If the AC compressor is switched on without freon

Should Auto Air Conditioner Run in Winter?

All cars and trucks since at least the 1970s turn on the air conditioning compressor when the defrost mode is selected to remove moisture in the passenger compartment. A / C oil is not hydroscopic, the only fluid I know of is brake fluid that absorbs water. Therefore, manufacturers recommend changing it or flushing it.

PS The only thing that would prevent this is a low level of freon. In this case, the low pressure cut-off switch prevents the compressor from running to prevent system damage. Jim

Jimmy Fix-it

Note: Not all vehicles with air conditioning have a shutdown switch to prevent the compressor from running with low refrigerant pressure.


Not all cars turn on the air conditioning when the defrost is on. My 2003 European Vauxhall MPV didn't. This is true even for buses, especially if they have just a single, large displacement compressor designed to cool the entire bus: you have to turn on the entire bus AC system to activate the driver loop, and this means that the alternating current is switched on also for the passengers, otherwise it is only thawed with heated air. Newer vehicles with internally controlled variable displacement compressors rarely have an LP sensor or switch. This is because the stroke of the compressor is controlled by the LP pressure.