A similar thing happened to my 300SD yesterday after I topped it off with R134a. At idle the AC was cool, not near as cold as I would like, but noticeably cooler than the outside air. I decided to drive the car around the parking lot for a few minutes to see if the AC would get cooler past idle. Well, it stayed the same until it stopped putting out cool air. Suddenly, I was getting ambient temp air. It wasn't hot like the heater was on but about the same as the outside temp.

I have yet to get cool air from the car again.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge