What Does a CDI Box Do?

In this article, you’ll learn the motorcycle’s CDI box’s role, a capacitor discharge ignition.

The CDI box is one of the main components for diagnosing and repairing a motorcycle. Accordingly, knowing what a CDI box does will help you greatly with a DIY repair job. Today, I hope to cover everything I know about this mysterious black box.

The CDI box generally decides when the engine needs to fire the spark plug. This is done by completing a series of calculations after receiving the pulse signal from the stator. Then, the CDI box sends a pulse signal to the ignition coil that fires the spark plug.

Don’t forget to read the below article for a detailed explanation.

Working Principle of a CDI Box

CDI box connected to an engine
Video | LearningZone

A CDI box, aka capacitor discharge ignition, is a significant part of modern motorcycles. This box is a part of most motorcycles created after the 1980s. Before explaining what a CDI box does, here are a few familiar names for CDI boxes.

  • Black box
  • Ignition box
  • Power pack
  • CDI module
  • Brainbox

As you can see, there are a few different names for the CDI box. Try to remember those names too. It will come in handy while you are working with different types of mechanics or clients.

What Does a CDI Box Do?

CDI box board
Video | LearningZone

The CDI box controls the bike’s ignition system. It has capacitors that are capable of working as batteries. So, they can store energy and release that energy for instant ignition. You can recognize this system as a trigger mechanism. This trigger mechanism includes internal circuits, coils, and capacitors. Here is how the CDI box works.

  1. The stator sends a pulse signal to the CDI box.
  2. Then, the CDI box calculates the exact moment the engine needs to fire the spark plug.
  3. Finally, it sends a pulse signal to the ignition coil to fire the spark plug.

Why Firing Spark Plugs at the Right Time is so Important?

spark plug in zoom
Video | Skill Lync

You now know the CDI box determines when the spark plug needs to fire. But have you ever thought about why this is so important? If you haven’t, don’t worry about it. I’ll explain it here.

You’ll hear pinging, constant rattling, or knocking when the spark plugs fire at the wrong time. Also, you’ll have a hard time starting the vehicle. In other words, it will lead to cylinder misfires.

Advantages and Disadvantages of the CDI Box

There are advantages and disadvantages to using a CDI box. CDI boxes are far superior compared to early ignition system contact breakers.

Quick Tip: A contact breaker is a mechanical component for ignition performance. This was used before inventing the CDI box. However, because of the wear and tear, using a contact breaker wasn’t the best idea then.


  • Less charging time
  • The fast rise in voltage


  • Electromagnetic noise
  • Unable to ignite lean mixtures

Known History of the CDI Box

capacitor discharge ignition
Video | leporP Autos

Nikola Tesla invented the first-ever capacitor discharge ignition in the 1890s. It was a groundbreaking invention at that time, and afterward, Tesla filed for a patent for his invention.

The Ford Model K was the first-ever vehicle that ran with a CDI box, which was accomplished in 1906. In the 1950s, the electronics CDI ignition system was introduced to the automobile industry.

CDI Box Troubleshooting

Proper knowledge of CDI box troubleshooting is never a bad thing. After all, the CDI box is one of the crucial parts of your motorcycle.

On the other hand, motorcycle repair and service costs are ridiculously high these days. So, why not learn a few things about troubleshooting? With that in mind, here are a few symptoms of a bad CDI box.

  • Engine or cylinder misfires
  • Difficulty starting
  • Rough running
  • Engine stalling
  • Other ignition issues

Don’t Forget: Above symptoms can result from another issue on your bike. For instance, an engine misfire can happen for many reasons, such as a bad fuel pump or faulty spark plug.

So, it is always better to check the parts of your bike first.

  • Battery: Check the battery voltage using a multimeter or voltmeter. Remember to check the battery’s wire connection, corrosion, and other issues.
  • Wiring: Check all the wire connections in the bike. Especially the ground wires.
  • Fuses: Check all of the main fuses in the bike.
  • Stator: If the stator is faulty, the bike’s battery won’t get the necessary power. So, check the stator’s resistance and voltage.
  • Regulator: The regulator converts AC into DC. So, use a multimeter and check the regulator’s functionality.

If you still get the previously discussed symptoms after all the above parts are working properly, you might be dealing with a faulty CDI box.

Luckily, you can check a CDI box with a multimeter. Here is how you can do that.

testing cdi box with multimeter
Video | LearningZone

  1. Pull out the CDI box from the bike.
  2. Set the multimeter to check continuity.
  3. Check for continuity between ground and other connections.
  4. A properly working CDI box won’t give any continuity for this test.

Make sure to run the CDI box test before and after troubleshooting.

Quick Tip: You can also test the CDI box while it still connected to the bike.

Reason for CDI Box Failure

Your bike exposes to lots of vibration, heat, and other environmental effects while you are riding the bike. Every mile and pothole you cover gives a considerable amount of beating to your bike.

Hence, the life of the black box is a rough one. Because of this, the CDI box’s internal parts might fail without any warnings. For instance, the box might show symptoms such as smoking. So, it might be time to replace the CDI box.

What is the Replacement Cost for a CDI Box?

When dealing with a faulty CDI box, you might do two things. First, you might try to repair it with the help of a professional. Or you might replace the entire box. Hence, knowing the price tag of these CDI boxes will help you immensely.

CDI boxes might range between $30 to $900. In some cases, it might pass the $900 mark. The price of the CDI box entirely depends on the model of the bike, the brand, and the features. Also, you’ll have to pay the mechanic for the replacement.

Quick Tip: CDI box replacement is usually the best option (rather than repairing).

Compatibility Issues

Compatibility is one of the most common issues in CDI box replacement. Sometimes, it might be challenging to find a CDI Box according to your model, make, and year of bike. It will be even more complicated when you try to do it alone. So, always seek professional help for this type of issue.

For instance, an experienced mechanic might know what to do in this situation. Otherwise, you’ll end up wasting your time and money.


What is the Primary Function of a CDI Box?

A CDI box controls the motorcycle’s ignition system. It helps the ignition starting process and combustion process. In other words, the CDI box conveys when to fire the spark plug.

Can I Start my Bike without Black Box?

No, you cannot start the bike without the black box. The black box is the spark control, one of three things the engine needs to run.

1. Spark
2. Compression
3. Fuel

Hence, without a black box, the engine won’t start. That means you cannot run the bike.

What are the Parts of a CDI Box?

Typically, a CDI box consists of a small transformer, main capacitor, triggering circuit, and charging circuit:

1. The power supply raises the system voltage to 250 to 600 volts.
2. The current flow to the charging circuit.
3. Then current charges the capacitor.

Where is the CDI Box Located?

Depending on the bike’s model, the CDI box’s location might vary. For instance, some are in the battery box, and some are under the seat.

Can Aftermarket CDI Boxes Go Faster?

Yes. Some aftermarket CDI boxes come with all the upgrades and applications. Most often, they will increase the power of your motorcycle. With a bit of good research, you’ll be able to find a powerful CDI box for your bike. (1)

Are CDI and TCI the Same?

They both have the same functionality (controlling the ignition system). However, the CDI uses capacitor control, and the TCI is transistor-controlled. (2)

Take a look at some of our related articles below.

(1) power of your motorcycle – https://newatlas.com/motorcycles/most-powerful-motorcycles/
(2) ignition system – https://www.britannica.com/technology/ignition-system

Video References


Skill Lync

leporP Autos

Rolling Wrench

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About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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