He probably does not need a new motor. I had the same problem with the SL and it turned out to be the multi-function switch. It was a very easy replacement.

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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!!!