Check out: http://www.mbz.org/info/articles/hvac/monovalve/

If your monovalve hasn't been replaced, it probably needs to be. It's an inexpensive thing to check and could be causing your problem.

Somebody sells a replacement kit on eBay.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino