Motorcycle Ignition Systems: What Are They and How Do They Work?

Key Points

• A motorcycle ignition system is crucial for the engine to run, as it generates the spark needed to ignite the air/fuel mixture within the engine. This system must produce high voltage and deliver the spark at precisely the right time for efficient operation.
• The ignition system is one of three key electrical systems on a motorcycle, alongside the starting and charging systems. It has evolved from mechanical points to electronic ignition systems for improved reliability and performance, especially at high RPMs.
• There are three main types of electronic ignition systems found in modern motorcycles: AC-CDI (Alternating Current, Capacitor Discharge Ignition), DC-CDI (Direct Current, Capacitor Discharge Ignition) and DC-TPI (Direct Current, Transistorized Pointless Ignition). These systems control the timing of the spark plug firing to meet engine demands.
• When it comes to ignition systems, you’ll commonly hear of a CDI box, or capacitor discharge ignition box, whichconsists of several key components that work together to ensure the engine receives a strong and timely spark, essential for efficient combustion and engine performance.
• Motorcycle ignition coils transform the battery's low voltage into the high voltage needed to jump the spark plug gap. Timing is critical, as the spark must be delivered at the right moment for optimal engine performance. Modern systems automatically adjust spark timing based on various engine parameters.
• Troubleshooting ignition systems involves several steps, including checking the battery, pulser coil, fuses, switches and ignition coil resistance. Understanding and being skilled in diagnosing and repairing ignition system issues is valuable for motorcycle technicians, highlighting the importance of training in this area.

From suspension systems to carburetors to alternators, there’s a lot to know when it comes to working on motorcycles. Not only is it critical to know these individual components, it’s important to understand how they all work together.

One of the most essential systems on a motorcycle is its ignition system. For a motorcycle to run, its ignition system must function properly. If it breaks down, a variety of issues can occur.  

So, what exactly are motorcycle ignition systems, and how do they work? Keep reading to learn about ignition system components, like the CDI box, and how they work to deliver the spark that engines need to power a motorcycle.

What Is a Motorcycle Ignition System?

A motorcycle ignition system’s main purpose is to create the spark, which ignites an air/fuel mixture within the motorcycle engine.

The spark has to occur when the piston is close to the top of the compression stroke, so the compressed air/fuel mixture burns properly, and the expanding gases force the piston down the cylinder. Not only does the ignition system produce high voltage needed to fire the spark plug, but it also must send the spark at exactly the right time!

The ignition system is one of three separate but related electrical systems on a motorcycle. Also present is the starting system and the charging system. The starting system contains the battery, ignition switch and starter motor. The charging system contains the stator, rectifier/regulator and battery.

A Brief History

Both points and electronic ignition systems control spark delivery to an engine. Electronic ignition is an evolution from a mechanical system to an electronic one. Before we go further, let’s break down what points are and why they had to evolve.

Motorcycle ignition points

Ignition points are a mechanical/analog method of controlling when the coil-generated spark is sent to the cylinder. Points work great, until they don’t. Maintaining the system is critical to proper function. Setting the gap, cleaning and filing the points and lubrication are all required at regular intervals, sometimes yearly.

Points are a mechanical switch. A spinning cam controls the opening and closing of the switch, which can be seen in the image below:

Image Credit: Honda Twins

In this image, there are two sets of mechanical points. As the center shaft spins at engine speed, the 90-degree rocker arms open and close the switch. This completes and breaks the circuit, resulting in a spark from the ignition coils.

From point to electronic ignition

Automotive and motorcycle manufacturers transitioned to electronic controlled ignition for two main reasons: reliability and performance (specifically during high RPM). While electronic ignition dates to the 1960s, its widespread acceptance occurred in the ’70s.

There are three main types of electronic ignition systems found on modern motorcycles:

1. AC-CDI: Alternating Current, Capacitor Discharge Ignition
2. DC-CDI: Direct Current, Capacitor Discharge Ignition
3. DC-TPI: Direct Current, Transistorized Pointless Ignition  

Regardless of the voltage type, alternating or direct current, these systems trigger the spark plug to fire at precisely timed intervals, allowing the motorcycle to run. Additionally, these ignition systems have the capacity to advance or retard ignition timing based on engine demand.

What Is a CDI Box?

When it comes to ignition systems, you’ll commonly hear of a CDI box or capacitor discharge ignition box. It's also known as an igniter box, black box or powerpack.

A CDI box consists of several components, each playing a crucial role in its operation:

• Capacitor: The capacitor stores electrical energy, charges up to a high voltage and then releases energy to create the spark needed for ignition.
• Charging circuit: The circuit is responsible for charging the capacitor. It typically receives power from the motorcycle's battery or a charging coil.
• Triggering circuit: This component receives signals from a sensor or mechanical switch that indicates the engine's position to determine the precise moment the capacitor should release its stored energy.
• Transformer: The transformer increases voltage from the capacitor to a level high enough to create a spark at the spark plug.
• Ignition coil: Although not housed within the CDI box, the ignition coil works closely with the CDI system to convert the high voltage from the transformer into a spark.

These components work together to ensure the engine receives the strong and timely spark essential for efficient combustion and engine performance.

In addition to the CDI box, other components of modern ignition systems include:

• Battery
• Coil
• Spark plugs/spark plug wires
• Switch
• Pickup coils or crank position sensors

How the Ignition Coil Works on a Motorcycle

As mentioned above, the ignition coil and CDI box on a motorcycle go hand-in-hand.

If a motorcycle battery only produces 12 volts, and the CDI box requires 12-20,000 (or more) volts to jump the spark plug gap and ignite the air/fuel mixture, how is that addressed?

Motorcycles use a coil to ‘step-up’ the battery’s voltage to the correct range. A motorcycle ignition coil is a transformer consisting of two separate coils of wire, wound around a central post.

The coil’s primary winding consists of a few turns of heavier-gauge wire. The secondary winding is much finer wire, wrapped many more times. This two-winding system greatly increases voltage up to the level needed to fire the spark plug.

Mutual induction is the phenomenon where voltage in one wire induces voltage in another wire, even when the wires are not connected. More importantly, mutual induction allows for changing power’s characteristics, such as a significant step up in voltage within the coil.

Timing is everything

Even the highest voltage spark isn’t effective if it is delivered to the engine at the wrong time. At best an improperly timed engine will suffer from poor performance, and at worst an improperly timed engine will be destructive to itself.

Modern motorcycle engine timing is computer controlled, using inputs such as:

• Engine RPM
• Percentage of throttle opening
• Engine temperature

To complicate matters, the precise time to deliver a spark of optimal performance varies. Ignition timing is not a “set it and forget it” task. As engine speed (RPM) increases, the spark plug needs to fire earlier in relation to the piston’s travel within the cylinder. To “advance” ignition timing is to deliver the spark earlier; to “retard” ignition timing is to deliver the spark later.

Older motorcycles, prior to electronic engine control and the CDI box, used simple mechanical devices to advance or retard time based on engine need.

While these basic systems worked, they were limited by their need for upkeep and their inability to deliver on increasingly stringent emissions requirements. Modern electronic ignition systems automatically adjust spark timing to compensate to a range of engine demands.

Stick coils like the one shown below are an all-in-one unit that sits directly on top of each spark plug. This eliminates the need for the heavy gauge high voltage plug wires that are found in older designs.

Image Credit: JDK Motorcycle

Troubleshooting Ignition Systems

Where does a technician start when it comes to troubleshooting or diagnosing a motorcycle ignition system?

The first step in any diagnostic procedure is to start with a known good battery. A battery that illuminates the headlight isn’t guaranteed to have the voltage required to start and run a vehicle. A tech may charge and test a battery before troubleshooting ignition system challenges or may simply install a new, known good battery.

Other common ignition system tests are:

• Checking the pulser coil for continuity.
• Checking the pulser coil for voltage output.
• Checking fuses, switches and ground wires.
• Checking ignition coil resistance on both circuits.
• CDI box: This can be tough to troubleshoot. If everything else checks out, swapping in a new CDI box may help identify the problem.

FAQs

Which ignition system is better: CDI or TPI?

The motorcycle CDI ignition system is known for delivering a quick and powerful spark, which is beneficial for high RPM performance and efficient combustion.

TPI systems offer more control over ignition timing, which can lead to better fuel efficiency and smoother engine operation. The choice between CDI and TPI depends on specific performance needs and vehicle design.

What is a CDI box on an ATV?

A CDI box on an ATV operates like one on a motorcycle. The electrical device controls the ignition system by storing and discharging electrical energy to ignite the air-fuel mixture in the engine.

Are CDI boxes interchangeable between different motorcycle models or brands?

CDI boxes are not interchangeable between motorcycle models or brands due to variations in engine configurations, sensors and ignition management requirements. Even within the same model, different versions might require specific CDI units to function correctly.

Learn About Motorcycle Ignition Systems at UTI

Along with starting and charging systems, ignition system components are critical to safe and reliable motorcycle operation. Motorcycle ignition system problems can be intimidating to diagnose, especially compared with mechanical failures that may be visible.

Technicians who understand and are skilled in this area are in demand. Taking the time to gain experience and develop the skill of troubleshooting and repairing ignition system issues can be a great benefit to your career down the road.

In Universal Technical Institute's 48-week Motorcycle Technician Training program, you can learn the foundations of motorcycles, ATVs, side-by-sides and personal watercrafts to prepare for a career as a motorcycle technician.^1,7 To learn more, request information and an Admissions Representative will reach out!

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