Does the compressor kick in? You will hear and feel it, and you can see the clutch plate on the front of the clutch spinning when it is engaged -- it will be stationary when not.

Check the following:

fuse for climate control (don't remember which one) -- if it is bad or corroded, you won't get AC.

Make sure you don't have the button marked "EC" pushed (next to to the off button) -- this is climate control without the AC compressor.

If fuse is good and the compressor doesn't run, unplug the two connectors to the low pressure switch and short together. If compressor runs, it's low on freon. You can also use a gauge set to check pressure.

If the compressor comes on once, runs for a few second, then shuts off and won't run again until you shut the car off and start it again, the speed sensor in the compressor is bad OR you have a bad serpentine belt, the belt tensioner is broken, or there is something else making it slip.

From here, you will need more expertise -- the operation of the compressor is controlled by a complex relay (usually a Klima), and you need to know what signals should be where before you start replacing things.

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!