Re: A/C compressor runs constantly

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Posted by Frankenbiker ( on 22:17:58 08/02/07

In Reply to: A/C compressor runs constantly posted by kcburb

: First question - should it be constantly running? Second question - if it shouldn't be - could I have too much freon??
: Thanks in advance for your help!!!!

How it runs/cycles is entirely dependent on environmental conditions -- temperate early evening temps (after the day's heat has broken) are often good conditions for a compressor to run constantly; the temp isn't hot enough to run the high-side pressures up (to hit the high-pressure cut-out) but warm enough to keep the low-side from dropping past the cut-out point. Environmental variables are fairly big ones in automotive A/C systems.

This is, of course, assuming at idle. Depending on your engine speed at freeway speeds, if you've got enough freon, you may be able to hear the compressor cycling (My R-4 was noisy enough to hear over the radio), especially if you're turning 2000 RPM (or more) at speed. If you're turning down in the 1500-1700 RPM range, you may not be able to hear it cycle, because the compressor flow rate may not have come up enough to get you into a cycling operating regimen.

One note: The '91's should have still been using R-12 refrigerant (i.e. the Freon(tm) brand name) from the factory. If you've converted to R-134a, the factory low-side pressure switch is NOT the correct one to be using. R-12 and R-134a have different low-side pressure points. You'll have to source one from a '94 or later truck to get the right one. They'll look the same, but they'll cut out at different pressures. This may help your cycling problem.

Also note that for the 94-96 (as I remember) model year trucks, they run amazing high-side pressure values; The Haynes manual I looked at showed well over 300 PSI for the high-side on R134A. I think this was before they switched to the REEELY expensive Sanden-style variable-displacement compressors.

(sorry if this got a little verbose. :-) )


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