Ignition Coil: Description and Operation

Ignition Module And Coils Replacement:

Ignition Schematic:

Ignition Current Flow:

Four separate coils are mounted to the module assembly. Each coil provides the spark for two spark plugs simultaneously (wasted spark ignition). Each coil can be replaced separately.



WASTE SPARK THEORY
When the coil discharges, both plugs fire at the same time by using the engine block to complete the electrical circuit. The cylinder on the compression stroke is called the "event" cylinder and the one on the exhaust stroke is the "waste" cylinder. The two cylinders "share" the energy available from the ignition coil to fire both spark plugs. This method of ignition is called "waste spark" ignition.

Since the polarity of the ignition coil primary and secondary windings does not change, one spark plug always fires with a forward current (center electrode to ground electrode) and its "companion" plug fires with a reverse current (ground electrode to center electrode). This is different from a convention distributor ignition system that fires all the plugs with the same forward current flow.

It is possible for one spark plug to fire even though a plug wire from the same coil may be disconnected from its "companion" spark plug. The disconnected plug wire acts as one plate of a capacitor and the engine block acts as the other plate. These two "capacitor plates" are charged as a spark first jumps across the gap of the connected spark plug. The "plates" are then discharged as the energy is dissipated as the spark continues. Voltage requirements are very high with an "open" spark plug or wire. The ignition coil may have enough reserve energy to fire the connected plug at idle, but possibly not under some engine load conditions. A more noticeable misfire may be evident under load; both spark plugs may then not fire.