For what it's worth, the chief mechanic at a MB dealer with a respected service department told us at a tech session that he had seen precisely two (2) 1980's era fan clutches go bad and leak during his years wrenching. It's a very expensive "shotgun" repair. I know this from personal experience, because I have a...uhhhh..."good spare"...

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm