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

Nov 23, 2020 ·

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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 in a motorcycle is its ignition system. In order for a motorcycle to run, its ignition system must function properly. If it breaks down, a variety of issues can occur, which is why the role of technicians who can diagnose these systems is so important. 

So what exactly are motorcycle ignition systems, and how do they work? Keep reading to learn about ignition system components and how they work together to deliver the spark that engines need to function properly.

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, it has to 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 need 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.

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 back to the 1960s, its widespread acceptance occurred in the ’70s.

There are three main types of electron 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, 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.

CDI Ignition System Motorcycle

When it comes to ignition systems, you’ll commonly hear of what’s called a CDI box, or capacitor discharge ignition box. Essentially, a CDI box controls a motorcycle’s ignition system by starting the ignition and combustion process. A pulse of voltage from the motorcycle battery passes through the CDI box to fire up the spark plug.

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

While very reliable, these components can fail, causing a motorcycle to stop running properly. A skilled technician can test and diagnose ignition system components quickly and reliably.

The two main objectives of a motorcycle ignition system are:

  • Generating the voltage needed to ignite the air/fuel mixture
  • Delivering the spark to the engine at precisely the right time

How Ignition Coil Works on a Motorcycle

If a motorcycle battery only produces 12 volts, and 12-20,000 (or more) volts are needed 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, 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

Ignition System Events

The image below identifies ignition system components and their function:

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 they 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.

How Does This Apply to Motorcycle Technicians?

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 to the eye.

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.

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