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

THere is no doubt that the car's original ICU and upgraded OVP relay were removed from the car. Whether the dealer or the independent shop did is unknown and is a separate problem; likely the vehicle was used as a parts donor car and the original parts were for whatever reason not reinstalled. It will be difficult to point a finger at this point since the two services occurred over 6 months ago.

The cost of the control units warrants confirmation of the defective status of the existing part prior to its renewal, so I'm at a bit of a standstill until I can obtain a unit of known quality to swap in.

Fresh paste was installed on the bottom of the ICU when it was removed for inspection. But it seems lack of paste would manifest itself as a potential probelm once the ICU was at operating temperature... which is not what is seen in this case: the vehicle misses when started cold; subsequent restarts when warm yield a smooth idle.

Tends to suggest an electrical problem, where something is not making contact when cold.

Hello Ferdman,
Would you happen to know the part number of the MB heat-conducting paste?
Thanks in advance,
Chicky