jgl1, I would track down the original ICU for your 500SL. From my experience with our daughter's 300CE the ICU's are very expensive (around $2,500) so I can't imagine any indie or MB dealer swapping one to help you without charging you. Something isn't right about that scenario. It makes no sense to swap the ICU's unless the other party wanted your good one ... sorry if I seem skeptical about the motive. Suggest you talk to the MB dealer and indie who worked on your 500SL when the family member had it.

As mentioned, the heat-conducting paste is critical to longevity of the ICU, as is maintaining the ignition system components (spark plugs, wiring, coils, etc.). Radio Shack sells it for computer applications.

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Fred Hoelzle