I noticed that the AC on our 91 745t seems to be short cycling. With the AC on it runs the compressor about five seconds before shutting off and then is off for a couple of seconds before coming back on. It seems to be cooling fine but it isn't all that hot and humid yet in the Ohio Valley

This system was successfully converted to 134a four or five years ago and has always done a good job.

Any thoughts? It seems that cycling that often will put additional wear on the compressor clutch.

Randy

Hi Randy,

I'm a rookie at A/C but looked at the wiring for the 89 740.


There's a delay relay behind the glovebox and a pressure switch on the receiver drier, and a solenoid valve.

Maybe that pressure switch isn't happy or you have a ratty relay?

Bill

Thanks, I will check into that possibility if I find the charge is correct this morning.

Randy

Classic symptom. It is bit low on R134. Not bad after 4 yrs. I susually have to top up on both my Volvos every year or 2.

My '93 940 has factory 134a, which I got working 4 years ago (lines, condensor, accumulator, O-valve etc.). For the past 2 years the clutch hasn't cycled at all -- compressor runs all the time (I wired up a tell-tale light).

Local shop says the amount of R134 is correct, but Low side pressure never gets down to the switch opening pressure (23±psi).

No complaints inside -- the vent temp can get down to 37-37°F at 55 mph, with blower on 3. At that point I usually raise the temp control up a bit.

My hunch is the the (original) compressor is about played out. Is this a possibility, given my symptoms?

I know this isn't much to go on, just trolling for an informed opinion.

My '93 940 has factory 134a, which I got working 4 years ago (lines, condensor, accumulator, O-valve etc.). For the past 2 years the clutch hasn't cycled at all -- compressor runs all the time (I wired up a tell-tale light).

Local shop says the amount of R134 is correct, but Low side pressure never gets down to the switch opening pressure (23±psi).

No complaints inside -- the vent temp can get down to 37-37°F at 55 mph, with blower on 3. At that point I usually raise the temp control up a bit.

My hunch is the the (original) compressor is about played out. Is this a possibility, given my symptoms?

I know this isn't much to go on, just trolling for an informed opinion.

The compressor doesn't cycle much on my cars unless the system is low or when used for defogging in cool weather.Here in the south,in the summer,the compressor is engaged 100% of the time.

Bill

Thanks Bill,

Maybe it's no big deal, but it did cycle during the first 2 seasons after repair. I could feel or sense a difference in engine sound when it cut in.

I may get one of those $19.95 gauge + can of R134 combos at W-M, just to see the pressure if nothing else. I know the Low Side was too high 2 years ago, but playing ± with amount of R134 had little effect.

Bruce

I am sorry I did not mean to mislead you on the time span of top ups. I have had to add 134a twice to this car, but both times it was indicated by not cooling properly.

Thanks for your reply; I will check out the pressure and correct the level if needed.

Randy