Bad AC compressor or clogged expansion valve.

Check for difficulty in rotation of the AC compressor with the engine off. Should be fairly easy to rotate it using the clutch plate on the front. If very stiff or rough, it's shot and absorbing mucho horsepower.

If the belt slips, the AC relay will drop the AC compressor off line until you restart.

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!