Ignition coils provide the high voltage needed by the ignition system to fire the spark plugs. Most engines that have a distributor ignition system have a single coil, but a few import applications have two coils. On distributorless ignition systems (DIS), multiple ignition coils are used. On “waste spark” systems, each pair of cylinders shares a coil. On other DIS and coil-on-plug (COP) ignition systems, each cylinder or spark plug has its own individual coil.

The ignition coil serves as a high voltage transformer. It steps up the Pencil Ignition Coils system’s primary voltage from 12 volts up to thousands of volts.

The actual firing voltage needed to create a spark across a spark plug’s electrode gap depends on the width of the gap, the electrical resistance in the spark plug and plug wires, the air/fuel mixture, the load on the engine and the temperature of the spark plug. The voltage required is constantly changing and can vary from as little as 5,000 volts up to 25,000 volts or more. Some systems can put out as much as 40,000 volts under peak demand.

HOW AN IGNITION COIL WORKS

Inside every ignition coil are two sets of windings around a laminated or segmented iron core. The “primary” windings, which number a few hundred, are connected to the two external low voltage terminals on the coil. The positive (+) primary terminal connects to the ignition switch and battery while the negative (-) primary terminal connects to the ignition module which provides ground. The “secondary” windings, which have thousands of turns, are connected at one end to the primary positive terminal and the high voltage secondary output terminal in the center of the coil at the other end.

The ratio of secondary to primary windings is typically around 80 to one. The higher the ratio, the higher the potential output voltage of the coil. Performance ignition coils typically have a higher ratio than standard coils.

When the ignition module closes the coil primary circuit and provides a ground, current flows through the primary windings. This creates a strong magnetic field around the iron core and charges up the coil. It takes about 10 to 15 milliseconds for the magnetic field to reach maximum strength.

The ignition module then opens the coil’s ground connection and turns the primary coil windings off. This causes the magnetic field to suddenly collapse. The energy stored in the magnetic field has to go somewhere so it induces a current in the coil’s secondary windings. Depending on the ratio of turns of wire, this multiplies the voltage up to 100 times or more until there is enough voltage to fire the spark plug.

IGNITION COIL FAILURES

Ignition coils are very rugged and reliable, but can fail for a variety of reasons. Heat and vibration can damage the coil’s windings and insulation causing shorts or opens in the primary or secondary windings. But the number one killer of ignition coils is voltage overload caused by bad spark plugs or plug wires.

If a spark plug or plug wire is open or has excessive resistance, the ignition coil’s output voltage can rise to the point where it burns through the coil’s internal insulation causing a short. The insulation in many coils can be damaged if output exceeds 35,000 volts. Once this happens, the coil’s output voltage may drop causing ignition misfire when the engine is under load, or the coil may cease to put out any voltage preventing the engine from starting or running.

If a coil has battery voltage at its positive terminal and is being grounded on and off by the ignition module or circuit but is not producing a spark, the coil is defective and needs to be replaced.

TIP: If the ignition module has failed more than once, it may be due to a bad ignition coil. Internal arcing or shorts in a coil can overload and damage the circuitry inside the ignition module.

IGNITION COIL DIAGNOSIS

When a coil failure occurs on a distributor ignition system, it affects all the cylinders. The engine may not start or it may misfire badly when under load. The misfire may also jump from cylinder to cylinder. But on an engine with a distributorless ignition system (DIS) or coil-on-plug (COP) ignition system, a single coil failure will only affect one cylinder (or two cylinders if it is a DIS waste spark system where two cylinders that are opposite each other in the firing order share the same coil).

If your engine is running rough (misfiring) and the Check Engine Light is on, use a code reader or scan tool to check for misfire codes.

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