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Electrical Help: KTM

  1. #1
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    Electrical Help: KTM


    2009 KTM 250 XC, 2 stroke.
    Racing last weekend at the JDay Sprint Enduro, all was fine, started a segment, felt a tiny change, maybe a bit of friction, then suddenly bike is running like crap, bogging under throttle, semi recovering off throttle, then just died.

    Track side diagnosis was no spark, checked all the obvious things, no Joy, headed home. (new plug, kill switch, grounds, harness connections, etc)

    Yesterday i dove in to it.
    Took the stator cover off and i was right, the nut on the flywheel had backed off too the end of the shaft (lock washer was smooshed flat, not exactly doing it's job). i'm assuming this was the friction I was feeling. it probably contacted the stator and caused a surge/short?

    Broke out the multimeter:
    all continuity in the stator is good.
    All resistance seems good also (using ranges found online, don't currently have a service manual, it's in the mail now).
    Tested the coil next and have continuity across the grounds and its good, but there is NO continuity from the CDI input to the coil and the plug cap (so power in to power out). No resistance either.

    To me that sounds like the coil is shot, which is great because its cheap (and already ordered), but in order for a short to GET to the coil, it had to travel through the CDI, which would NOT be great.

    I HATE just throwing good parts at it trying to fix things. Any way i can test the CDI? Any other thoughts on wtf is going on? hopefully i'll have the manual by the weekend and have actual resistance values to confirm, but could a few ohm difference in the stator really cause a no spark condition?

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  2. #2
    Lifer Kurlon's Avatar
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    Re: Electrical Help: KTM

    Unfortunately the only real way to test a CDI is to swap in a known good one and see if the problem persists. There isn't anything conclusive you can do with your meter to fully and reliably validate one.

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  3. #3
    Lifer ZX-12R's Avatar
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    Re: Electrical Help: KTM

    Quote Originally Posted by iTGbuDeeV View Post
    Tested the coil next and have continuity across the grounds and its good
    What exactly do you mean by this?

    but there is NO continuity from the CDI input to the coil and the plug cap (so power in to power out). No resistance either.
    The coil is a transformer and there is no electrical connection from the primary side (low voltage from the CDI) to the secondary side (high voltage spark plug side). What you measured would be expected.

    but in order for a short to GET to the coil, it had to travel through the CDI
    Electricity doesn't work that way. If something was overpowered, the weakest link in the circuit will blow-up first. That does not mean that everything else in the path has to break as well. Ignition parts are generally fairly robust.

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  4. #4
    Lifer ZX-12R's Avatar
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    Re: Electrical Help: KTM

    Like Kurlon said, there isn't a good way to test a CDI box. The parts inside them that are most likely to fail don't have connections to the outside of the unit that you can readily test.

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  5. #5
    Backwoods lobster boy number9's Avatar
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    Re: Electrical Help: KTM

    Quote Originally Posted by Kurlon View Post
    Unfortunately the only real way to test a CDI is to swap in a known good one and see if the problem persists. There isn't anything conclusive you can do with your meter to fully and reliably validate one.
    This. The service manual for my 300 says something along the same lines, i.e. a CDI cannot be tested without specialized equipment.

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  6. #6
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    Re: Electrical Help: KTM

    Thanks Guys.

    That was my thought/ concern over the CDI. now i have to find someone local with a good one i can test.

    This is the 2nd time i've gotten information about the CDI input to the coil and the plug cap NOT having continuity, so maybe i'm misspeaking here. But I have watched no less than 3 diagnosis videos and ALL of them say there should be resistance between the coil input and the plug cap. I'm getting nothing there. I also removed the plug wire from the coil and tested resistance of just the wire/cap and got no reading.

    I have a repair manual, coil, and wire/cap on the way to the house already. i'll try to duplicate tests on the new parts vs the old ones and see if i get anything different.

    I'll update when i have info.

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  7. #7
    Lifer ZX-12R's Avatar
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    Re: Electrical Help: KTM

    Now that I'm looking a little more closely at how the coils are wired on the bike, the primary and secondary windings share a common ground so you are measuring both the windings at the same time. The primary is around 0.3 Ohms while the secondary is about 6.5K Ohms. If you dont have an auto-ranging meter, make sure you select the correct range.

    The more useful information would be to measure the coil's CDI input to ground (primary) and the plug cap to ground (secondary) making sure your meter is in the correct range for both.

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