How Distributor Ignition Systems Work

The Purpose of the Ignition System

Your vehicle relies on combustion in the engine's cylinders to make it go. Thousands of little explosions put your engine and your vehicle in motion. Combustion requires three things: fuel, compression, and a spark. That spark is provided by the ignition system, and you can't start your car or truck without one.

The system that creates that spark may seem very complicated to you at first glance, but once we've walked you through it, you should have a much better understanding of how your engine gets fired up.

The Types of Ignition Systems

Most ignition systems in automotive history have used a distributor to send electricity to the right cylinder at the right time. More recent distributor-less or direct ignition systems forego the mechanical distributor and use electronic sensors and the vehicle's computer, the engine control unit (or ECU), to control the spark timing. In both these systems, the goal is to get the spark plugs to spark in the correct order and at the ideal time to keep combustion occurring smoothly, in each cylinder, in sequence.

If you're willing to accept that computers are boxes full of magic, the direct ignition system can be understood easily enough. Electronic sensors relay information about the position of the crankshaft and the camshaft to the ECU. This allows the ECU to determine the right time for the ignition coils (which receive power from the battery) to supply power to the spark plugs. The ignition coils in these types of systems may be mounted directly onto the plug in the valve cover, in what is known as a coil-on-plug setup, or may be ignition coil packs that send power to the spark plugs via spark plug wires.

How Mechanical Ignition Systems With Distributors Work

Mechanical ignition systems, those that use distributors, are slightly more complicated and possess a greater number of parts. The best way to understand it will be if we walk through each part, starting at the battery, and ending at the spark plugs.

If you just want the basics, here's a much briefer step-by-step run down of the ignition system. The battery provides low voltage electricity to the ignition coil. The ignition coil converts the low voltage electricity into high voltage power in timed pulses. The distributor has a shaft that gets spun by the camshaft. That moves other distributor parts that cause the ignition coil to pulse, and sends the electricity down each spark plug wire in order. The power travels down the spark plug wires to the spark plugs and causes sparks. The sparks ignite the fuel and air in the engine cylinders. That's it. Hopefully you found that briefer and easier to understand.

What The Battery Does

The battery in your vehicle works like many other rechargeable batteries. It can store electricity and dissipate it as direct current. When the engine is running, it runs the alternator which generates electricity to recharge the battery. To simplify things a bit, electrical energy causes a chemical reaction in the battery. So the electrical energy gets stored as chemical energy. When the battery needs to discharge, another chemical reaction occurs, which releases the electric charge. The battery supplies twelve volts of direct current. In order to achieve combustion, though, there needs to be about 40,000 to 100,000 volts at the spark plug. How does the voltage get so much higher? The answer lies in the ignition coil.

What The Ignition Coil Does

While newer, coil-on-plug systems use one ignition coil per cylinder, mechanical ignition systems rely on one coil to convert the power from the battery to the high voltage needed by the spark plugs.

The ignition coil is a transformer. Transformers are used in many electrical applications. Step-up transformers convert power from power stations to higher voltage so it can travel farther distances. Step-down performers in household appliances decrease the voltage supplied to your house to a smaller one that your appliances can tolerate. Your ignition coil is a step-up transformer.

Transformers work on a principle called magnetic induction. A magnetic field moving across a coiled wire will create a voltage in that wire. As it turns out, voltage in a coiled wire also produced a magnetic field. In a traditional transformer, one coil, the primary coil, receives power from an outside source. Since alternating current is constantly changing, the magnetic field produced by the primary coil is constantly moving. That produces a voltage in the secondary coil.

What that voltage is depends on the ratio of the number of turns in the primary coil to the number of turns in the secondary coil. If the secondary coil has twice as many turns as the primary coil, the output voltage will be twice the input voltage. If the primary coil has twice as many turns as the secondary coil, then the output voltage will be half of the input voltage. In an automotive ignition coil, the secondary coil has tens of thousands of times as many turns as the primary coil. This provides the big step-up in voltage that the spark plugs need.

If you noticed that our discussion of transformers hinged on alternating current and that automotive batteries provide direct current, then we commend you on your astuteness. The transformers we discussed before aim to put out a constant flow of energy. An ignition coil is designed to put out discrete jolts of electricity. To this aim, the charge through the primary coil is periodically disrupted. This collapses the magnetic field that the primary coil produces. This acts as one big movement of the magnetic field and causes the secondary coil to create one burst of high voltage energy at a time.

