Not sure what I'll be able to offer as I suspect you have a 104 motor and all of the gadgetry that came with that configuration, but I'd like to clarify a few thing(s).

You mentioned that the engine seemed to disconnect from the transmission about the time the dash lights came on. When a cars engine does, often times the dash light come on. Since the eng/tranny seemed disconnected, I wonder if the motor had actually died when the dash lights came on?

If not, how fast were you able to drive with the accelerator pedal depressed and was there any knocking, missing out, etc. if there was movement with pedal deprssed?

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'91 300-SEL