Ignition coil (mechanical breaker ignition)

1 Coils normally last the life of a car. The most usual reason for a coil to fail is after being left with the ignition switched on but the engine not running. There is then constant current flowing, instead of the intermittent flow when the contact breaker is opening. The coil then overheats, and the insulation is damaged (photo).

2 If the coil seems suspect after fault finding, the measurement of the resistance of the primary and secondary windings (usually an ohmmeter) can establish its condition. If an ohmmeter is not available, it will be necessary to try a new coil.