Do you understand how a coil works?

There is nothing that can fry a pack related to the other wiring. Allow me to explain exactly how a coil works.

A coil is a bunch of wires that are insulated from touching itself or other wires. It just wraps around a magnetic core, usually iron. In these vehicles one side of the coil is hooked up to a constant Hot 12V power source anytime the engine is on. The other end of each coil is connected to the ignition computer, or more specifically solid state relays or drivers within the computer. They function as an on-off switch, they are normally interrupted or open, and when they close, the coil energizes. As the coil energizes, it builds up a magnetic field. It doesn't just suddenly become magnetized. the field actually takes a certain amount of time to build, and during this building phase, the lines of magnetism are in a state known as flux (change). As these lines cross and intersect with other wires, it causes electrons within those wires to move and follow the direction of flow. This principle allows the secondary coil, which has no power source, The secondary coil instead just forms a loop.

How coils produce high voltage is simple. The primary coil has just a few windings and a heavier gauge wire. The secondary coil then has more windings, depending on how much voltage it is supposed to produce. The magnetic flux basically produces a high electrical potential in the second coil. However, current only flows through the secondary coil when the primary coil is in a state of change. If the coil is left off, the secondary coil does nothing, and when it is left on, the secondary coil does nothing. When the coil is first energized, the secondary coil generates a voltage relative to the voltage and current input, and when the coil is first shut off, the secondary coil also generates a voltage as the magnetic field collapses.

At the high voltages produces, the electricity wants to take the path of least resistance, and it will jump through gaps and insulators if it has to in order to complete its circuit. In the case of these cars, that circuit consists of two spark plugs, two ignition wires, the coil, and the engine block. The total resistance would be the combined resistance of both wires, plus the spark gap of both plugs. The block itself offers very little resistance, neither does the coil.

So if the plugs are worn, the spark gap that has to be jumped if the gap of both plugs. This means if each plug wears .010 then the coil has to jump and extra .020. This puts a lot of strain on the insulators in the various parts of the ignition system, because not only is the coil sparking on two plugs, the way waste spark systems are set up, it fires twice each revolution, once on the end of the compression stroke, and the other on the end of the exhaust stroke.

So what happens if the resistance is too high because the spark gaps are too great, or the wires have deteriorated? The current will try to find a new path that is easier. This new path could include, jumping through the insulator on the plug wires themselves, or it could jump between windings inside the coil pack. When the energy does jump through the insulators, it scorches the insulator, which makes it easier to jump through it in the future. Pretty soon the insulator has broken down, and the coil doesn't work anymore because it has internally shorted.

Often the path of least resistance is jumping through the coil casing and grounding out on the block.

So thats a quick rundown on how a coil works, and how a coil fails.

So if you are blowing up coil packs, in means the electrical resistance it is trying to overcome is too great, or they are cheap coil packs. Using all OEM parts helps ensure proper operation. Using proper spark plugs with a correctly set gap prevents the coil from being strained, and using good OEM grade wires helps ensure that there is enough current flow and the superior insulators used prevents the wires from shorting against the block. And the Motorcraft coil packs themselves are exceptional quality. I have used aftermarket coils, including the performance coils, and the quality is inferior. In fact, performance coils often highlight any weaknesses in the ignition system, and very often that weakness is the coil itself. Most aftermarket coils are budget price competitive units, that use inferior insulators, and thinner wire. And often the aftermarket coils do not fit properly, and the wires won't clip on correctly due to sloppy molding. I have found if you want the best most reliable operation, use OEM or better parts.

There is nothing in the electrical system that can fry a coil pack from the primary side. Again, the coil is designed to handle full 12V+ current. It will not draw more current than it can handle. The ignition drivers are just on-off switches.

On most of these vehicles, the ignition functions are handled by the EDIS module, some later years they are handled by the engine computer directly. The EDIS module is still a computer, its just not the main engine computer, instead it is a dedicated computer for running the ignition system. It is rare that the EDIS modules fail, even if they did, they should not hurt the coil. If in doubt, the module can be tested at most auto parts stores. This is not a part you would replace lightly, last I priced one out they were around $200. They are a part you would probably want to replace with either a Motorcraft or Standard brand.