the pressure switch contacts open, the CCRM opens the clutch electrical circuit to de-energize the clutch and compressor operation stops. 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 which prevents evaporator icing and the blockage of airflow.