[CobraDan]

The usual cause for a failing or failed coil is overheating. This is usually caused by spark plugs with too large a gap, which causes excessive KV to jump the gap. The bigger the gap, the bigger the jump, and the more power necessary to initiate.

A coil that is failing will cause a low-spark intensity. When this happens, the combustion is not efficient, and a driver will experience a drop in power. A weak coil will also have a more difficult time jumping the gap in the plugs and will also have the possibility of no spark in lean fuel conditions within the cylinders, resulting in an engine misfire. Rich conditions are conductive and promote ignition sequence, while lean conditions require more voltage to jump the gap. If the coil is too weak there will be no spark.

Certain coils are designed to operate at 6 volts rather than 12 volts. They are designed this way to prevent heat build-up in their particular situation. These coils require a ballast resister in line that drops the voltage.

A total coil failure will cause a no spark, no run condition. This can be diagnosed by pulling off one of the spark plug wires at the point at which it is attached to the spark plugs. In the event of a coil on plug situation, remove the two 10mm bolts and pull the coil off of the plug. Install a tester spark plug in either unit and lay it on a good ground on the engine. Have a helper attempt to start the engine and observe the plug for a good spark. If you do not see any spark, the coil is bad.