I had posted this on the SL forum but didn't get a response so I'll try here.. What should the voltage read at the + terminal of the coil with the key on and engine not running on a '77 450SL? Mine reads 4 volts. If I remove the wires from the - terminal of the coil the voltage reads 12 volts. I suspect the transistorized switching unit may be at fault. I removed the unit and did a check on the power transistor, it appears to be OK. Anyone have a schematic of the unit or a way to better check the unit? I'm 75 miles from the nearest MB shop and don't know if they can check the unit off the car. Also I removed the plugs and some were wet indicating they were not firing.