When the AC compressor on my 500SL locked up it would stall the motor when I kicked it on. Made a terrible racket too. I would yank the belt and see how easy it is to turn the compressor. If it won't budge, then you've found the problem for the stalling.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!