My AC compressor clutch is engaging every 4-5 seconds whenever the selector is in either AC position, as it should, but also when in vent or heat. It does not engage in OFF or defrost. Does this seem normal to anyone else?
Thinking further, I remembered that the temp lever was in the cold position in all settings. I'll double check what happens with the temp set higher.

Anyone have an idea?
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Also, it doesn't matter where the temp is set.

It shouldn't run with HEAT or VENT, but it should run with DEFOG. Don't know what the problem would be though. Also, the compressor shouldn't cycle that often unless the outside temp is cool or cold. Probably low on freon.

So there is a lo pressure switch to maintain pressure at all times? I know there is also a hi pressure cut-off.

If the A/C clutch is cycling like that then you have low pressure in the refrigerrant lines.There is a low pressure switch in the receiver/drier to protect the compressor and the high pressure switch(if there is one)is in the compressor itself.As 86blackiroc stated the copressor should run in defrost setting also for humidity control.

I just finished checking things out. I am a propane tech with some AC experience. Lo side cycles from 20-50 psi. The guage set I was given has large schrader fittings on all hose ends and I don't have an adaptor for the small schrader on the hi side port. That sucks. I'm thinking I have a restriction now. I'll check with some friends for a vac pump and collector bottle before I evacuate. I can get the hose adaptor at our parts distributor. The lo pressure switch is the one away from the accumulator, right? I'm trying to relate to residential systems, with 3 years since I've had to play with this.

I have an 84 Trans Am doing the same thing. A/C was working last season, so probably a tiny leak. If I recharge with a can or R-134, how high should I bring up the pressure? According to the gauge that came with the R-134, 55 psi is the max safe range, but the compressor keeps cycling on and off every few seconds. I somehow recall hearing/reading somewhere that the R-4 to R134 switchovers in these vehicles require more pressure than many systems. Any truth to that? So bottom line, does anybody know what to charge pressure to at approx 80 degrees F ambient temperature? Thanks.

Probably need to add more r134, that is usually the case when the pump is frequently cycling.