Hi Raymond, My 117 engined 500SEL did the very same once but without the soot. I found a small vacuum pipe behind the RH cylinder head off. Seems this small bore pipe sends a signal to the transmission for gear shifting, fixed it back on and hey presto, problem solved !

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1993 190E 2.0L (Euro.) sold
1991. 500 SEL (Euro.)
1991. 300 CE (Euro.) sold
1993. 500 E (U.S. spec.)