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CDI unit faulty - symptoms & how to repair.


CDI unit faults are not easy to spot. You must make sure other common issues are not the reason for the trouble.

CDI in advanced vehicles (with fuel injectors and all) are like little computers.

Even the “not so advanced CDI Units by today’s standards are” are highly tech involved.

AC CDI & DC CDI

Both usually have same number of connecting port. Only difference is that while AC CDI is directly powered by pickup coil, DC CDI is powered by battery.

Therefore ‘DC CDI’ which is little bit bigger than AC Counterpart, need battery power to work.

And a scooter equipped with ‘AC CDI’ can work without battery.


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CDI unit fault symptoms.

Fully faulty CDI unit - symptoms.

When CDI unit goes wrong, it is a good thing. Compared to partially faulty one at least.

Because it is easy to spot and fix when it is completely gone. You can first test spark plug, then ignition coil & then ensure that battery is good.

Then, check wires from pick up coil with multi-meter & if you have reading, then most likely it is CDI unit.

This is kind of the worst case. Because it is not easy to spot the culprit. Because you will face problems starting from misfire, lose power, sputtering etc. to scooter not starting at all.

CDI unit partial fault symptoms.

Partially working CDI unit - symptoms.

  • Miss firing and lose of power
  • Scooter not starting
  • Problems after riding few kilometers.
  • No spark
  • spitting & sputtering.
  • losing power

As you can see, any other part can cause these problems . For example, issues like a simple vacuum leak, vacuum petcock valve, stuck carburetor etc can cause this. So not that easy to spot.

Most common CDI issues

Wiring short. - This is a simple fix if you can find the exact spot. Sometimes corrosion or a chipped insulation will cause the wires from the pickup coil or to the battery/ignition coil shorting.

CDI getting faulty because of stator issues (charging system fault) - This one is tricky because if you change the CDI unit it might work for few months/days but will again stat acting up. Then if you replace the pickup coil, new CDI with be faulty by that time.

Old CDI. - CDI units go bad when they are very old.

Repairing vs replacing.

Easiest way is of course replacing the part if it is faulty.

If you want to try DIY, you can open it and re solder every point in the system.

Written on November 3, 2021
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