Non Delorean AC question. Compressor runs backwards when clutch disengages

[Jason]

While trying to track down a leak in the AC system in my Toyota Sera, I noticed that when the AC clutch disengages the compressor slowly spin backwards for 5 to 10 seconds. Does anyone know if this is normal or not for 1990s Toyota compressors? If this were on the DeLorean I would assume the valves in the back of the compressor were bad and that its time for a new compressor.

[Josh]

I think its merely an optical illusion of the compressor slowly spinning to a stop. i would not worry about it.

https://io9.gizmodo.com/why-do-wheels-sometimes-appear-to-spin-backwards-1593807400

[Jason]

I think its merely an optical illusion of the compressor slowly spinning to a stop. i would not worry about it.

https://io9.gizmodo.com/why-do-wheels-sometimes-appear-to-spin-backwards-1593807400

In this case its not an illusion. It's definitely spinning backwards.

[Rich]

In this case its not an illusion. It's definitely spinning backwards.

I found a few other such reports on the web.

See linked thread for one of them - Suzuki Sidekick compressor runs backward after disengaging (https://www.suzuki-forums.com/suzuki-sidekick-escudo-vitara-geo-x/51909-ac-compressor-spinning-backwards-when-vehicle.html)

Posts #5 and #8 mention possible cause.

[SS Spoiler]

line pressure backfeeding?

[David T]

Residual pressure should not make the compressor rotate backwards. The check valves inside should prevent that. It could be that they leak or it could be peculiar to just that model. In any case it is not a good thing because there could be liquid and liquid should not go through the compressor. If it makes knocking noises when you start it up you are doing damage to it from the liquid in the compressor.

[Michael]

Did you mean to say the compressor itself is turning backwards or are ypu seeing the clutch plate? The clutch plate which is not engaged? Sure on some compressors you can move the clutch by hand if it is not energized but maybe you have one that's just a little free. As long as it works I would not worry myself with it.

[Ron]

Did you mean to say the compressor itself is turning backwards or are ypu seeing the clutch plate? The clutch plate which is not engaged? Sure on some compressors you can move the clutch by hand if it is not energized but maybe you have one that's just a little free. As long as it works I would not worry myself with it.The clutch plate mounts to the compressor (shaft), solid. So, they always turn the same way...

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It's probably a weak valve allowing residual pressure to drive the compressor until it equalizes, as suggested.
FWIW, I have seen this last a long time...but note it also causes a reduction in performance.

[Jason]

Did you mean to say the compressor itself is turning backwards or are ypu seeing the clutch plate? The clutch plate which is not engaged? Sure on some compressors you can move the clutch by hand if it is not energized but maybe you have one that's just a little free. As long as it works I would not worry myself with it.

It's the compressor shaft and clutch assembly. From the way it moves and watching the high and low side pressures its pretty clear it's back pressure pushing the compressor backwards.

The reason I'm trying to figure out if this is normal is that I have a slow leak I'm having trouble finding. If I need a new compressor anyway then I might as well replace the compressor and see if the leak goes away. Unfortunately finding a new compressor looks like its going to be a major endeavor, as the part isn't available anymore. I'll probably have to either rebuild it or have custom hoses made to connect to a different compressor. If spinning backwards is normal than I'll concentrate on finding the leak. I asked on a Toyota Sera group, but no one there seems to know.

I've got UV dye in the system, and have also been using a sniffer probe to look for the leak, but so far no luck. I also tried soapy water on all the fittings, the compressor, and the condenser.

I'm getting close to giving up, which leads to the next question. Does anyone know of a shop in the Philadelphia PA/Trenton NJ area that's good at doing AC work on unusual cars?

[Bitsyncmaster]

I would think maybe a little reverse movement would be normal because the pistons are under pressure so a little backwards movement would be normal when the clutch is released.

[David T]

You can always send the compressor to Classic Air in Florida to get it rebuilt. Any internal leakage will not show up externally with UV. Internal leakage will definitely have an effect on the performance of the compressor. Leaky valves are no good but have nothing to do with loss of refrigerant. Loss of refrigerant is a leaky component, leaky seal, or leaky hose.

[Jason]

https://www.youtube.com/watch?v=v0UjpGcJyww

It's hard to tell because of the camera frame rate, but the crankshaft and accessories are normally spinning clockwise.

[DMC-Ron]

Well, if that is not the frame rate "wagon wheel" illusion, then it's pretty odd that it rotates in reverse as much as it does. There should be check valves in the compressor that prevent back-flow. I guess one thing that makes the DeLorean AC different from "traditional" auto AC is the amount of refrigerant piping. The extra tubing would increase the available compressed gas that can bleed back thru the compressor. Still, the compressor valves must be leaking for this to happen.

Ron