I am back from the shop and here is what I found. For the record. My AC system is not a VW system. It is an aftermarket unit from Vintage Air for custom cars and hot rods. Their specs are 6 - 12 psi steady state at 1500 rpm on the suction side. High side should be 160 - 250 psi depending on ambient and condensor efficency. The vintage air expansion valve is designed to operate at the lower pressures which would have the effect of lowering the evaportator core temp. So better cooling. There is a relationship between the low side pressure and the evaporator core temp. The system has a conventional ac thermostat with a pig tail into the evaporator core that shuts the compressor off before the evap ices up. The expansion valve also has a pig tail attached to the low side of the evap line externally to regulate the opening in the expansion valve.
I could not get below 25 psi even with recovering half of the freon out of the system. I contacted Vintage Air Tech support and we were going over the usual problems when I mentioned the Compressor was not a fixed displacement system. The tech advised their a/c systems will not work with variable displacement compressors. The TDI compressor seems to keep compensating for what it thinks the low side should be.
As luck would have it, we had no 7/16ths bolts in the shop. I decided to chuck the valve mentioned in the article into a lathe, cut it down removed the innards tapped the inner bore and installed a 1/4 20 set screw to plug it. To make it solid. It is machined with a shoulder to center the highside reed plate and the reed plate retainer. We also took the time to examine the compressor. There is another peice in the very center of the compressor that seems to be a part of the displacement valve that is not mentioned in the article. So, I put it all back together with my disabled valve. It was a bit difficult to tighten the center bolt because it holds the high side reed plate and the retainer. Nothing to keep it from spinning as you tighten it. After a couple tries, I did get it centered and tight. For now, I reused the gaskets.
I pulled a vacumn on the system for about an hour and charged the system with the recommended 2 pounds of r134. There is definitely an improvement in the pressures. I can now get down to 10 psi on the low side and was seeing 40 degress at the center vents. Not sure why, but the high side was creeping up to the point the the radiator fan went to the second speed, which means high side is above 260 psi. I let a little freon out to drop the high side and then noticed that when the radiator is on the second speed, it is back feeding power to the relay that turns the compressor on an off and the 1st speed fan. That bypasses the compressor thermostat. So, I may need to add a diode or an additional relay to isolate the two. Seemed like a good idea when I wired it. Did not take into consideration power back feeding.
Andy, I am still not exactly 100% sure if the TDI compressor is still compatible even with the valve disabled. Doing my reading, The Sanden 508 and the later Sanden SD-709 for r12 and the SD-7H15 for r134 all have the same external dimensions. Some have ears and some are bolt through. Nostalgic Air in Tampa lists a lot of different head plate options. I emailed them to ask if the had a compressor that might be adapted. That was late Friday. Supposedly the SD 7 series fixed rate compressors are an improvement over originial SD 508. I am going to sort out my electical problem tomorrow and also bypass the heater core to be sure that is not causing the higher than expected vent temps.
Andy, thank you for checking the compressor ear dimensions. We had a bad TDI compressor at the shop. I pulled it apart before working on mine. Sort of a trial run. So I have the mounting bushings to adapt to a replacement compressor and if you have a problem, a spare TDI rear compressor cover plate. If I can't get my pressures to Vintage Air specs, I may go ahead and adapt the SD-7H15 to my engine. Since you are using the Vanagon AC system, you may not run into the same pressure problem I am seeing. My back is killing me. Time to hit the showers. p.s. DanG, you were a tremendous amount of help and if we make it to TDI fest and you are there, I owe you a drink. mark