Perhaps the diesel and the gas systems operate different, but I recently had the same problem with my SL. Turns out the problem was in the climate control unit which mounts behind my glove box. Had replaced the OEM one with another of a different part # which the vendor said would work. The replacement part had one less pin on the right side than the OEM one. Couldn't figure out what the difference was until I put the OEM part back in after replacing the voltage regulator.

So, my guess is your problem could be in the CCU box, but it's just a guess.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000