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Can You Bypass a CDI for Spark? – Quick CDI Troubleshooting Guide

Diagnosing and fixing electrical faults on a motorcycle is not fun, especially if you don’t have much experience with electrical systems. One of the most annoying and confusing parts of the motorcycle electrical system is the CDI. A lot of riders don’t even know what it is, let alone how it works or why it’s needed.

Now, readers often ask us whether it’s possible to bypass the CDI for spark in order to confirm that the CDI has failed and is the root cause of the no start condition.

To answer this question, we put together a quick article discussing everything you need to know about the CDI and whether it’s possible to bypass it. Moreover, we’ll include a guide on how to diagnose a simple 5-pin CDI, and a few tips on how to diagnose a more complex CDI.

Can You Bypass a CDI for Spark? 

In short, it’s impossible to bypass the CDI for spark, as the bike cannot run without the CDI. The CDI is what times produces the spark and controls its timing, therefore it’s not possible to bypass the CDI.

Consider the CDI as the Engine Control Unit of a car. There’s no way to make a motorcycle run without the CDI the same way as it would be impossible to make a car run without a CDI.

The CDI uses a transformer to convert low voltage from the stator or the battery to 200-600 V and stores the energy in a capacitor. When it gets a timing signal from the stator, the CDI quickly discharges the high voltage energy to the ignition coil, which then produces a spark.

In order to bypass the CDI, not only would you need to perfectly time the spark, but you would also need to convert 12 V to 600 V and discharge it instantaneously every 20 milliseconds or so.

What Is a CDI and How Does It Work?

Before we begin diagnosing the CDI, let’s first understand what it is and how it works.

The CDI (Capacitor Discharge Ignition) is an electronic ignition system that is used in motorcycles, ATVs, and other small engines. The CDI is what produces and times the spark that ignites the air-fuel mixture in the engine’s cylinders.

cdi schematic

A typical CDI consists of the power supply circuit, the charging circuit, the triggering circuit, and the main capacitor. More complex CDIs have a more sophisticated timing signal circuit and a CPU to adjust timing.

The power supply circuit of the CDI boosts the input voltage to 200-600 V and then transfers it to the charging circuit. The charging circuit charges up a high voltage capacitor and keeps it fully charged using a rectifier.

When the triggering circuit receives a signal from the signal generator (Hall effect sensor near the stator), it quickly discharges the high voltage capacitor and dumps the high voltage into the primary winding of the ignition coil.

This high voltage induces a much higher voltage (around 40kV) in the secondary winding of the ignition coil, which then produces a spark at the spark plug.

How To Diagnose a 5-pin CDI

The following only applies to the classic 5 Pin CDI.

5 pin cdi pinout

The best way to diagnose a CDI is by using the elimination method. The CDI contains a lot of circuitry inside, which is proprietary to the manufacturer – this makes it extremely difficult to test it using the continuity setting on a multimeter. So much so, that even nearly all motorcycle service manuals tell you that the only way to diagnose a CDI is by using a proprietary CDI tester. Not only are they incredibly expensive, but they’re almost never available for purchase.

Instead, we’ll confirm that the CDI is getting good power, ground, and everything else it needs to function. If the CDI is getting good power, ground, and all of the other inputs are working properly, then the only logical conclusion is that the CDI itself is bad.

1. Verify That the Ignition Coils and Spark Plugs Are Good

If you haven’t done so already, first, check whether the ignition coils are good. To do this, you’ll need an Ohm meter.

Test the continuity on the primary side of the ignition coil – it should be less than 1 Ohms.

Next, measure the continuity of the secondary winding. The multimeter should read a resistance of around 6k Ohms.

Check the service manual for the exact specifications.

If the coils are good, then move on to checking the spark plugs. The spark plugs should be replaced every 20,000 miles or so – it’s a good idea to replace them while you’re diagnosing the CDI, just in case they’re the cause of the problem.

To check the spark plugs, remove them from the engine and inspect them. Look for any cracks, carbon buildup, or other damage.

2. Bypass the Kill Switch

This is the most common reason why a CDI won’t work. The kill switch is a safety feature that kills the engine when it’s engaged.

You can test whether the kill switch circuit is good, but you can also bypass it. To do that, remove the center pin from the 5-pin CDI connector. You can also cut the wire temporarily, but please remember to reconnect it after you’re done troubleshooting.

3. Verify Whether the Cdi Is Getting a Good Ground

If the kill switch bypass above did not work, then it’s time to check the grounds.

Use a test light to test the ground wire going to the CDI. Why a test light? A test light will load the circuit and expose any bad grounds. A multimeter, on the other hand, will not load the circuit, thus it’s unreliable.

4. Verify the AC Power Feed

Next, it’s time to check whether the CDI is getting enough power to work. Be extremely careful when checking the power feed as it does contain high AC voltage, which could shock you. If you’re not comfortable working with high voltage, then please take the bike to a qualified mechanic.

Start cranking the engine and measure the AC voltage at the ac power feed pin. You should be seeing at least 50 VAC – if it’s less than that then the AC won’t work and you need to replace the stator.

Also read: 6 Most Common Bad Stator Symptoms

5. Verify the Timing Signal

Lastly, it’s time to check the timing signal circuit.

