The refrigerant pressure switch is installed in the high pressure line of the vehicle's air conditioning system.

When the air conditioner is running, heat removed from the air is absorbed by the refrigerant (e.g., freon, R-12 and R-134a). As the refrigerant heats, pressure builds in the air conditioner high pressure line until the refrigerant pressure reaches the switch's set point. The switch then sends a signal to the solenoid valve to engage the fan drive. The fan pulls air through the air conditioner condenser coils, which cools the condenser/refrigerant and reduces refrigerant pressure.

NOTE:
Air conditioning refrigerant pressure switches are available as either normally-open or normally-closed.