I am not all that knowledgable in this area for Mercedes, but I would believe that you should have battery voltage ( about 13V ) at the coil (+). This would certainly be the case with an older standard K points ignition. In those systems there was usually a resistor in series with the coil primary voltage which was bypassed for starting, but in the circuit with the engine running. When the resistor failed, the engine would spark at crank, but fail as soon as you stopped cranking. I think with Mercedes the OVP has a similar function. If your engine has a simple ignition you need to check the points and capacitor as well as the coil and the coil ground. The coil is usually grounded through the shell via the mount and corrosion of the shell or mount can cause problems. I've also seen a case where a bad high tension wire between the distributor and the coil was bad and caused weak spark. In general, the high tension wires can break down over time, particually the carbon core type. Since the head was off the engine, recheck all the wiring and plugs. It looks like you have checked most of the obvious stuff. I hope somebody with insite on your particular engine chimes in here to help.
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