Ignition Coils

• Ignition coils are made from a silicon-iron alloy that has a tolerance to heat. That being said, its internal components and joints where it has been soldered to the rest of the ignition system may not have been, and these parts may overheat at a much lower temperature than the coil itself. As temperature rises, the coil becomes less able to conduct electricity, which will contribute to the diminished functionality and eventual failure of the unit.

• Ignition coils are generally reliable, though their everyday wear and tear will eventually begin to break them down. Large amounts of electricity constantly run through the coil during the ignition process, and over time this current may contribute to the degradation of the insulation between coil windings, coil tower or the coil housing. This loss of insulation leads to mechanical failure as well as overheating.

• The resistance levels working in the ignition coil have to maintain at a mid-level constant in order to ensure proper function. A lower than normal or short resistance in the coil windings creates greater electricity flow through the ignition coil, which will quickly damage the entire ignition module. Variances in resistance levels within the coil may also cause a weak spark, which will result in the vehicle not starting as well as failure of the ignition coil and the mechanical parts surrounding it.

http://www.ehow.com/list_5954461_causes-ignition-coil-fail_.html