BAd fuse, only works sometimes (usually you get only defrost then, too.
Stuck monovalve, check for warm heater hoses, should be cool with AC on when it's hot.
Bad Klima relay, won't run AC compressor.
Bad serpentine belt, slips on AC compressor, Klima relay shuts off AC because the compressor is slow (this it to prevent the compressor siezing and stopping the engine).
Stuck expansion valve, or clogged orifice, so that no freon is moving (need gauges to verify).
Bad PBCU, won't run temp properly.
Bad hose to the interior temp sensor, so that the climate control is reading the temp under the dash next to the evaporator instead of the air temp - see if you have gentle suction at the little grille next to the dome light, if not , the hose is behind the glove box.
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!