AC running not so cold

[Kent McLean]

When I bought my '94 100 S Avant, the AC worked fine. It wasn't ice cold,
but it was cold. Now, 3 years on, it doesn't blow cold, and summer is here.
My new-to-me '91 200 20V Avant, on the other hand, didn't blow cold or
even cool when I got it. So...

- Is there a good on-line tutorial for how automotive AC works?
- Does my '94 have a low-pressure valve for adding R134a? (I know
  some Audis only have a high-pressure valve.)
- I don't know that the '91 has been converted to R134a. Will a
  cheap R134a conversion kit let me use R134a? Or do I need to
  change things (hoses, compressor, etc.)?

The '94 is a non-automatic climate control. Yup, it has 3 big
old-fashioned knobs :) So debugging its system using the CC
won't work there.

Thanks,

-- 
Kent McLean
'91 200 TQA #1, for sale
'91 200 TQA #2, no name yet
'94 100 S Avant, "Moody"
'89 200 TQ, "Bad Puppy", up in smoke