Low refrigerant, bad low pressure switch, bad AC clutch wire, bad clutch. No speed sensor on the V8s, separate belt, but the belt could be broken, too. Bad relay for the AC clutch.

What I would do:

Check the belt. If missing or slipping badly, replace.

Check the low pressure switch by pulling the wires off (top one, I think, on the reciever/dryer) and touching the wires together. If the compressor comes on, you have a leak and no freon or a bad switch (if it cools properly with the wires touching, it's a bad switch). If the aux fan comes on, you have the wrong switch, try the other one. Bad switch requires venting the freon, have a shop replace it so they can recover your freon for you.

If nothing happens, fine the wire to the compressor clutch and disconnect it, then jumper the compressor end directly to the positive battery terminal. If the clutch engages, relay is probably bad. If it clicks but the compressor doesn't turn, either the clutch is bad or the compressor is locked up (this will stall the engine).

Peter

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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!