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VW AC not working - confusing pressures

TIMSCH on Mon May 03, 2010 3:55 PM User is offline

Year: 96
Make: vw
Model: passat
Engine Size: 1.9
Refrigerant Type: 134a
Ambient Temp: 88
Pressure Low: 25
Pressure High: 25

Bought the car in November, so don't know it's history. AC seems to engage initially and blows cooler than normal air for ~30 seconds. Gauges on high & low sides showed ~60 psi with engine off. Upon starting engine & AC/fan on max, pressures drop to 25 psi on both high & low. My research (browsing forums) indicated that it could be the expansion valve, low charge, or the control valve in the compressor.

My first step was to add some r134a to the low-side port. I added one can - no change in pressure (hmmm....interesting). Added a second - again, 25 psi on both high & low sides (scratching head). No cool air from the vents.

After adding the refrigerant, I did notice that both the high & low lines near the fill/bleed ports were sweating & cool.

How can I add 2 cans of refrigerant & see no change in pressure on either side?

Before taking the next step, I'd like some feedback from someone more knowledgeable about this. Thanks.

mk378 on Mon May 03, 2010 4:09 PM User is offline

Clearly with only 60 psi static pressure, the system was about empty when you started. It should be checked for leaks. You are not measuring the high side properly, or there is a restriction somewhere between the compressor and the high side port. If the lines to the receiver drier are warm on the inlet and cold on the outlet, it is blocked. The lines should not get cold until after the TXV.

This car has a variable compressor which means the low side will stay at 25 over a wide range of operating conditions both normal and abnormal. There is an internal regulator trying to keep it at 25 by changing the compressor displacement. Charging to specified weight is essential for variable compressor systems, and optimal on others.

iceman2555 on Mon May 03, 2010 4:37 PM User is offlineView users profile

From the posted pressures and reported static pressures, I wonder if there is not an issue with the operation of your gauges. At 88 degrees ambient, the static pressures should have been higher than 60 psi. The posted equalization pressures would normally indicated a failed compressor.....variable or not....however,this pressure/temperature reading does not reflect the posted ambient temp nor the possible elevated temperature of the engine compartment.
As stated by mk378m it is essential that these system be properly charged and the most efficient method is using the correct equipment....not merely a set of gauges and cans of refrigerant.
Either service the system correctly or have someone service the system and report the findings.
Good luck!!!

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

TIMSCH on Mon May 03, 2010 5:29 PM User is offline

Thinking about it, I may be mis-remembering the static pressure. 80 psi on both sides may have been the actual static pressure. I will recheck it again this evening and report findings. This being an old car and not knowing if it had ever been recharged, I did not want to assume a leak that was significant enough to warrant breaking into the system. Not knowing much about AC (even less about these variable compression systems), I was trying to make a fairly safe first step. Shows what I know....

TIMSCH on Tue May 04, 2010 9:02 AM User is offline

While driving home yesterday, I turned on the AC, and much to my surprise it blew cold air for several minutes until I turned the car off at home. I put the gauges on the system again after a couple of hours of sitting and the static pressures were 150 psi (engine components still very warm). I turned on the engine & AC and my low-side was ~33 psi & high side fluctuated from 200-250. Prior to adding 134 the other day, I had added a shot of leak dye, so I will check for leaks tonight after running AC for 20 miles home today.

With this VW AC system, is it common to add refrigerant and see no change in pressures at that time, only to have it work itself through the system in due time?

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