It is easy to understand the primary circuit, but many are confused by the high voltage secondary. Since each end of the coil goes to a spark plug (insuring they both fire at the same time) the only way to complete the circuit is if both plugs are grounded to the block. Obviously if one wire is grounded the other plug will then get twice the voltage, but the grounded side will not fire its spark plug. So the way to ensure that the secondary is properly grounded is to be sure both plugs are firmly screwed into there proper holes. . . 😀
