Pressure Switches

LOW REFRIGERANT PRESSURE SWITCH
The low refrigerant pressure switch is located in the low pressure refrigerant line. When pressurized, the switch is closed and allows the compressor clutch relay to energize, feeding battery voltage to the compressor clutch coil. If the pressure drops below 10 ± 2 psi, the switch opens and cuts off voltage to the clutch. There is a circuit to the Air Conditioning Programmer (ACP) which monitors the state of the switch for diagnostic purposes.

HIGH REFRIGERANT PRESSURE SWITCH
The high refrigerant pressure switch is located in the compressor discharge line and monitors the compressor discharge pressure. This switch acts as a safeguard to prevent refrigerant "blow-off" from the compressor pressure relief valve and disengages the compressor in the case of refrigerant pressure above 2965 kPa (430 psi). In this function it supplements the A/C high side temperature sensor.

Should pressures reach that level, the switch will open cutting off the power supply to the relay and disengage the compressor clutch. As soon as the compressor shuts "OFF", the pressures will begin to drop. When they drop to 1379 Kpa (200 psi), the switch will close and allow the compressor to engage. Compressor cycling due to pressure switch cycling can occur under conditions such as high ambient temperature, high humidity, extended idle, slow vehicle speed or tailwind, and is considered normal. Should cycling occur without the above conditions, a blocked line or condenser is possible and appropriate service should be performed.