How does Ignition Coils Work

When you turn on your car's ignition, battery voltage goes to the coil and the coil converts it to high-voltage power in timed pulses. The pulses then go to the distributor and get sent to the right spark plug for each cylinder in proper firing order.

The old-style ignition systems that were used on cars and trucks from the 1920s through the early 1970s all had one common part: a mechanically-timed ignition coil. The ignition coil gets power from the ignition system battery and turns it into high-voltage electricity for each spark plug in the engine cylinders at just the right time, when the piston in that cylinder is at top dead center (TDC).

Inside the distributor, there's a rotor arm attached to the central shaft of the coil. The arm has a series of numbered segments on it, usually named for the cylinders in the order they are fired. As the rotor arm spins, the segments (which are actually contact-breaker points) open and close in a synchronized way with the cam on the distributor shaft to pass a 12 volt low-tension (LT) circuit back and forth between the coil and the engine's spark plug wires.

A small gap exists between each segment and the rotor arm's tip. As the rotor passes each spark plug lead (which leads to the spark plugs in the cylinders), its electrified tip 'jumps' across each gap and creates a burst of electricity that can fire the spark needed to ignite the air-fuel mixture in that cylinder. Then, the combustion can continue to burn the gasoline and build up pressure in the cylinder, allowing the piston to reach bottom dead center (BDC).

Newer vehicles have replaced these older designs with an electronic ignition. The newer ignition systems are based on pickup coils and an electronic control module that are very different from the old-style distributor. They're much less likely to break down than the old-style systems, and they have much lower maintenance needs, with some automakers specifying 100,000 miles between service intervals. This type of ignition system also uses a smaller and simpler coil.