How Does a Motorcycle Capacitor Discharge Ignition Work?


Most motorcycles built after 1980 use a Capacitor Discharge Ignition System to power their engines, replacing the older points-type mechanical contact breaker ignition. CDI ignitions provide the engine with powerful, reliable power and require very little maintenance. Understanding what the CDI is and how it works can help you weed out problems within your motorcycle's ignition system.

Capacitor Discharge Ignition Construction

  • A basic Capacitor Discharge Ignition system is composed of four separate components: the CDI box, pickup coil, source coil and an ignition coil. The CDI box is the heart of a motorcycle's ignition system, containing a series of diodes and semiconductors that amplify the 12-volt current supplied by the motorcycle's battery between a 400- to 600-volt range. The amplified current transfers to a capacitor, a device used to store an electrical current for short periods, housed within the CDI box. The pickup and source coils, a pair of hall-effect sensors responsible for charging and discharging the capacitor, mount near the magnetic flywheel attached to the end of the engine crankshaft. At the output side of the ignition system is the ignition coil, which amplifies the current further before releasing it into a spark plug.

Basic Operation

  • As the motorcycle's engine runs, a notch cut into the side of the magnetic engine flywheel passes between the source and pickup coils. The sudden drop in the magnetic field triggers the source coil, allowing a 12-volt current to flow into the CDI box. The current amplifies to a higher voltage, usually between 400- and 600-volts, and is stored within the capacitor. A drop in the magnetic field triggers the pickup coil and the capacitor discharges its stored current into the ignition coil. The current passes through a tightly wound wire within the ignition coil, called a primary coil, where it is amplified further to an excess of 10,000 volts. The final output current feeds through the ignition coil spark plug cable and into the spark plug, where it discharges within the engine's combustion chamber. This cycle repeats itself at every revolution of the engine.

Maintaining a CDI System

  • Under normal circumstances, a CDI-type ignition system requires very little attention aside from regular spark plug changes. Some motorcycles allow for some adjustment in the ignition timing, which changes the points at which the source and pickup coils are triggered, by using a movable coil plate. Ideally, the pickup coil should trigger the ignition coil just before the engine's piston reaches the top of its stroke. Moving the plate against the flywheel's rotational direction advances the timing and creates a spark sooner. Alternatively, moving the plate in the same direction of the flywheel retards the timing, generating the spark later. A mark along the edge of the coil plate will indicate the timing position relative to the position of the piston. Adjustments to the ignition timing are not usually required unless the motorcycle is being used for competition. Such adjustments can affect the machine's reliability unless performed carefully.

Troubleshooting A CDI ignition System

  • Most CDI ignition problems experienced in a street-ridden motorcycle originate within the charging system or battery. In order to generate the current needed to produce a spark, the battery must supply the CDI box with a full 12-volt current. Unfortunately, the battery must also power the motorcycle's lights as well. If your motorcycle's engine turn over, but doesn't start, completely charge the battery with an automatic battery charger before checking anything else. If the motorcycle still refuses to start, perform tests with a multimeter to determine if the CDI box incurred damage. Factory service manuals provide the testing procedures and CDI box specifications needed by your motorcycle.


  • "Haynes Motorcycle Workshop PracticeTechbook"; John Fidell; 1999
  • "The Essential Guide to Motorcycle Maintenance"; Mark Zimmerman; 2004
  • The Professional Motorcycle Repair Program; Professional Career Development Institute; 1995
  • Photo Credit Jupiterimages/BananaStock/Getty Images
Promoted By Zergnet


You May Also Like

  • How to Test Ignition Capacitors

    How Does a Motorcycle Capacitor Discharge Ignition Work? What Is an Ignition Coil Capacitor? How to Troubleshoot a Poulan 4200 Ignition;

  • How do I Eliminate a Motorcycle Battery?

    Secure the capacitor to the motorcycle's frame using the kit's metal strap. Select a suitable bolt within the battery compartment and remove...

  • How Does a Motorcycle Engine Work?

    Motorcycle engines work essentially as a big air pump. Discover how this four stroke engine works as explained by a certified motorcycle...

  • How Does a Starter Relay Work?

    Like most other motorized or powerful electric devices on your car, the starter doesn't draw its power directly from the switch used...

  • How to Test the CDI Ignition of a Motorcycle

    Capacitor discharge ignition systems are usually a reliable means of generating high-energy sparks for motorcycle engines. Common configurations of these systems ...

  • How to Check CDI on a Yamaha Warrior

    A weak or absent spark is a symptom of problems occurring within the CDI unit or the ignition system. ... How Does...

  • How Does Motorcycle Point Ignition Work?

    Primary voltage to the coil runs through the points. The points are closed most of the time, and this is when the...

  • Motorcycle Ignition Problems

    Motorcycle ignition systems are exposed to very harsh operating conditions. Many components are exposed to the elements. High engine temperatures effect the...

Related Searches

Read Article

How To Travel For Free With Reward Points

Is DIY in your DNA? Become part of our maker community.
Submit Your Work!