Magneto problems

Discussion in 'Electrical' started by xd45owner, Nov 23, 2013.

  1. xd45owner

    xd45owner New Member

    I'm having a bit of a problem with my magneto and any help is greatly appreciated as I am about to scratch a raw spot in my head trying to figure this darn thing out..I took the cover off the motor and put my voltmeter on the Blue and Black wires and get between 3v & 6v depending on how fast I am running my drill which I am using to spin the motor over with, now when I disconnect everything and do an OHM's reading (set at 2k) I get nothing between the white & blue wire..Is my magneto shorted out? I tested my CDI and I also get nothing in resistance on that rascal, but am ordering a new one Monday morning..Can I fix the magneto myself? Is the hard plastic like tape around the windings something I need to try to keep intact? Any help is greatly appreciated :)

  2. bluegoatwoods

    bluegoatwoods Well-Known Member

    I'm no authority but I'm guessing that, yes, you should try not to booger up that tape stuff. I'd bet it's insulation. And probably important.

    Why don't you try testing Ohms between the blue and the black lead? If you get zero, then it's likely that the solder connection on the blue lead has gone bad. Or was never very good in the first place. They have a reputation for that. You'll be able to re-solder that if you'd like. Or you can get a new magneto for about twenty bucks.

    On the other hand you did say that you got voltage between blue and black when the engine was turning over. If that's the case, then what I just said is not going to be an issue. And now I'm getting stumped.

    But others will come along who know more than I do. And if you search 'magneto' on this page, then you're sure to come up with more suggestions.

    Good luck.
  3. BigBlue

    BigBlue Active Member

    Welcome to the forum.

    Please tell us what your exact problem is. Why do you think it is the magneto coil? Is your engine new? Or have you had it for a while and now it doesn't run? It could be other electrical issues other than the magneto coil: Bad spark plug; bad spark plug wire; faulty kill switch; or bad ground. Most problems don't involve the CDI unit, unless you didn't ground the spark plug against the head when turning the engine over to check for spark.

    If the magneto coil got wet, more than likely it has corrosion and will cause problems.

    If it is the magneto coil, they're so cheap it's not worth trying to fix.

    Here's a link on the magneto coil with information on Ohms readings: Mystery.html

    You should be getting 300 to 350 Ohms across the the blue and black wires.

    Please go to the Introduce Yourself section of the forum out of courtesy.

    AKA: BigBlue
    Last edited: Nov 24, 2013
  4. xd45owner

    xd45owner New Member

    I honestly am not sure as to what happened, this is a bike I built & sold(Everything New) less than a 2 weeks old, the customer had it for less than a week and gave me a call about it not starting..When I sold him the bike it looked new, when he brought it back for me to check out it looked like he owned it for 5yrs..I checked the resistance in the CDI module, and I got no reading between the Black & Blue wires, I replaced the plug with a new NGK and still no fire, checked the plug wire,boot and connection at CDI and no spark..I took the kill switch out of the equation all together and wired directly and it's still dead..I Ohm checked the magneto and got no reading either, but yet when I turn it over with my drill I get between 3 - 6v reading depending on the speed I am running my drill at..
  5. BigBlue

    BigBlue Active Member

    Here's how to check the CDI:
    Put your Ohms at 200K scale.

    1. Positive lead on Blue wire and negative on black wire should be infinite (no activity).

    2. Positive on black wire and negative on blue wire should be 130 - 150 ohms.

    3. Positive lead on spark plug wire and negative on blue wire should be about 135 - 155 ohms.

    Switch scale to 20k to check spark plug wire with black lead should be about 2.5 - 2.7k ohms.

    You don't need to test the white wire. A lot of people cut them off and newer magneto coils don't have them. What does the magneto coil look like? Post a picture if you can. If the customer rode the bike or let it sit in the rain, the magneto coil is toast. If the magneto coil is faulty, it may look and run o.k., but will give you problems, especially when compression is thrown into the equation - that is why you may get some voltage, but not enough to fire the engine.

    Good Luck,

    AKA: BigBlue
    Last edited: Nov 24, 2013
  6. HeadSmess

    HeadSmess Well-Known Member

    welcome to the joys of magnetics and electricity.

    if theres no reading between blue and white, the coil is internally open circuit. dead. almost impossible to fix unless your patient and want to make a coil winder. theres 2500 turns of .05mm wire last time i checked...

    after all the time and effort...just buy a new coil.

    the coil between the blue and white wires delivers the main voltage to charge the capacitor, and should read, even with a lousy multi meter that is NOT intended for this use, somewhere in the 60-70 type volt range...

    if you get a reading of only a few ohms between white and black, thats normal. and that coil rarely burns out as its heavy guage wire.

    and the voltage between blue and black should be around 6 volts.

    yes, the two coils are in series with each other. black (earth) to white, in heavy guage, then white to blue, in light guage.

    the reason you DO get a reading when turning the engine over, is because the coils are still there. theyre still conductors, cutting lines of magnetic force. just because one end is waving in the breeze doesnt matter. theres a voltage produced in the conductor. being AC, electrons surge that way! then back again! and the conductor, having some resistance, allows a tiny lil current to flow, despite there being no complete circuit!

    (Tesla, the leading man of AC, made many devices that operated with only one wire, and no apparent "return circuit"s. dc is hard enough. once you deal with hi frequency AC things get weird!)

    while you might read 2v or so, the current available is sweet f all... if you tried inserting a 10ohm resistor, added some load to the circuit...well, you still would only read 2v or so as the circuit isnt a full circle due to that open circuit inside.

    like a mains transformer... say its rated at 12va. thats 12 volts at one amp. if you increase the load, (less resistance) the voltage drops. the current increases proportianally to the change in resistance or load. if you remove the load, infinite resistance, the voltage "theoretically" is infinite too. youll find an unloaded transformer of 12v may produce 15 or 20v across the secondary.
    in this case, the multimeter has its own resistance of a few megohms, which still allows some current to pass, that due to ohms law... lowers the voltage being read. your measuring device affects the measurement. thats why the impedance of a voltage tester is so high. less current flowing, less effect on the voltage. current is the reverse. you want as much current to flow through the meter as possible, so it has an extremely LOW impedance.why you NEVER attach an ammeter across the outputs but wire it in SERIES with the "load". an ammeter is basically a short circuit.

    you would more than likely get the same reading if you just disconnected one lead on the multimeter...

    on the cdi unit itself... you shouldnt get anything between blue and black wires. the diodes are reverse biased and wont allow any current to flow. ( one direction. i forgot about the trigger current flowing the other way... )

    if you had one of these aftermarket cdi's where you can access the wire that feeds the ignition coil... between that wire, and the blue wire, with the multimeter set to one direction, you would get a constantly increasing resistance, as the main cap charges. reverse leads, and you would get nothing, as the diode prevents the cap from discharging.

    if you measure the resistance of a will increase, until cap is charged fully, then by reversing the wires, you get NEGATIVE resistance as the cap discharges!. same deal with certain types of mosfets ;)

    basically? just get a new magneto :)
  7. xd45owner

    xd45owner New Member

  8. xd45owner

    xd45owner New Member

    Checked the CDI, it's dead all the way across the board (No readings) on everything you mentioned for me to check.
  9. xd45owner

    xd45owner New Member

    Well, just because my curiosity was driving me crazy I took the fiberglass tape off the mag and discovered some burnt wires, I'm thinking that evidently when the guy rode through water and shorted out the mag it got the CDI unit as well..So I will be replacing them both..Thank you everyone for your input :)