before running diagnostics I recommend you do this first (this causes most of the rough running cases):


1. Remove the hard buildup on the posts of the inside of the distributor cap with a hard tool (I have to do this every 2 months, same engine)

2. measure the resistance of your ignition wires 1k ohms just the wire, 2k if you measure from inside the distributor cap to plug end, wire to the coil no resistance

3. If you don't have records then replace the ignition coil (insulation inside breaks down and you can't test it)

4. remove the plugs and see what they look like, clean


once you do this then you can start diagnosing the round plug (called x11) between pins 3 and 2 with a warmed up engine idling.

you will need a meter that supports duty cycle or an oscilloscope

more info on page 16:

http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/07.3-0121.pdf