Urgent - Magneto and CDI Testing

Discussion in 'Electrical' started by Blaze_Fox89, May 13, 2009.

  1. Blaze_Fox89

    Blaze_Fox89 New Member

    I PROMISE I did a search, I have searched for 2 days and I cannot find any thread that can tell me how to test it or where and what ohms should be. I've only found stuff saying it should be over 300, but what?

    If anyone can help me that will be great as i am ordering a new magneto nmw. I REALLY can't afford to get it wrong. :(

    This is what my multimeter reads:
    Blue to Black > 327
    Blue to White > 329
    White to Black > 3
    I'm guessing white to black is the culprit. 3 ohms usually means near shorted i think. But I still get 320 odd between blue and white.

    CDI Unit
    I get a breif connection from Black to the HT output but then it goes open.

    Also, my kill switch is wired to the white and ground, so my guess is after a hot run, turning off the engine blew the coil. :icon_cry:

    I'll appreciate any help as I need it to get around. Even if it's just a link to the right page. :sweatdrop:

    Thanks in advance, Blaze.

  2. crazeehorse

    crazeehorse Member

    according to your readings, your magneto is good. the cdi stands for capacitor discharge ignition. the reading you got on your cdi sounds normal. what is the problem you are having? have you tried to start it with the kill switch unhooked from the circuit? this is a common problem, (a bad or stuck kill switch) also check your plug wire for resistance, it should be very low reading on the ohm scale. your kill switch should be wired to the black wire from the magneto , or grounded to the frame, & the other lead from the kill switch should be hooked to the blue wire from the magneto.
    Last edited: May 13, 2009
  3. duivendyk

    duivendyk Guest

    Your coil is OK,the white wire is a tap on this coil close to the ground end (black wire)that's why it's only a few Ohms.If the WW worked before as a kill switch don't change it.The blue wiire can be used but carries dangerous voltages (above 100 V) and is much more susceptible to moisture leakage etc.You can't bust the coil by using the kill switch on theWW, that's just nonsense.Your CD unit probably has bitten the dust.Look up a post I wrote yesterday about making this unit less susceptible to failure, it gives you chapter &verse.It also tells you that sometimes the generator coil spacing to the flywheel magnet is the problem.Look me up in the Members list&you can find it.The Forum has a terrible search engine IMHO.
  4. Blaze_Fox89

    Blaze_Fox89 New Member

    Well, this is what i've tried:
    The coil spacing I thought was gonna grind the magnet away it was that close, so its not that.
    Sparkplug and cap.
    Killswitch unhooked.
    HT lead still works with no holes.
    uhh..... I'm fairly sure it's not fuel.
    There is a spark sometimes, it ran as a 4 stroke for awhile it so i guess the timing in the cdi must be gone. I just didn't know what the readings were for the magneto.

    It's was trying to start but it just kept doing that 4 stroke thing or nothing at all.

    I'll order a new cdi then.
  5. crazeehorse

    crazeehorse Member

    i agree with duivendky, i think it is your cdi unit.i checked one that is verified good, here's the readings i got on the 40 k ohm scale,( blue to sparkplug lead, open, no reading.) (black to spark plug lead, 2.40.) (blue to black, open, no reading.) so based on this, i'd go with the replacing the cdi. i had never checked one of these before, so when i saw the reading on your old one, i just assumed that you were getting a reading on the capacitor, sorry for the confusion.
  6. duivendyk

    duivendyk Guest

    I think he might have an intermittent connection somewhere (not necessarily in the CD unit),that 4 stroking could be a result of that,it indicates that things work part of the time.Have you replaced the plug.?If the output from the CD is low it might cause that too.Reducing the gap to less than 0.020 helps firing the plug.The thing to remember is that the firing voltage increased quite a lot with compression pressure,so if the plug fires on open air that does not guarantee it works inside the engine,but usually causes high speed misfiring.I don't know of any resistance test results on the CD.The output to the plug to ground (black) wire should be in the kilo ohm range (between 1000-10000 Ohms) but NOT infinite) ,that's where the intermittent is likely to be,between the plug wire and the pulse transformer output,a very fine wire that get's eroded(vaporized) by internal arcing,and when the gap gets large enough,goodbye CDI .
    Last edited by a moderator: May 14, 2009
  7. Blaze_Fox89

    Blaze_Fox89 New Member

    The reading I get from Black to Blue is I think 2500 ohms. Ive tried jiggiling the wires on the magneto and the black wire from the magneto just then.

    NOW, the fox has a theory.
    You know how the plug cap has the brass peice that connects to the top of the spark plug right? Well, that fell out because it was really loose. And theres a tiny spring behind it. That went too. Didn't see it till the next day after trying to start it 39 times. I read that you shouldn't crank the engine with the cdi hooked up with no sparkplug.

    SO, the brass bit in the plug cap came loose, stopped contacting, lost the screw and I blew up the cdi trying to start it so it didn't work then it was fixed.

    I'm sorry if thats mega long but It's a pretty logical explanation I reckon. I have also ordered a cdi unit, expecting it Monday. ^^
    BTW, I say, have a look at yours, it doesn't look easy to make them tight so there all pretty loose. My spare was loose too. :(
    Last edited: May 15, 2009
  8. duivendyk

    duivendyk Guest

    That's probably it.I have never set eye on any of these things, looks like there is a spring contact on the plug wire to the output of the pulse transformer,a very crappy proposition with regard to moisture exposure. Plug cable should be silicon rubbered in and if it looks crappy to begin with try grafting the spring on a bettercable&plug cap and do the same moisture is DEADLY to ignition coils.Your blue/black reading should be around 350 Ohms,certainly NOT 2500 !!.If you don't have spark plug or similar discharge path the output voltage goes sky high &might zap a marginal transformer.This whole thing appears to be under designed and the more that fail the more they will sell,until people stop buying them alltogether.Can you trust that ohm meter?Remember it is NOT certain the coil is bad,it may be just a connection problem!
    Last edited by a moderator: May 15, 2009
  9. Blaze_Fox89

    Blaze_Fox89 New Member

    No! Your RIGHT! I don't get 2500 ohms across the black and blue. Ment to type black to ht screw, or something.

    I just tested it and I get nothing between the blue and black as long as the red probe of the multimeter is on the blue wire. The other way, 4270 ohms.
    (I think, Iforgot how do this. The meter is set to 20k.)

    I USED to know how to use a multimeter lol! :sweatdrop:
  10. duivendyk

    duivendyk Guest

    Put it on 'resistance' or 'ohms' scale 1k(ohms) or higher,first short leads should read "0" (or should be able to set to 0 with adj if provided).Then put between Blue&black should read 300/400 Ohms (nominal around 350) irrespective of polarity.If high check white to black should read about 2.5 Ohm.Also check between blue&white should read very close to blue &black reading.These are Readings on the generator coil in the ENGINE, NOT on the CD unit
    Last edited by a moderator: May 16, 2009
  11. duivendyk

    duivendyk Guest

    On second thoughts there is likely to be a diode onthe input of the CD unit someehere (blue/blk) that explains your $4k reading and the oc in the other direction, red is +pos.These multimeters nowadays use low currents for resistance measurements so diodes can actually show fairly high resistances,this can lead to confusion in interpreting readings.I am more concerned about the reading to ground (blk) at the plug, should be 2-5 k range but not above 10k.
  12. wanderer33

    wanderer33 New Member

    Hey Blaze, talking about that little brass thing and spring, I had that problem wit my first bike I don't know to date why they are supplying that type of plug cap in three parts.
    I did simple thing, the cap on lawn movers are exactly same size and it's one solid unit I changed to this and never had a problem again.