The CDI gets a timing signal from the magneto inside the alternator. To test the timing signal use an oscilloscope, or use a multimeter to measure the VAC at the timing signal pin.

You should see a fluctuation of 0.2 to 0.4 VAC.

6. Replace the Cdi (If the Inputs Are Ok)

If you’ve checked and verified that the CDI is getting everything it needs to work, but it’s still not working, then the only logical conclusion is that the CDI itself is faulty and needs to be replaced.

There are aftermarket replacement CDIs available, but they might not be as good as the OEM ones. If you decide to get an aftermarket CDI, make sure it’s from a reputable manufacturer.

How to Diagnose More Complex CDIs

Unfortunately, since more complex CDIs have more pins, they are virtually impossible to test using a multimeter.

Your best bet is to apply the same elimination method above. Different CDIs will have different pinouts, so check the wiring diagram on your service manual for guidance.

Once you figured out which pin is which, verify that every single input to the CDI is working correctly. If even one input is not working, then the CDI won’t work.

If everything looks good, but the CDI still doesn’t work, then the only logical conclusion is that the CDI itself is faulty and needs to be replaced.

Conclusion

As you can see, troubleshooting a CDI is not easy, but it’s still possible to do it yourself if you have the right tools and know-how.

The CDI is an essential component of the motorcycle’s ignition system, and without it, the engine will not run. Unfortunately, it is impossible to bypass the CDI for spark, so if it’s not working then the only option is to replace it.

By following the simple steps above, you should be able to diagnose and fix most CDI problems. However, if you’re not comfortable working with high voltage or complex electrical systems, then it’s best to take the bike to a qualified mechanic.

7 thoughts on “Can You Bypass a CDI for Spark? – Quick CDI Troubleshooting Guide”

  1. Hello have a good day.
    I am working with1775 Honda cb125s motorcycle.I going to convert it to DC cdi system.my program is its not given spark.when I check I found some suspect thing.cdi ignition coil supply line provide continuously current to the ignition coil.with out rotating the engine.I want to know is it normal or my cdi is bad .thank you…

    1. Hi!

      It’s normal for the ignition coils to have a constant power feed, but the ground side should be pulsed by the CDI. Check out the Ignition diagram above for a better idea of what I mean. The coils in the diagram are receiving constant power with the key on.

      With that said, I’m not familiar with the CB125s ignition system, so you’ll have to double-check it with a service manual.

      Hope this helps!

  2. Hi Nick.

    I’m struggling with the CDI on my 1986 Honda XBR500 (single cylinder 500cc). The CDI seems to be a hybrid between AC and DC as it should start the bike on the kickstart with no battery fitted but when using the electric start there is a 12V DC applied to the unit.

    The original symptoms were that the engine would start and idle quite well but would start missing at around 3,000 rpm. This got progressively worse and eventually the engine died and I had to get recovered home about 10 miles.

    I have a replacement CDI unit from a Dutch company but am still not getting a spark. I’ve tried going through what you suggest above – all earths are OK, resistances on pulser, exciter and ignition coil are correct. I get approx 50 volts AC from the exciter when cranking and what looks like .1 – .2 volts (varying) from the pulser. I have tried putting the latter through an android oscilloscope app but have to say I have no idea what I’m looking for. There is a signal but it seems quite random and jagged to me.

    Any ideas would be most welcome.

    1. Hi Ian,

      You’ve done well diagnosing the issue, but there’s probably some small detail that you’ve missed or need to double check. There’s a free service manual for your bike on manualslib.com – it has lots of great information specific to your bike. It even shows how to test your CDI so you can be pretty sure that it’s good.

      With that said, the CDI should be good if it’s new and from a Dutch manufacturer, but since you have an oscilloscope (HScope app I assume), you can check if the CDI is outputting a signal to the ignition coils. If there’s nothing coming out of the CDI, then there’s a problem with the inputs to the CDI itself, or the CDI is bad. That way you can narrow it down. You can also compare the signal to a good one (google for motorcycle primary ignition waveforms).

      Be careful though, the signal to the ignition coils is over 200V and you need a 20:1 attenuator for your scope in order to see the signal safely.

      Lastly, make sure that the battery is good and also check the air gap between the pulse generator and the rotor tooth – it should be around 0.8mm (it’s all in the manual linked above).

      Hope this helps.

  3. Hi Nick.

    Many thanks for the very prompt reply.

    I have a copy of the manual and have been using it to check resistance values etc.

    I’m a bit reluctant to connect the CDI output direct to my android device – although the connecting cable is attenuated it’s some years since I put that together. Ironically chasing down a similar problem on a Honda 400 4 which turned out to be a defective plug cap eventually – which is why I’ve fitted a new cap here….

    If I connect my multimeter on a high voltage setting across the CDI output would I see something? I have both digital and analogue meters.

  4. The answer to my last question was “No” but I don’t know whether it was because there was nothing to see, there was something but it didn’t last long enough to show up (I use the analogue meter for testing capacitors) or for some other reason…..

  5. In desperation I’ve bought this: https://www.ebay.co.uk/itm/394249416594 – initially I’ll try just the coil with my existing setup. If unsuccessful will try the CDI unit. The pinouts are the same except the XBR supplies 12 volts to the unit while cranking but I have cut this lead and inserted a connector so I can disconnect it.

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