Coil voltage

Sounds like your wiring is messed up. Your “start” circuit is at 12V cranking, once started, car uses the “run” circuit and a lower voltage. So depending on what ignition system your using, will dictate what voltage hit your coil. MSD box handles power to the coil and works with one circuit. Mopar uses the ballast resistor and the 2 circuits, “start and “run”

So more detail is necessary for a clear Anwser…