Now you may be wondering what disrupts the primary coil. In more recent systems, this is handled by a computer to achieve more precise timing. Originally, it was achieved through mechanical means in the distributor.

What The Distributor Does

While the battery and ignition coil provide the power, the distributor determines where that power goes and when. The distributor is like a traffic cop for electricity.

The distributor contains, among other parts, a rotor that spins, and a number of contacts mounted to the distributor cap. Power from the ignition coil is supplied to the rotor. The rotor spins in time with the engine. When the end of the rotor is near one of the contacts, electricity arcs to the contact. From there, the power travels down a spark plug wire to the associated spark plug. This is what times the charge to each spark plug.

The rotor must turn in time with the engine. How this is achieved is fairly simple and elegant. The distributor rotor is turned to by a distributor shaft which is connected to the camshaft. The faster the camshaft is spinning—and by extension, the faster the valves are opening and closing—the faster the distributor will spin, and the faster the sparking sequence will go. This neatly times the valves that feed fuel and air to the cylinders to the distributor that supplies the spark.

Now that you understand how the distributor turns, we can get back to the matter of what creates the periodic, discrete charges from the ignition coil. The distributor contains a breaker point that grounds the primary coil's circuit. This point is connected to the ground by a lever. The lever gets moved by a cam connected to the distributor shaft. That opens the primary coil circuit and causes the collapse that triggers the high voltage pulse in the secondary coil.

What The Spark Plugs and Their Wires Do

You will probably be thankful to hear that these last few parts are pretty simple. The spark plug wires or ignition wires are insulated wires that carry the power to the spark plugs. The spark plugs are what finally produce the spark that causes combustion.

A conductive metal core runs through the insulating ceramic body of the spark plug. There is also an electrode that grounds to the metal base of the spark plug, which grounds into the engine block. There is a gap between the central core and the electrode tip. Electricity arcs across that gap, causing the spark.

Some Common Symptoms of a Bad Distributor

Any of the parts in the ignition system can become damaged and wear out over time. Many of them are even expected to have limited lifetimes. Problems with different ignition parts can cause similar problems. If you have problems with the distributor, the spark plug wires or the spark plugs, the results will be the same. The spark timing will be thrown off or one or more spark plugs might not produce a spark at all. This can result in misfires, poor power production, poor gas mileage, and rough idling. In some cases you may not be able to start your engine at all. A malfunctioning ignition coil or a dead battery can also keep your engine from starting.

Since the results are so similar, it may be hard to tell what part is causing your problem. A good first step in determining your problem is to visually inspect the parts. You may be able to identify wear to the distributor or cracks in the distributor cap. You may notice cracks in the spark plug wire insulation. If you remove the spark plugs, you may see that they are pitted, fouled, covered in soot, or otherwise in poor shape.

You should also keep in mind that many of these parts are supposed to be periodically replaced. The most commonly used types of spark plugs (iridium and platinum) usually have a lifespan of 60,000 to 100,000 miles. For distributor ignition systems, many car manufacturers recommend a complete tune-up every 30,000 miles, which involves replacing the distributor cap and rotor. In general, it is a good idea to replace the spark plug wires and distributor when you are replacing your spark plugs.

Do It Yourself Ignition Repair

Working on your own distributor ignition system is probably within your grasp. As we mentioned, the parts are expected to be periodically replaced. They are conveniently located on top of your engine, making them easy to access.

One important thing to remember is that your spark plug wires must be replaced in the correct configuration or else the spark plugs will spark out of order. This will cause a whole new set of ignition coil problems. One way to avoid this is to put a labeled flag of tape on each spark plug wire, which will help you remember which is which. You could also replace your spark plug wires one at a time. One important safety tip is that you should disconnect your battery before working with the electrical system. So long as you work in a cautious and organized fashion, tuning up your ignition should be a relatively painless and straightforward job, especially now that you understand how the whole thing works together.

Having Problems with your Distributor Ignition System?

If you are having issues with your distributor ignition system, then you have come to the right place. 1A Auto is your source for replacement parts to get your distributor ignition system back in working order again! Below is a list of common distributor ignition system parts that you may need to replace.