As pointed out, you should have battery voltage at one wire on the + side of the coil. Since you have that, that isn't the problem.
It also would be extremely unlikely for power to the coil to be the problem; there are no fuses for that circuit and only tyhe ignition switch would be able to drop the voltage.
In triggering the coil its the negative side that is importnat and changing. The positive side always stays hooked up if the car is switched on.
On most cars I might agree with Larry that any visable spark is probably all you need check for. As it turns out, the coils on 300Es fail in output quite often. I have probably replaced more 300E coils in the last year than all other MB coils combined in the whole of my career. (maybe it would take two years worth).
With that said, it also sounds like you might be getting too much fuel.
Bosch Master, ASE Master, L1
33 years MB technician