Low Pressure Sensor / Switch: Description and Operation

The A/C low pressure switch is a single pole, single throw, pressure actuated switch that is used to control evaporator freeze up.

The A/C low pressure switch is mounted on a fitting located on the top of the A/C accumulator. The A/C low pressure switch fitting on the accumulator is equipped with an O-ring and contains a Schrader-type valve, which allows the switch to be serviced without discharging the refrigerant system.

The A/C low pressure switch monitors the pressure of the refrigerant leaving the accumulator to the A/C compressor. When equipped with a diesel engine, the A/C low pressure switch is electrically connected in series with the A/C high pressure switch, between ground and the engine control module (ECM).

The contacts in the A/C low pressure switch open or close the path to ground, signaling the powertrain control module (PCM) or the ECM (depending on engine application) to turn the A/C compressor clutch on and off. This regulates the refrigerant system pressure and controls A/C evaporator temperature. Controlling the evaporator temperature prevents con-densate water on the evaporator fins from freezing and obstructing A/C system air flow.

The A/C low pressure switch contacts are open when the A/C suction line pressure is approximately 141 kPa (20.5 psi) or lower. The A/C low pressure switch contacts will close when the A/C suction line pressure rises to approximately 234 to 262 kPa (34 to 38 psi) or above. Lower ambient temperatures, below approximately -1° C (30° F), will also cause the A/C low pressure switch contacts to open. This is due to the pressure/temperature relationship of the refrigerant in the system.

The A/C low pressure switch is factory-calibrated and cannot be adjusted or repaired. If the A/C low pressure switch is faulty or damaged, it must be replaced.