AC compressor pulley turns, key out not running

[flybyu]

2006 3500 5.9 , Ignition is off and motor is not running.. when I look underneath at AC compressor, the pulley is turning. Nothing is powered so how is this possible.

 

[Yo Hoot]

All I can think of is it's being driven by the refrigerant in reverse. Something wrong with the AC system. Check valve or something.

 

[Tuesdak]

2006 3500 5.9 , Ignition is off and motor is not running.. when I look underneath at AC compressor, the pulley is turning. Nothing is powered so how is this possible.

Assumed the A/C was running prior to you looking even in Defrost. Slowly turning? Less than one revolution is somewhat normal as one cylinder's volume of compressed gas can push the compressor backwards. If it's making more than one revolution you have bad or leaking valves in the compressor. Generally broken valves or held open from debris. @Yo Hoot Correct, the only check valves in the A/C system are the valves in a compressor.

I would also suggest because the engine cools off last and the compressor being a low point in the system collects liquid refrigerant because of this (Known as liquid slugging and the "THUMP!" you get sometimes when turning on the A/C) : if you heat up the compressor (engine heat running) without having run the A/C: the refrigerant boiling off in the compressor can cause it to turn as well although I have never seen it myself.

 

[flybyu]

Assumed the A/C was running prior to you looking even in Defrost. Slowly turning? Less than one revolution is somewhat normal as one cylinder's volume of compressed gas can push the compressor backwards. If it's making more than one revolution you have bad or leaking valves in the compressor. Generally broken valves or held open from debris. @Yo Hoot Correct, the only check valves in the A/C system are the valves in a compressor.

I would also suggest because the engine cools off last and the compressor being a low point in the system collects liquid refrigerant because of this (Known as liquid slugging and the "THUMP!" you get sometimes when turning on the A/C) : if you heat up the compressor (engine heat running) without having run the A/C: the refrigerant boiling off in the compressor can cause it to turn as well although I have never seen it myself.

I live in California and we may have short periods off high heat so like seven years ago AC stopped blowing cool. I tried to recharge and never followed up on it. A month ago my batteries started showing signs of needing replacing. how is underneath the compressor? The truck wasn’t running the keys running or not in the ignition and I was looking at the compressor and I thought my eyes were just playing games on me. I had to video it and I could see it turning real slow. I think I even tried to pull the fuse to it. So I threw in some new batteries and I just looked underneath there and I can see it turning now but you can really see it turn moving pretty good.

 

[Crunch]

I had to video it and I could see it turning real slow.

Well, #1 on the list is post that video! A picture is worth a thousand words, but a video is worth 1,538 words...

 

[Tuesdak]

So the A/C quit 7 years ago, you added an unknown charge and have not used it since?
With the engine running I would see if the A/C clutch is spinning. (No Cold Air may be a busted blend door.) If you have the A/C controls "off" and it's spinning with the engine running it would be like a Ghost of GM A/C controls esp 1995 that like to fail and kick the compressor on...

I assume you mean the clutch hub on the front of the compressor vs. belt pulley that has the fan belt on it?

If the above is correct: you don't use the A/C then it's the hot engine and then cooled off cold engine moving any gas or refrigerant around the A/C system. This can slowly turn the compressor.

Now I am curious what direction it's spinning? Backwards or the normal direction.

I assume the clutch hasn't broken and is driving the compressor with the engine on. I have had one of three drive tangs break and the A/C clutch was slipping but turning even when A/C was "off". It ground it up good.

End of the day if you don't use the A/C all you care about is the pulley bearing.