Volkswagen: Air conditioning problem, jetta 1, receiver dryer

Question
Ok, this may be a little out of your expertise, but here goes.
The car is a 91 Jetta 1.8 8-valve 5 speed.
The a/c has been converted to 134a by the previous owner. When I got it last year, it would cool very well untill the engine got up to operating temp, then the compressor cuts off. I added a little 134a, and not immediatly, but several days later, it started cooling all the time! This spring, it does the same as before, so I added a very litle 134, as the pressure on the low side indicated it was full, and it still cuts off at operating temp. I have tried driving it till the engine is at operating temp, then turning on the a/c. It cools great for a couple of minutes, then compressor cuts off. I am getting the feeling it is a sensor somewhere, but any advice is appreciated.
Thanks,
Norm

Answer
Hi Norm,

This is totally indicative of moisture in the lines.  When you change over the system, the receiver/dryer must be replaced.  I would check to make sure yours was.

What I believe to be happening is that when the A/C refrigerant gets hot, the moisture in the lines turns to steam.

Since the H20 steam is not as thick as the water, it gives the compressor the 'empty' signal and cuts it out.

The best way to fix this is to evacuate the system, and have all the moisture removed, along with replacing the receiver/dryer.

Best of Luck and Stay Cool!

Ron