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Common Tread

What is CDI? What does a CDI box do?

Jul 19, 2018

Black box, brain box, pulse pack, igniter box, CDI module, power pack, whatever you want to call it: if your motorcycle is newer than 1980, it probably has a black rectangle spouting wires under the seat that makes the magic happen. If you're lucky, you're reading this for your own education. If you're not, you're wondering what a CDI does... because you suspect yours doesn't. Let's take a look inside.

What is CDI?

CDI stands for capacitor discharge ignition (alternatively, “capacitive.”) If you’re new to capacitors, they’re similar to batteries in that they can store energy for later. What sets them apart is their ability to release all that energy near instantaneously, which is ideal for an ignition circuit.

Suzuki DRZ250 CDI box
Here's a CDI box in a dirt bike. You'll often find them lurking under your seat. Andy Greaser photo.

The basic CDI system is a trigger mechanism, coils, and a box, often black, with capacitors and other circuitry inside. The trigger tells the box to fire, the box determines when to fire which coil with the capacitors, and zap goes the spark plug, ad infinitum. In addition to dischargin’ those capacitors, the box may also influence your rev limit, timing advance, and other variables related to spark, but that’s it.

A popular variation of this system is TCI, which doesn't use capacitors in the same way. For the purposes of this article, I'll refer to all the black box ignitions as CDI, since they all use the same basic components: box, coils, sensor.

Yamaha FZR600 TCI box
This is a TCI box. On an electrical engineering level, it's different from CDI, but the troubleshooting process is the same. Andy Greaser photo.

A little CDI history

CDI's complexity is in its circuitry, but its ancestors relied on clever mechanical systems to fire the coils. Springs, weights, cam lobes, and other simple machines worked together to deliver spark at the right time, even as revs rose. These ignitions did just fine for decades, but those moving parts wear out over time. Motorcyclists accepted occasional maintenance in exchange for rugged reliability, easy adjustment, and cheap cost of replacement.

KZ400 points ignition.
In a points system, the crank basically opens and closes a switch. Eventually, your switch wears out. Not so with modern sensors. Andy Greaser photo.

That's all great, but the jump to CDI was like going from carburetors to fuel injection. Electronics gave manufacturers new control over their engines. The ignition advance, once mechanically limited to a curve, could be tuned to squiggle any way you liked all the way to redline (read Lemmy's timing article if you'd like more on this). Increased power, better efficiency, less maintenance, and fewer moving parts earned CDI, and systems like it, a good reputation. But that doesn't mean they stay problem-free.

Troubleshooting CDI systems

So you’ve got a spark problem on your CDI-equipped motorcycle. Symptoms could include misfiring, dead cylinders, backfiring, bizarre tach behavior, and countless other things related to how your engine's running. The problem may get worse as the bike warms up. It might not even hold low revs at all.

You’ve tested everything but the CDI unit, including the charge of your battery, your spark plugs, the health of your coils, and the triggering mechanism itself. Could it be your black box? Sure, particularly if your motorcycle is older, but still new enough to have CDI. Black box ignitions tend to fail all at once, unlike the gradual decrease in performance you’d expect from a points-ignition motorcycle. (Some riders retrofit points-based systems onto their newer motorcycles to avoid the shortcomings of CDI systems. Conversely, plenty of other riders swap factory points-and-condenser setups out for more modern ignition.)

The key is eliminating every variable until only the box remains, because they’re not easy to troubleshoot. Most are sealed up like a sarcophagus. These systems are supposed to be maintenance-free, but even a loose trigger sensor can mess with your bike’s little brain. Some manufacturers give specs for testing your box pin-by-pin, but don’t count on it. The best solution when troubleshooting black boxes is to replace a suspicious one with a known good part after checking the other possibilities first. Be careful when buying replacements. An eBay special may have the same issue as yours, and a new replacement may not be returnable once installed.

Yamaha Vision CDI sensor pickup
See that rectangular bump on the flywheel rotor of this Yamaha Vision? A magnetic pickup senses it as it goes by. photo.

Why do black boxes stop working in the first place?

The box itself is packed with electronics, and delicate components inside can fall victim to vibration and moisture over time. Thousands of miles of bouncing down the road can take its toll. Heat can also damage or destroy a black box. Finally, if you see the Magic Smoke come out, time for a new one. 

I heard you can buy an aftermarket CDI/TCI box and go faster. Is this true?

Sure, there are aftermarket options for all kinds of applications. Most offer more power, and the best units come from reputable tuners. Do your research before buying one. You can also gain power by keeping your stock box, but using an ignition advance kit to alter the trigger mechanism. Again, read up on safe options for your motorcycle before attempting. I've tried both with good results.

If you’d like to know more about black boxes, the engineering behind them, and troubleshooting for your specific motorcycle, there are endless resources online to check out. We owe countless miles to these creations. Might as well know a little about what’s happening under your seat.