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moetheshmoe

1978 DT 125 Did the factory wire it this way?

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Hi, I'm new to the club and hope someone can help me. I'm going bald and can't afford to pull out anymore of my hair. I just inherited a 1978 DT125E, with points and magneto. Trying to get it started and decide whether it's worth restoring. It has no spark. Could not get it to generate any ac voltage, even when I hooked up a drill to the crank and spun it. Took the flywheel off and the coils and points look good. But here's the strange thing - one end of the source coil was hooked to the condenser and then exited via the B/W wire which leads to the ignition coil. The other end was pigtailed with 2 leads coming off the lighting coil and exited via the Yellow wire. The lighting coil seems to be made up of 2 little coils each with 2 leads. One is G/W, the Yellow has 2 as I said and the 4th is grounded to the plate. So anything connected to the lighting coil is also grounded! So the points can not work. This can't be right, can it? Did it come from the factory this way? Or did someone butcher the wiring? It has no lights or accessories, set up for dirt only. I have looked high and low for a schematic of the internal wirings of a points magneto and it doesn't exist as far as I can tell. Common sense told me to disconnect the source coil from the lighting coil and when I did the points worked properly, when closed it's grounded and when they're open it's not. I checked the resistance of the source coil and got none with points open and 3.2 ohms with points closed. Also checked the lighting coil and got 1.4 ohms when static. When the engine turns it's erratic - 12, 24, 16, etc. I kicked it over again to check ac voltage and didn't show anything but as soon as I disconnect the negative lead on my meter I saw voltage dropping - 12, 8, 6 ,4 . Does anyone have any idea what my problem is? Thanks for any and all help.

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both generator coils should be grounded to the baseplate at one end only. You wont find a wiring diagram for inside of the magneto...there isnt one!

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Ok I think this is starting to make sense now. Not knowing much about electronics I thought the coil had to be isolated(not continuously grounded) and the points would provide on and off grounding. If I understand this right..not so. One end of the source coil is constantly grounded(thru the yellow wire-lighting coil) the other black n white wire is grounded thru the points when they are closed. During this time a charge is built up in the coil. When the points open and disconnect the ground the charge is sent up the black n white wire to the ignition coil. Is that right? If so, then I need to rewire back the way it was. I did check for continuity in the source coil when it was disconnected and there are no breaks in the coil. When it's hooked up I get 2.7 ohms yet my manual says it should be .27. I'm guessing that's ok, just a matter of the meter setting? Also the lighting coil measured 1 ohm, which may be a little low, should be .19. But as a general rule less resistance is good, is it not? By the way, the green n white wire which comes off of one of the two small coils that make up the lightening coil does not show continuity or an ohm reading. I won't be needing them unless I reconnect the complete harness but that sure puzzles me even more. So at this point I'm going to rewire back the way it was and I can only think of the condenser as the possible problem. While I'm messing around I'll try and bypass it and see what happens.

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Forget the lighting cct for now concentrate on the ign cct, here is a schematic of a basic magneto with points (contact breaker) ignition, yours will be similar but the connection the ignition coil low voltage terminal will branch off also to the kill switch and key switch

MagnetoIgnition_zpsd034e347.png

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Well I tried it without the condenser and couldn't get a spark. I did hear that without a condenser you can't build up enough voltage for the ignition coil. I've got a new set of points and condenser coming in a couple of days. I'll put those in and let you know what happens. Please stay tuned........

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Well I put in the new points and condenser, rewired back the way it was and still nothing. When I hook up a drill to the crank and spin it the source coil averages about 6 volts ac being generated and the lighting coil averages about 4. I double checked the source coil resistance and it's 2.7 ohms, 10 times what the manual says - .27 and I don't think it's my meter. I set it on the lowest setting. Also, I disconnected the source coil to check again for continuity and there's a complete circuit there, no breaks. And the coil unconnected still shows 2.7 ohms. What does it mean when resistance in a coil is high but there's no breaks in the winding. And these windings look almost brand new. The lighting coil is also 10 times higher than the book says, unless the book is wrong? Could the magnets be too far away? There doesn't look like any adjustment. Help? Thanks

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UPDATE: I completely removed the ligghting coil and connected one end of the source coil to it's mounting post(ground). The other end is connected to the wire that comes from the condenser and goes to the ignition coil. Surprisingly, the resistance has gone down from 2.7 ohm to .7 ohms! And when I touch the 2 leads of my meter to check resistance it reads .6 ohms. If I adjust the coil reading(.7) by subtracting that .6 I get a net resistance of .1 ohms, hardly any. I hooked up the drill and checked for AC voltage generated and I get an average of only 4 to 5 volts. So I have very low resistance in the coil, along with no breaks and it does generate volatge but nowhere what it should be, which I believe is around 40 volts. The magneto as a whole seems tight when mounted and doesn't wobble. There is a little scoring on the cam surface but the points open properly and the gap is .14 so I don't think that's an issue. The question is "What's the issue?".

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Strangest thing. Before I removed the lighting coil the source coil had a resistance of 2.7(-.6) and after I removed it went to .7(-.6). It's been sitting in the sun for a while and I decided to reverse rotation with my drill and see if I could get more voltage, in case, like you said, I attached the wrong end of the coil. Still got about 5 volts, no difference. But I rechecked resistance and it's back up to 2.7, same as before.

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UPDATE No. 2

I switched the wiring as you said and I'm getting 6 to 7 volts. But I did find this out - the difference between coil resistance figures depends on if the points are open or closed. 2.7 when open and .6 when closed. So, when the points are open it's measuring resistance thru the coil? Any thoughts?

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I out of ideas if you still dont have a spark!

the figure I saw for the source coil was 1.7 ohms

It seems to me that you meter reads 0.6 ohms when you touch the probes together, same as when the points are closed.

so if this is the case 2.7 - 0.6 = 2.1 ..slightly over the 1.7 +- 10% but not much

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Just got off the phone with Yamaha Corporate office in So. Calif. and the only info they could find were some general specs that confirmed what my Haynes manual has said - .27 ohms for the source coil. Where did you find 1.7? Also do you have any info. on AC voltage output as it's cranking? And finally, does your bike vary resistance as the points oopen and close? Thanks.

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