Motorcycle CDI Unit

Motorcycle CDI Units Simplified

Most motorcycle CDI units are plug & play devices. But they are not all the same. When sourcing a new regulator rectifier there are two essential things to find out first:

1. The number of wires & connector type needed
2. The power supply type (AC or DC)
3. Desired RPM limit
4. Timing advance curve

Motorcycle CDI Wiring Basics

CDI units come in many different formats. The most important thing to remember is that AC input CDI units take their power supply from the engines stator. Whereas DC input CDI units take their power supply from the motorcycle battery.

CDI Standard Wire Colours

Most OEM’s have adopted subtle variations of Honda’s standard wiring colours from the classic motorcycles of the 1970’s. This is a breakdown of the most commonly used wire colours and what they mean…

Red or Red&Black = AC Power Supply Input (from the stator to the CDI)

Green = V- (from the CDI to the engine ground)

Blue or Blue&Yellow or Blue&White = Trigger (from the pickup coil to the to the CDI)

Yellow = Ignition Coil HV Output (from the CDI to the ignition coil

Black or Black&White = Kill Switch (from the CDI to a kill switch)

Of course, not all manufacturers use these same wire colours. Especially in their wiring looms. It is highly advisable to cross reference your bikes wiring manual. Alternatively, talk to your local auto-electrician before starting. However, the majority of aftermarket wired CDI units have adopted these same wire colours.

CDI Standard Pin Layouts

Check out our electrical wiring diagrams for the generic pin layout configuration of each CDI unit connector type. Some manufacturers may design their own components to their own specifications. Therefore, always check the wiring carefully. It’s always best to trace wires back to their source.

Different Regulator Rectifier Specifications

Although the internal electronics and overall build quality of motorcycle CDI units vary between manufacturers. When sourcing a new CDI unit, it’s important to consider evaluating the following before diving straight in.

Power supply type

A DC CDI unit requires a Direct Current electrical input. This is normally supplied from the motorcycles on board 12 Volt battery.

An AC CDI unit requires an Analog Current electrical input. This is normally supplied from the motorcycles stator coils. The electronic circuitry of AC and DC input CDI units is very different.

DC CDI units require many more components, so they are larger in size. Connecting the wrong supply type to a CDI will cause irreparable damage.

RPM limiter

Many motorcycle CDI units have a built in RPM limiter to prevent damage caused by the engine revving to high. It’s important to match this to the motorcycles engine specifications to ensure smooth running and prevent engine damage.

However, there are racing CDI units available on the market that do not have a built in RPM limiter. Use these at your own risk.

Timing advance curve

This is where it gets technical. Timing advance is required in some applications to ensure that the engine runs smooth throughout its rev range. Our best advice is to check the owner’s manual of the motorcycle engine and reference the ignition specifications.

Mechanical dimensions

Getting the dimensions right when buying a new CDI unit will save you whole lot of headache when it comes to installation. Consider these two things:

1. The space that’s available
2. The type of fitting
   • Bolt hole
   • Rubber strap

It’s also wise to check the length of cable that’s available on your loom. Just to make sure that it will reach.

Connectors

Unless you plan on crafting your own electrical connections this can make sourcing a replacement rather problematic. CDI units are commonly found with one of the following connector types:

Top Tip

Most motorcycle connectors have replaceable terminal pins. This means that the pins can be relocated in the connector housing. So, there’s no need to worry don’t worry about wire positioning in connectors. This can be remedied easily with a sewing needle at home…

• Lightly bend down the latch on the terminal pin to remove it from its connector housing.
• Then, make sure that the latch is still in good condition, before finally clicking it into the location you need.

Motorcycle CDI Diagrams & Schematics

Click on a pane below to find links to our motorcycle regulator rectifier mechanical and electrical diagrams.

Motorcycle CDI unit Guide Links

Tap or click on a button below to navigate to one of our motorbike regulator rectifier and associated parts guides.

How do CDI units work? – Under Development

Motorcycle CDI Testing

Properly testing a motorcycle CDI requires some technical thinking. Understanding some of the results requires an understanding of some basic engineering principles.

That’s why we’ve created an entire webpage, devoted to giving you the low down on CDI unit testing. Click on the button below to get a thorough understanding of how to test motorcycle CDI units and diagnose faults.

How to test a CDI unit? – Under Development

CDI unit Compatibility

Here are a few things to extra points to consider regarding the cross compatibility of motorcycle regulator rectifiers.

AC vs DC

As previously discussed, AC and DC supplied CDI units are not cross compatible. Getting this wrong will damage the CDI and potentially burn the bikes wiring loom. If in doubt, consult your local auto-electrician.

Old system conversions

It is possible to swap an old points-based ignition system for a modern CDI unit. The key is being able to install a hall sensor in the right place to provide the trigger signal.

It’s not an easy conversion but it has been done, with great success, by a number of bike building experts. You can even buy conversion kits for popular classic bikes.