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Last Updated: October 12, 2014

Testing Motorcycle CDI Ignition Coil

CD ignition coil is a step up transformer that transform the voltage generated by the internal HV converter of a CDI into 20 to 40kv in order to achieve a spark at the tip of the plug. Do not be mistaken, CDI coil differs from TCI coil, and almost all available Ignition coil has only one purpose, To produce an arc to spark plug inside the combustion. Photo shown are some variants of ignition coil used on AC-CDI and DC-CDI. At top is china made ignition coil derived from a certain pioneer brand bike, center coil came from my GY6 scooter, and the bottom taken from my existing suzuki shogun 125cc commuter bike. The article will tell us the basic test of ignition coil, that is, resistance and inductance test. The benefits of knowing their resistance and inductance will tell us if the coil will be providing enough energy for our ignition beyond optimum level.

Primary winding compose of few turns thus resistance reading will show milliohms reading and almost zero  when using ordinary DVM, thus inductance may be a better choice for an instrument to test it, while as the secondary compose of a hundred turns, it is capable of resistance test.

here are the variation of reading of those three mentioned CDI coil above

The primary winding of the China made CD coil is about 34 uh, when i tried to take resistance test on this side, i only gain 0.3 ohms










for the GY6 coil, i got 29 uh still with a resistance of 0.3 ohms











and for my OEM ignition coil (suzuki shogun 125cc coil) got a reading of 66 uh but still with a reading of 0.3 ohms.











I did this test to be able to determine how it will affect my ignition before i tried to test it on board my motorcycle with the exception of my OEM. Yes they will all work since they are made for a CD ignition. and this inductance will primarily affect the transfer of energy towards the secondary winding of the coil.

For the secondary winding basic test, i use the resistance check, for my homemade inductance tester can only reach the 1 mh range.

Secondary resistance of the china made was 5.65 kohms











GY6 secondary winding resistance of 3.00 kohms












and OEM stands at 6.57 kohms












Higher resistance reading of the secondary means more number of turns and may transform it to a higher voltage output when the primary is being switched by the ignition controller, but then the primary winding also has an adverse effect on the secondary winding. Only by using HIGH voltage tester, we can assure of the reading, but because ignition coil are HIGHLY shield by epoxy resin, we cannot determine if the theory is in fact true that the more number of turns and higher resistance reading will produce more hotter spark.

Soon i will try to make an IGNITION COIL tester to see how the spark is generated by this three outside the combustion..

but then what we will see outside will differ if its inside due the pressure of the combustion. as of now resistance and inductance test is highly recommended for those who wants to try out different CDI coil available.

I am using by the way the china made coil, rather than my SHOGUN OEM, for a change. :-)

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