Low Pressure Sensor / Switch: Description and Operation

FIN SENSING CYCLING CLUTCH SWITCH
The fin sensing cycling clutch switch is used on this model to signal the Powertrain Control Module (PCM) to cycle the compressor clutch on and off in order to control the evaporator temperature. Controlling the evaporator temperature prevents condensate water on the evaporator fins from freezing and obstructing air conditioning system air flow.

Fin Sensing Cycling Clutch Switch:

The fin sensing cycling clutch switch consists of a probe and a switch unit. The probe, which is a Negative Temperature Coefficient (NTC) thermistor in a capillary tube, is inserted between the front evaporator coil fins to monitor evaporator temperature. The switch unit contains internal control logic that monitors the input from the probe in order to switch an internal transistor that controls the output to the PCM.

When the fin sensing cycling clutch switch sees a temperature input below about 1.6 °C (35 °F), it signals the PCM to cycle the compressor off. When the temperature input reaches above about 3.9 °C (39 °F), the switch signals the PCM to cycle the compressor back on.

The fin sensing cycling clutch switch can be accessed for service by removing the screws and care- fully lifting the front of the top cover up from the heater-A/C housing. The fin sensing cycling clutch switch is a sealed unit and cannot be adjusted or repaired. If faulty or damaged, the switch must be replaced.

LOW PRESSURE CUT-OFF SWITCH
The low pressure cut-off switch is located on the suction line jumper near the filter-drier in the engine compartment. The switch is screwed onto a fitting on the suction line jumper that contains a Schrader-type valve, which allows the switch to be serviced without discharging the refrigerant system. The suction line jumper fitting is equipped with an O-ring to seal the switch connection.

The low pressure cut-off switch is connected in series electrically between the fin sensing cycling clutch switch and the Powertrain Control Module (PCM). The switch contacts open and close causing the PCM to turn the compressor clutch on and off. This helps to regulate the refrigerant system pressure and protects the compressor from damage due to loss of refrigerant and refrigerant oil.

The low pressure cut-off switch contacts are open when the suction pressure is about 48 - 90 kPa (7 - 13 psi) or lower. The switch contacts will close when the suction pressure rises to about 103 - 172 kPa (15 - 25 psi) or above. Lower ambient temperatures, below about -1 °C (30 °F), will also cause the switch contacts to open. This is due to the pressure/temperature relationship of the refrigerant in the system.

The low pressure cut-off switch is a factory-calibrated unit. It cannot be adjusted or repaired and, if faulty or damaged, it must be replaced.