To prevent system freeze-up, the fresh air blower and the radiator come on at low speed whenever the A/C is turned on. A low-pressure switch prevents engagement of the compressor clutch if the R-12 charge is too low. A high-pressure switch runs the radiator cooling fan at high speed to cool the R-12 if the pressure in the system is excessive. A blow-out plug in the receiver/drier releases if system pressure exceeds approximately 500 psi. On diesel-engine cars, a thermoswitch turns the compressor off when the engine coolant temperature rises above 248°F (120°C).