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Differential pressures not changing

fdryer on Sat June 18, 2011 10:15 PM User is offline

Year: 1997
Make: Volkswagon
Model: Jetta GL
Engine Size: 1.9L
Refrigerant Type: R134a
Ambient Temp: 76F
Pressure Low: 47
Pressure High: 105
Country of Origin: United States

Placed gauges on this car and raised rpm to 1500; pressures never changed. Added one can of R134a, low side remained while high side barely went up above 110psi. This was refilled only once two years ago. Compressor or txv? No leaks anywhere.

bromodragonfly on Sat June 18, 2011 10:24 PM User is offlineView users profile

Is the AC not blowing cold at all? Compressor shaft seal will leak over time, not much you can do about that.

I'm not too familiar with automotive AC compressors, but I've been told some are designed with larger clearance volume and smaller valves, which will provide sufficient compression at lower RPMS, but keep head pressures steady on rise of RPMS.

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Edited: Sat June 18, 2011 at 11:28 PM by bromodragonfly

TRB on Sat June 18, 2011 11:08 PM User is offlineView users profile

Sanden variable compressor. No or little pressure with a full charge. Bad control vale is something to consider.

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bromodragonfly on Sat June 18, 2011 11:44 PM User is offlineView users profile

Found a nice little PDF that gave me a much clearer image of how a variable displacement compressor works. Hopefully it'll be interesting to others too:

http://www.polarbearinc.com/Articles/Variable%20Displacement%20Compressor.pdf

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There is no knowledge that is not power

fdryer on Sun June 19, 2011 1:41 AM User is offline

I'm aware of variable displacement compressors. Thanks. On another forum a member posted a final outcome to an issue that was traced to a failed txv but much higher than normal pressures were found on the high side (450psi). The service manual specs for the Jetta calls for 17psi low, 175 high @1500rpm. I'm not familiar with faulty txv's and their characteristics when they fail.

mk378 on Sun June 19, 2011 7:24 AM User is offline

More likely, it's the compressor. Every summer two or three VW owners come here in the same situation. When you know it's fully charged but revving the engine doesn't change the pressure at all, you have a compressor that is refusing to pump.

bromodragonfly on Sun June 19, 2011 1:26 PM User is offlineView users profile

I haven't seen too many TXV's that have outright failed. But when they do, the most common problem is that they fail to feed enough liquid into the evaporator. This could be due to something foreign blocking the internal port, or sometimes the power element loses its charge, which will result in zero bulb pressure and a closed valve. Both these situations will result in a backup of liquid at the TXV inlet, which will drop the low side pressure. The backup, and the fact that gas coming back to the compressor is superheated more than normal, will result in higher head pressures.

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There is no knowledge that is not power

fasto on Mon June 20, 2011 2:48 PM User is offline

Quote
Originally posted by: bromodragonfly
I haven't seen too many TXV's that have outright failed.

The VW TXV's usually fail by corrosion of a steel cover plate on the side of the TXV. This shows as leaking oil & dye, of course. Two or so years ago there was a bad batch of aftermarket VW TXV's (I bought 3 of them from 3 different suppliers). These would cause skyrocketing high side pressure because they'd close completely.

None of that fits the OP's problem; he's got a bad control valve in the SD7V16 compressor. Sometimes it can be temporarily fixed by gentle tapping on the compressor body. When that doesn't work I suggest a new compressor from the site sponsor.

fdryer on Thu July 28, 2011 4:54 PM User is offline

To close this thread I spoke to the owner of the VW and he decided not to invest any more money into this repair. I presented several options but with sound advice from here I relayed this faulty compressor replacement to him. His choice to not proceed any further. Thank you all for your help.

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