How a Car Ignition System Works

Honda Civic EX engine bay

Adrian Gable

Your engine is like a big pump. It pumps air and gas in, then pumps exhaust out. The byproduct is a lot of energy that is sent to your wheels (and exhaust out the tailpipe. That's the basic of all basic descriptions. A little detail helps complete the picture. Your engine mixes air and fuel, then adds a spark to make the explosion. This spark ignites the air-fuel mixture and is referred to as the ignition.

The Ignition System: The Basics

Ignition System Diagram
This diagram shows the parts of your ignition system. Auto Repair Library

This ignition takes place thanks to a group of components working together, otherwise known as the ignition system. The ignition system consists of an ignition coil, distributor, distributor cap, rotor, plug wires and spark plugs. Older systems used a points-and-condenser system in the distributor, newer (as in most we'll ever see anymore) use an ECU, a little brain in a box, to control the spark and make slight changes in ignition timing. 

The Ignition Coil

The ignition coil is the unit that takes your relatively weak battery power and turns it into a spark powerful enough to ignite fuel vapor. Inside ​a traditional ignition coil are two coils of wire on top of each other. These coils are called windings. One winding is called the primary winding, the other is the secondary. The primary winding gets the juice together to make a spark and the secondary sends it out the door to the distributor.

You'll see three contacts on an ignition coil unless it has an external plug, in which case the contacts are hidden inside the case. The large contact in the middle is where the coil wire goes (the wire that links the coil to the distributor cap. There is also a 12V+ wire that connects to a positive power source. The third contact communicates information to the rest of the car, like the tachometer.

You can test your ignition coil right on the car in many cases.

The Distributor, Distributor Cap, and Rotor

Once the coil generates that very powerful spark, it needs to send it someplace. That someplace takes the spark and sends it out to the spark plugs, and that someplace is the distributor.

The distributor is basically a very precise spinner. As it spins, it distributes the sparks to the individual spark plugs at exactly the right time. It distributes the sparks by taking the powerful spark that came in via the coil wire and sending it through a spinning electrical contact known as the rotor. The rotor spins because it's connected directly to the shaft of the distributor. As the rotor spins, it makes contact with a number of points (4, 6, 8 or 12 depending on how many cylinders your engine has) and sends the spark through that point to the plug wire on the other end. Modern distributors have electronic assistance that can do things like alter the ignition timing.

Spark Plugs and Wires

Holding old and new car spark plugs on engine

Jorge Villalba/Getty Images

After the coil takes the weaker juice and makes a high powered spark and the distributor takes the powerful spark and spins it to the right outlet, we need a way to take the spark to the spark plug. This is done through the spark plug wires. Each contact point on the distributor cap is connected to a plug wire that takes the spark to the spark plug.

The spark plugs are screwed into the cylinder head, which means that the end of the plug is sitting at the top of the cylinder where the action happens. At just the right time (thanks to the distributor), when the intake valves have let the right amount of fuel vapor and air into the cylinder, the spark plug makes a nice, blue, hot spark that ignites the mixture and creates combustion.

At this point, the ignition system has done its job, a job it can do thousands of times per minute.

The Ignition Module

In the old days, a distributor relied on a lot of its own "mechanical intuition" to keep the spark timed perfectly. It did this through a setup called a points-and-condenser system. Ignition points were set to a specific gap that created an optimal spark while the condenser regulated.

These days this is all handled by computers. The computer that directly regulates your ignition system is called the ignition module, or ignition control module. There is no maintenance or repair procedure for the module aside from replacement.