Originally Posted by MB-Dude

False. I’m sorry Mike, but the addition of resistance “in series” to the secondary side of any transformer (which is what a simple automotive coil is) does not place additional demand on the secondary voltage. This simply causes less current to flow from the secondary side of the coil. In essence, less spark energy would be created. If anything, it would be less stressful on the coil because the increased resistance from secondary through the high voltage wire through the resistor spark plug allows less current to flow. Less secondary current flow equates to less energy being coupled through the coil from the primary side. The coil runs cooler and less stressful.

To illustrate through an extreme case, imagine removing all the high voltage wires from your engine and attempting to start it. Of course it will never fire, but if the battery could stay charged forever and you kept cranking the starter, you would never see current flow from the secondary – it’s impossible… no path for the current. The coil would last forever because there is no current being generated in the secondary. In fact, the primary side is also protected, but without getting into electrical/magnetic theory, you’ll have to accept my word on it.

Originally Posted by MB-Dude

All automotive coils generate energy by allowing 12v current to be pulsed through their primary windings. The pulsing action (timed to the when the spark needs to fire) transfers energy to the secondary windings of the coil. The ‘voltage’ created on the secondary side of the coil is in direct relation to the ratio of wire windings of the primary to the secondary. The amount of energy transferred is completely dependent upon the "resistance to ground" of the secondary circuit.

In a perfect zero resistance scenario, the energy builds in the coil and the output voltage of the secondary rises rapidly. When sufficient energy is achieved to overcome the spark gap, the gap arcs and suddenly there is current flow until the energy in the coil drains enough to not sustain an arc. The spark gap – only when arcing – is considered a resistor and has a certain limiting effect on the high voltage current. It is important to remember that other than when arcing, the spark gap offers infinite resistance. The current flow is indirectly proportional to the resistance – higher resistance to the spark gap, less current. Thus, resistor plugs should create a weaker spark, but be less stressful on the coil. It is this 'weaker spark' that greatly reduces the infamous radio interference noise.

However, your example of a bad high voltage wire is valid for coil stress, but for a very different reason. When a high voltage cable begins to fail, it allows current through/across its insulator jacket. This equates to current flow directly to ground, so a portion of the secondary energy bypasses the spark plug gap. This means that the coil will produce more current (i.e.: work harder) to generate sufficient energy for the spark. In extreme cases, enough secondary energy may be drained off through the insulating jacket to the extent that the sparking arc may not be sufficient for initiating a complete fuel/air mixture burn – i.e.: misfiring. This scenario can cause stress on the coil, secondary side, in particular. Whether that stress causes ignition module issues is debatable and is very dependent upon the electrical engineering design of the ignition module.

Originally Posted by MB-Dude

It is a common misconception that increased spark plug resistance causes coil stress. High voltage cable breakdown does. A partially shorted spark plug does. But spark plug internal resistance should lower spark current, not increase it.