I cover all this in my book. Here's a nutshell of the issues.
--The a/c in German cars was usually marginal with R12, and will be worse if it's converted to R134a without other modifications.
--Those other modifications are a rotary-style compressor (Sanden 508 clone) and the largest parallel-flow condenser that'll fit in the nose, and the largest fan that'll fit on the condenser.
--You'll also replace the receiver-drier, as you do anytime an a/c system is opened up.
--The odds that all of the original hoses are still good is small, so count on having new custom hoses built. When you do that, you can change as many of the hose ends as possible from flare fittings to o-ring fittings, as they seal better. The ones to the evaporator assembly will need to remain flare fittings.
--The lubricating oils used by R12 and R134a are different, so the remaining parts of the system (the evaporator assembly) will need to be flushed out. Unfortunately, since the expansion valve is inside the evaporator assembly, the best way to do that is to remove the console and evaporator assembly, take it apart, and flush it. That is a lot of work. You can try to "pop-flush" it without removing it and hope you get all the oil out.
--Yes, there are other refrigerants than R134a and R12, but the problem is that almost no shops work with them. So if you choose to use an alternative refrigerant, even an EPA-approved alternative refrigerant, you will always need to be the one servicing your a/c system. For this reason, most people gravitate toward R134a, and with the modifications described above, find it acceptable.
Here's a link to the book on Amazon:
--Rob
