Ignition Coils

The ignition coil is used to create a strong enough spark to jump the gap between the spark plug positive and negative electrodes causing the fuel-air mixture to ignite.

It has a primary winding that is controlled by the ICM that uses low voltage DC to charge the coil(create a magnetic field in the core) for a specific amount of time(dwell time). Then the ICM that controls the coil disconnects the primary winding ground, causing the magnetic field to collapse and generate a ~35,000 volt charge on the secondary side of the coil when it's time to fire a spark plug. That high voltage charge goes from the secondary winding, through the coil tower, carried by the coil wire to the distributor cap, through the cap to the rotor, back to the spark plug side of the cap, all the way down the spark plug wire, through the spark plug, jumping the gap and is finally grounded through the engine block.

The coil gets a lot of blame as it's the last thing to get replaced after you have repaired/replaced the rest of the secondary ignition system.

When any of the secondary ignition parts are worn/marginal, your coil has to work harder to complete that circuit. High resistance in the wires/connections, worn cap/rotor, too large a spark plug gap(incorrect gap or worn), wrong or contaminated plugs, a larger gap between the cap and rotor interface because of a worn distributor drive gear(Pancake Distributor Disease), or even a lean fuel mixture from a vacuum leak are all possible issues.

When the computer detects a misfire condition, it will increase the dwell time of the coil, leading to a higher coil charge being produced, more heat, more chance of damaging the coil. The ignition charge always takes the path of least resistance. When the secondary issues are bad enough, then the path of least resistance becomes through the coil insulation or even back through the ICM. As the spark energy travels through the coils insulation, it breaks it down a little bit at a time, thus making it easier for it to take that same path in the future.

If the ICM is damaged, it can cause the dwell time to increase, causing a stronger spark and more heat/wear on the coil.

This is why it will often be recommended that you replace the coil and ICM together after a heavy misfire repair.

Cool video about how ignition coils work. In our vehicles the switch in the video is part of the ICM, which is controlled by the PCM.