How to Diagnose and Test Ignition Coils

The ignition coil core supplier's ignition coil provides the high voltage required by the ignition system to ignite the spark plug. Most engines with distributor ignition systems have one coil, but a few imported applications have two coils. On a distributorless ignition system (DIS), multiple ignition coils are used. In a "waste spark" system, each pair of cylinders shares a single coil. On other DIS and spark plug (COP) ignition systems, each cylinder or spark plug has its own individual coil.

The ignition coil is used as a high voltage transformer. It boosts the primary voltage of the ignition system from 12 volts to thousands of volts.

The actual ignition voltage required to produce a spark across the electrode gap of the spark plug depends on the width of the gap, the 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 required voltage varies continuously and can vary from 5,000 volts to 25,000 volts or more. Some systems can output up to 40,000 volts at peak demand.

How ignition coils work
Inside each ignition coil are two sets of windings surrounding a laminated or segmented core. Hundreds of "primary" windings are connected to two external low voltage terminals on the coil. The positive (+) primary terminal is connected to the ignition switch and battery, while the negative (-) primary terminal is connected to the ignition module that provides ground. A "secondary" winding with thousands of turns is connected on one end to the primary positive terminal and the other end to the high voltage secondary output in the center of the coil.

The ratio of secondary to primary is typically about 80 to 1. The higher the ratio, the higher the potential output voltage of the coil. High performance ignition rails typically have higher ratios than standard coils.

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

The ignition module then opens the coil's ground connection and closes the primary coil winding. This causes the magnetic field to suddenly collapse. The energy stored in the magnetic field has to go somewhere in order to induce a current in the secondary winding of the coil. Depending on the ratio of wire turns, this will multiply the voltage by a factor of 100 or more until there is enough voltage to ignite the spark plug.