Thought I Was Good At Wiring

Hi guys, last night I was trying to get the lights going on my Motobecane 50v. I have been restoring it and so far I have had it running without any lights plugged in (see the top of the attached pic). Then when I plugged the headlight in, it wouldn't start how it was, so I swapped around the wiring (as per the bottom picture in my attached pic). Then when I was trying to start it there were some sparks from the wiring before it reached the spark plug. Now the coil doesn't work. I know this because I substituted the coil for another one, disconnected the headlight and it works. I have used a multimeter to test both coils and it appears there is no resistance between the terminals on the faulty coil, so looks like I killed it. The headlight still works though.

MY QUESTION IS: how could I kill the coil just by messing up the wiring??? Are they that sensitive? Can anyone give me a clue of what I did wrong from my picture?

Now I am hesitant to continue because I only have one working coil, especially if it's that easy to break it while I am trying to figure out the lights (plus adding indicators/blinkers).

Re: Thought I Was Good At Wiring

Attached is a .jpg picture (quicker to download).


Re: Thought I Was Good At Wiring

Ok, from looking at the pictures, I think I can tell you what happened.

Basically, by connecting the light in that fashion, you put both of the magneto coils in series with each other, the headlight, and the ignition coil. This caused the coil to see way more primary voltage (and current) than it should ever see, which caused an internal arc-over and burnout.

Since I don't know if this is a 6V or 12V system, that explanation could be wrong. What year is the bike?

So, use one of the diagrams available on (articles section), and put everything back to stock. Then, figure out what's wrong with the stock setup. Don't just go randomly connecting wires until something works or breaks.

Re: Thought I Was Good At Wiring

Thanks for the info! I'm still not sure what I should be doing. I have got the schematics from the site for both 12v and 6v but can't really understand the 12v one.

I have two mopeds, one is a 51v (28-04-78) and the other is a 51SU (30-05-79). I thought they were both 12v. I have put the engine from the 51v onto the frame of the 51su but I don't think that should make a difference. The headlight on the 51v was 6v but other globes said 12v, and all the globes on the 51 su are 12v.

As far as putting everything back to stock, that is my problem, it wasn't connected when I got the moped so I am kind of guessing. I sort of understand what you said, but could you please explain how I could put both the magneto coils in series? I would be eternally grateful if you could draw on my picture how I should have set it up with the headlight : )

Re: Thought I Was Good At Wiring

Also which wire is the 'grey' wire, is it the one attached to the magneto?

Re: Thought I Was Good At Wiring

Jason Luther /

yes, that should just go to the coil. the other terminal on the back of the magneto is for lighting

Re: Thought I Was Good At Wiring

I tested the bike again yesterday and found out I even blew the spark plug cap! Must have been a fair bit of electricity going through that coil! The blue sparks down near my legs should have been a tip off : /

I was wondering if I connect up a 12v fuse between the magneto and the coil, that should prevent it from blowing again if I mess it up right?

Re: Thought I Was Good At Wiring

The grey connected wire goes to the coil (spark plug) and kill switch.

The white wire (running lights) goes in the small connector.

The brown wire (brake lights and horn) goes in the larger connector.

Never tried the fuse thing, but if you follow the diagram on you should be fine.

Re: Thought I Was Good At Wiring

Fuses are rated to protect against current, not voltage. I'm not really sure how much current the coil draws under normal conditions, so I can't suggest a fuse rating to protect it.

Both of those bikes should be 12V systems. The 6V is easy to identify, as there will be a finned aluminum heatsink mounted below the headlight, with a zener diode. No zener, no 6V system.

I don't have a 12V magneto here, so I can't tell you exactly what happened when you made the connections you describe. The 12V wiring diagram is as bad as the 6V diagram - much missing information about how things actually work. It should be a good guide on how to connect the wires, though.

Re: Thought I Was Good At Wiring

Danny Emerson /

make sure ur lights are good if it is like my tomos it is wired in series and an opening in the circiut will kill the motor

Re: Thought I Was Good At Wiring

I have 6v, and 12v motobecanes, and I think only the lighting system is different. the ignition system seems to be the same

Re: Thought I Was Good At Wiring


From what I've been able to gather, that seems about right. Only thing is, on the 6V systems, the exciter coil also does duty for the tail lamps. I'm not sure if the 12V system works that way.

Oh, I just noticed something.. my drawing of the 6V magneto has an error in it. Oops..Better fix that.

Re: Thought I Was Good At Wiring

Ok, here is a corrected drawing..

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