AC Compressor acting up?

[GrndNatnl]

I have another question. Since this is the first time I've experienced running the car in cold weather, I'm not sure if this is normal or not.

I fired the GN up today, and its about 45 degrees or so (not too bad), and the heat was already on and set at defrost from the last time I was in it. (Never put it back to off position) While I was letting it warm up, the AC compressor kept engaging and disengaging. I took it for a quick spin, and when I came back and turned it off (with the heat still on) I heard a hissing from the AC unit. I figured it was the heater core at first. I started it up, let it run 30 seconds, cut it off, same thing. So then I ran it again, only this time I turned the heat OFF, let it idle a few secs, turned it off. No hissing, and the compressor never engaged. Then I did it the other way again one last time, sure enough, same results as before. When I walked around and touched the AC accumulator, it was ice cold, and one of the smaller lines that runs in front of the radiator was cold, and you could feel it vibrating a little with the hissing as it gradually went away.

Why is the AC compressor kicking on/off and pumping freon when the HEAT is turned on? I find this quite odd. Some sort of electrical or relay problem? This cant possibly be normal.

 

[Roc87]

Anytime the a/c controls are put on defrost, the compressor kicks on, it helps get rid of humidity in the car.

 

[GrndNatnl]

omg... You've got to be kidding me. I should have known that. I never had a car that did that though. Usually the defrost setting just directs the regular hot air at the windshield. How exactly does the AC system remove humidity?

 

[Roc87]

Not too sure how it works, but my Wifes accord doesn't do it (only car that I've seen with a/c stock and it doesn't kick on the compressor with the defrost) and for the defrost to work at all, you have to manually push the "A/C button".

 

[GrndNatnl]

Makes sense I guess. I just dont get why it kicks on and off so freaquently when its warming up. It clicks on, runs about 3-4 seconds, kicks off. Kicks on, runs about 8 seconds, kicks off. Kicks on again for about HALF a second, then right back off. Just seems odd. Could be the compressor going. The AC worked great during the hot days, but the compressor is very old. Could that be a reason its on & off so much? I would think it would stay on for longer increments.

I just want to fully understand it, or as much as possible to avoid freaking out thinking I have some sort of problem with the heating/ac.

 

[scottyb]

Sounds to me like it is functioning properly. It will cycle more in cold weather because it doesn't have much work to do. I'm not exactly sure how it decides this, maybe the pressure switch and the fact that it doesn't absorb as much heat causes pressure to drop slower than summertime. (maybe other way around?)

The hissing is the oriface tube and it always does this, you just can't hear it when the motor is on. It is located where the freon enters the car (evaporator). I think the compressed freon turns to gas as it passes thru, and gives it the ability to absorb heat. It will hiss until the freon pressure is equalized.

 

[scottyb]

BTW, I don't think it matters now, but if it cycles like that on a hot day with AC on max, then you have a problem.

Cycling like that really is not good for the compressor. Maybe an expert will chime in on if it is normal to cycle that fast on cold days.

Turn it on max AC, fan on high to see if it runs longer, just for grins.

 

[FlyinGN]

Matt, ac will remove moisture from cabin. Thats the main reason for ac. To supply cool DRY air. Dry air defrosts MUCH faster.

 

[GrndNatnl]

Yeah Frank, but when the heat is on? Should be hot dry air?

 

[FlyinGN]

nope Matt. The ac compressor should only come on in these modes
* ac
* max ac
* bi level
* def

all other modes no ac compressor should be engaged

 

[GrndNatnl]

Gotcha. Thanks for explaining this. I'll check it in the different modes and make sure it functions properly. I just wanted a better understanding, so I dont misdiagnose something as a problem. I've never had a car that had the same method of drying the air, so I was curious as to what the AC was doing.

 

[FlyinGN]

actually Matt most cars do the same thing. My father in laws 87 Olds, my 90 Olds, my wifes old 95 Neon, her new Suzuki XL7, my father in laws new Sebring. They all have compressors that come on when you put the defroster on.