CDI Charging and Triggering

Discussion in 'The Garage' started by Zahnarzt, Aug 21, 2019.

  1. Zahnarzt

    Zahnarzt Crashes Much

    Joined:
    Sep 10, 2009
    Oddometer:
    643
    Location:
    Sierra Vista Arizona
    All modern circa 1980 and newer bikes, quads, snowmobiles etc use a CDI to supply the momentary electrical current / voltage to the coil subsequently firing the plug.

    Where and how does the mystical "Black Box" get the timing and power to do its work?

    First: There are two power sources that charge the capacitors that will eventually discharge into the coil.
    1.The older of the two is an AC voltage provided by a coil located (usually) on an arm of the stator complex inside the engine

    2. A newer source is a 12V DC voltage provided by the battery. (There is circuitry inside the CDI that changes this into the high voltage needed to fire the coil.

    Next, there is a trigger coil (Sometimes a Hall effect sensor) that sends a "triggering Voltage" to the CDI to tell it to release the stored energy in the capacitor.

    OK... So what? In the decades I have been working on bikes and quads, the above mentioned system has always been a guessing game to diagnose. Most service manuals give resistance specs for the charging and triggering coils, BUT I, as well as others have replaced these parts that were out of spec only to have no resolution to the issue... WTH.
    For the last few years I have been testing the resistance of ALL the source and trigger coils of machines that come into the garage for any type of repair - usually carb cleaning.
    I would say, about 25% have out of spec values but were running just fine.

    My request (for the curiousness of it) and to help others, is for those that feel comfortable measuring these values on their own bikes and post it up so I can distill it into actual data.
    #1
  2. Zahnarzt

    Zahnarzt Crashes Much

    Joined:
    Sep 10, 2009
    Oddometer:
    643
    Location:
    Sierra Vista Arizona
    Another piece of information I use in diagnosis is the output voltage of these coils.

    I purchased a Direct Voltage Adapter (it measures peak AC voltage and converts it to a stable DC voltage that can be read on a standard Multi-Meter)
    I use a Electronic Specialties 640DVA purchased from Amazon for around $35
    I know this is not as precise as an oscilloscope but it gets me in the ball park.

    If you have access to a DVA that would be good information as well.
    If you work on any CDI equipped machines they will save you the investment cost the first time you use it.

    An example:

    1993 Yamaha Warrior/ Specified Resistance Charge Coil (270-330ohm), Measured 317ohms. DVA output 134V. Specified resistance trigger coil (171-209ohms), measured 217ohms, DVA output 3.8V

    Quad runs great

    2003 Yamaha Warrior/ No charging coil (DC CDI) Specified resistance trigger coil (459-561ohms), measured 432ohms, DVA output 5.5V

    Quad does not run - no spark. Coil test good... do I risk replacing the stator or the CDI?

    I assumed (with my limited knowledge that 5.5V was enough to trigger the CDI and ordered one.
    Plugged it in and it fired up first bump.

    I have an excel spreadsheet i am dumping all this data into.
    Any input is appreciated.

    Z
    #2
  3. tntmo

    tntmo Oops, I did it again.

    Joined:
    Jan 31, 2012
    Oddometer:
    1,205
    Location:
    San Diego, CA
    Interesting concept, I was just reading all kinds of data while trouleshooting my Vulcan Nomad trying to ensure that my ECU was ok. For that bike, there's no readings on the ECU, just the wires leading to it.
    #3