Casting light on the secrets of the black box

Published: 01 September 2000

YOU know what the problem with that is... that’s your cerebral oscillator confounding your collectivisor, that is...

Modern bikes can be such conundrums of electronic complexity that you won’t be alone if your eyes start glazing over when your mechanic begins explaining why yours won’t start.

It’s easy to give up, hand over the key and wander off in the hope that there’s a taxi rank nearby, without ever knowing what the fault really is – or what it is going to take to fix it.

Ignition and fuel injection systems are becoming increasingly baffling. But if you understand the terms, at least you’ll know the right places to nod when your local tuning guru talks you through what he’s going to do.

We turned to Superchips (01280-816781) to give you the basics on the three most commonly-found ignition control systems in use on bikes today – the black boxes often found lurking in the shadows under your seat.


CDI is the elder statesman of current electronic systems, and you’ll still find it on small-capacity bikes. CDI stands for Capacity Discharge Ignition. It works by recording and reacting to each turn of the crank to fire the required spark into the combustion chamber. The whole CDI system includes the ignition windings and a pick-up on the end of the crank. The information the pick-up receives is interpreted by the CDI unit itself – a black box full of transistors and diodes. After assembly it is sealed in a tamper-proof resin. You can’t repair them, you just replace them when they go wrong.


Electronic Control Units are a more state-of-the-art version of the old CDI. Using digital technology they can react a lot faster and deal with more complex demands – of the kind you get with a modern high-revving bike engine. They are controlled by a programmable silicon chip, or EPROM (Erasable Programmable Read Only Memory). By changing the chip you can change the engine’s fuel/ignition characteristics. The right chip is sometimes all you need to raise power. ECUs can also run ABS brake systems and traction control.


Some bikes already have this, and more are going to get it, as an Electronic Management System is the next generation of ignition control. They run with even higher-speed chips and you don’t have to replace them when you make changes. Plug into the bike’s diagnostic ports and you can update the ignition curve fix a fault or retune for more power at the stroke of a few keys. If an ECU can be likened to a pocket calculator then an EMS is a programmable computer.