You can run the evaporator several degrees below freezing if your car is in a hot climate. The warm air across the fins will keep the evap from freezing. Surely, you've been in a car with extremely cold vents haven't you? It is my belief that foreign cars and maybe ford incorporate the outside temp as a switch/relay that adjusts the evaporator temp sensor to cut the compressor out at a lower, below freezing temperature in the evaporator.

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JHaburay

1972 280se 4.5 "princess"
1986 2.3-16v first 100 (099) USA