Try energizing the monovalve with 12V and see if you have any difference. If you do, the silver box is probably on its way out.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

I did that on the car in question. No difference, But maybe I should try it on the stock 560SL. I rebuilt that system 3 years ago and that and the evaporator are the only parts I didn't replace.

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To see my 129 parts for sale visit:
http://stores.ebay.com/The-Mercedes-SL-Store
John Roncallo