Step 5: Coil Damage Due to Overvoltage
In typical automotive use, the coil only needs to generate a short spark to ignite the fuel-air mixture in the engine. A higher voltage output allows the generation of a sufficient spark, even if the spark plugs have deteriorated and become fouled. It is generally not suggested to run with the distributer disconnected, as the output voltage of the coil can then rise high enough to damage the coil internally. In normal operation the coil output voltage is limited due to the shorter gap of the spark plug. Even with wear and fouling, the output does not rise high enough to damage the coil. Many automotive diagnostic sites and articles list poor high voltage connections as a possible cause of ignition coil failure.
I damaged one coil in this manner. Initially, arcs as long as 1.5 inch could be generated, but after a relatively short length of time producing arcs of that length the output reduced to the point where the arcs were only about one half that length. This kind of damage is caused when a pathway of carbonized material forms internal to the coil.
This kind of issue can be confusing, as measurements of the primary and secondary winding resistances may look the same as they did before the damage. The meter will not produce a high enough voltage on its resistance measurement setting to cause the carbonized pathway to conduct, so the resistance measurement may look normal.
The picture below shows a coil with a visible scar on the center HV shroud, cause by arcing. The carbon track then provided a lower resistance path to the HV output, so that the arc would no longer jump across the desired spark gap, but instead jump directly from the coils HV output to its “-“ terminal. This coil operated properly again after cleaning up the carbon residue and covering it up with tape, but if the damage had been internal the coil would be irreparable. The possibility for damage of this nature is more pronounced when overdriving the coil, as the arcs tend to be hotter.