I'm trying to remember way back to when I owned a 123... Mine was the very common 1984 300d - 123.133.

It has a low pressure cut-out switch to disable the compressor at low ambient temperatures or in the case of refrigerant loss.

The electric fan was actually run by a freon temperature switch on the dryer, rather than a pressure switch. It activated whenever the temperature of the liquid freon in the dryer reached about 70C, as I recall. Really unusual way to do things, but I guess measuring the temperature is just as good as measuring the pressure, pv=nrt and all.

- JimY