Question re electronic ignition

[280Sbenz]

among the stack of mercedes i have accumilated of late i have a 126 280S (hong Kong import) with electronic ignition.

upon inspection for the trouble it has i found that :
cylinders 3 & 4 only fire when the lead is off, if i place it on the spark plug properly i have no power

i have checked compression, lead resistance, new plugs etc

i am guessing it is something to do with the electric ignition ???? someone said about the control lead could be busted, it has 7.9 volts power running through both wires with the ignition on, earth lead connected on distributor although no earth at control box end ..


any ideas ? or has someone else had this problem ??