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trinary switch problem or something else?

ploughna on Sun July 12, 2009 12:51 PM User is offline

Year: 1979
Make: Fiat
Model: Spider
Ambient Temp: 75
Pressure Low: 40
Pressure High: 250


I had my newly installed A/C system evacuated and charged yesterday - it seems to be working well from a cooling standpoint, and compressor is cycling on and off correctly based on the thermostat setting. However, I seem to have a problem with the fan operation when the car is idling. The trinary switch which controls the fan relay has this spec:

Fan on at 227 PSIG, off at 185 PSIG.

As I watched the pressure on the gauges, I saw it come on at about 240psi, but it always goes off well before the pressure gets down to the 185psi spec. Initially it was going off at about 200-210, but then the on/off pressure came closer and closer together so that a while later it started going off with only the smallest pressure drop (5-10psi), so the fan would come on for 1-2 seconds, go off, pressure rises and it almost immediately comes on again. So, it cycles continuously at a 1-2 second frequency.

My instinct is to suspect the switch, but it's a new part and I've been around strange problems long enough to know that the obvious is not always the case! I also wanted to check with the experts here before going any further because if I replace the switch I'll incur the cost of evacuating and charging the system all over again

Best Regards


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