Ac Question

[police interceptor]

98 Ford Crown Vic. I know there is a hot or high side and a cold/low side to a veh. ac system. (I'm not sure of the values), but I watched as the tech connected the gauges to the system and saw that the pressure would rise to 55, then the radiator electric cooling fan would come on, then the pressure would fall to 25 then the fan would cut off, then the pressure would immediately begin to rise to 55 again and the process started over again and again and again.

The ac is not cooling properly. Is the pressure rising because not enough air is passing over the coils to cool the hot/high side, then falling when the fan comes on and cools it?, and, is it normal for the pressure to flucuate so much?

Is the fan going on and off because of the pressure change in the AC system?, or is the pressure changing because the fan is coming on and going off?..or it is "what came first the chicken or the egg?"??

Bottom Line, IS THIS A FACTOR IN THE FACT THE AC IS NOT COOLING PROPERLY, AND WHAT NEEDS TO BE DONE TO FIX IT?

Thanks in advance!

[dxrflyboy]

What you describe is usually a symptom of an undercharged A/C system. The compressor will operate only if there is adequate system pressure. When the compressor engages, the cooling fan also comes on to ensure there is air flow across the condenser. If the low side pressure drops too low, the compressor disengages and the fan turns off. This will occur even if the system is fully charged, since the low side pressure will drop off when the refrigerant is very cold. But if there is insufficient refrigerant charge, the high side pressure will also be low and there will be inadequate cooling.

[Doug Rodrigues]

If you have gauges, your high side pressure should be at least 250 lbs. but not over 300. With R-134, I'd keep it at 250-260.