Refrigerant Pressure Sensor / Switch: Description and Operation

A/C Cycling Switch
The A/C cycling switch is mounted on a Schrader valve-type fitting on the top of the suction accumulator.
- A valve depressor, located inside the threaded end of the A/C cycling switch, presses in on the Schrader valve stem.
- This allows the suction pressure inside the suction accumulator to control the operation of the A/C cycling switch.
- The electrical switch contacts open when the suction pressure drops. The contacts close when the suction pressure rises.
- When the A/C cycling switch contacts close, the signal to energize the A/C clutch is sent to the constant control relay module.
- When the A/C cycling switch contacts open, the A/C clutch field coil is de-energized and compressor operation stops.
- The A/C cycling switch will control the A/C evaporator core pressure at a point where the plate/fin surface temperature will be maintained slightly above freezing.
- This prevents icing of the A/C evaporator core and blockage of air flow.
- It is not necessary to discharge the refrigerant system to remove the A/C cycling switch.

Dual Function Pressure Switch
The dual function pressure switch is used to interrupt A/C compressor operation in the event of high system discharge pressures.
- The dual function pressure switch is mounted on a Schrader valve-type fitting on the high pressure side of the condenser to evaporator tube.
- A valve depressor, located inside the threaded end of the dual function pressure switch, presses on the Schrader valve stem.
- This allows the dual function pressure switch to monitor the compressor discharge pressure.
- It is not necessary to discharge the refrigerant system to remove the dual function pressure switch.
- The dual function pressure switch has two sets of contacts. One electrical contact is normally closed.
- When the compressor discharge pressure rises, the switch contacts open, disengaging the A/C compressor. When the pressure drops, the contacts close to allow operation of the A/C compressor.

The switch contains a second set of electrical contacts used for high-speed fan control.
- When the compressor discharge pressure rises, the contacts close and engage the high-speed fan control. When the pressure drops, the contacts again open and the high speed fan control is disengaged.