grober
MB Master
Nothing lost by trying out a new bosch rotor arm with a BOSCH distributor cap. Looks as if you have the correct coil and transistor unit and wiring for your car then. Can't see how a faulty coil could lead to overvoltage if the primary and secondary coil continuity and impedance check out. Which leaves the transistor switching unit- MERCEDES never divulge the internal circuitry of these boxes but I note its got a lid with screws- might be worth a quick internal visual check? all else fails and the arcing continues a swap to the newer smaller design [ same part no] of transistor switching unit shouldn't break the bank?