There is a Hall effect pickup on the back of the compressor that generates a speed signal for the climate control.
Either the belt is slipping (check condition -- if a lot of the rubber ribbing on the inside is missing, it will slip on the AC compressor) or the compressor clutch is bad or dirty. The tensioner can be bad, too, but that usually wont' result in a belt slipping without a screech or whine. Belt usually goes to the bad place pretty fast, too!
It is possible that the speed sensor is bad, but that is usually only a result of a worn out compressor or a bad rebuild.
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!