OK, I can explain… It’s all about those little electron guys always wanting to go back home. In electronic terms there always has to be a complete circuit for electricity to flow. In the case of the usually encountered single spark plug wire ignition coil what is not oblivious, and you usually can’t see, is that one end of the secondary winding is connected to ground. The other end goes to the spark plug, thus forming a loop circuit with the SP gap in the middle. So, the little electrons running the “loop” have to jump that gap to get back home. On your two-lead coil, neither end of the secondary winding is directly connected to ground… we call this an “ungrounded coil” or a floating coil. Now thinking about the electrons running the “loop” , visualize it this way… the coil fires, the electrons leave one end of the winding run to one spark plug, jump the gap to the SP body and the engine block, run through the engine block to the other SP jump that gap ( but in the opposite direction) and run “home” to the other end of the secondary winding.
That is why one MUST have one SP wire grounded to check for spark on the other wire. Firing the coil absent a complete ground path loop risks damaging the coil, as that high voltage will seek a path to ground,.. internally to the coil.
Hope this helps.
Joe B
