I've never experienced this particular problem on my w140, but might take a look at http://pages.prodigy.net/jforgione/MB_AAC.html for help. Jim offers some excellent information on Air Conditioner Diagnostics for your model.

Good luck.
Russell

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Russell McMahon
Tulsa, OK
2001 CLK 320
1992 400se
1993 400e
2000 320ML