And do you have the AC off but with it in Defrost??? If so most settings run the AC to help keep the humidity down even in the winter months, with the heat on.
Also please include Year, make model as well.
When the pump is running you should have no less than 30PSI and no more than 35PSI. but it sounds like you are just low on Freon, and with those trucks its not uncommon. I have worked on at least a dozen with this issue, and it comes down to adding some once a year. Since it is rather cheap compared to going thru the entire system trying ti find a very very small leak.
Just what other members have said. Also it will turn on if the heat is on as well as defrost being on. My dads dodge truck always does this and his ac system is discharged and not in use anymore and when the defroster is on and/or the heat it cycles on and off every 30sec. or so.
Just for the sake of information the way the system works is there is a pressure switch just downstream of the cold coil in the big A/C line that returns to the compressor. You will usually find it within inches of where that line comes out of the housing at the firewall. The temperature of the unit is controlled by the clutch cycling and that switch is what cycles the clutch. As the temperature of the coil drops the outlet pressure also drops. when it gets down to 15-20 psi it will cycle the clutch off to keep the unit from freezing up. as the pressure comes back up the clutch will cycle back on. This balancing act works well until the charge gets low, with a low charge there is not enough freon on the inlet to maintain the outlet pressure and you get the short cycling issue. With regards to the charge forget the gauges, just go down to walmart (the cheapest place I have found to buy it) and get a can of 134 with the stop leak in it. Dont laugh, I had the same reaction the first time I tried it on my old S15. It has something in it that causes all the rubber to swell, the o rings on all the connections, the hose ends etc. A good share of the time this will stop the small leaks, or at least slow them down. Also you might try checking all the connections to see if there is one that is loose. While it is rare I have found it, I think maybe two or three times in about 40 years of working on A/C,s. A CAUTION if you do try to tighten up any connection double wrench it. all the tubing on the units is aluminum and if you try to tighten anything without supporting both sides it gets expensive. Good luck, The Grump
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