Dynamo Polarity Change

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
Diana3899
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Dynamo Polarity Change

Post by Diana3899 »

Hi Chaps, My 1946 M16 is wired for 6 Volts positive (+) earth and I want to change to 6 Volts (-) negative earth. Can some please explain in simple terms what I have to do to re-polarise the dynamo to get a negative earth system.
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dave16mct
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Re: Dynamo Polarity Change

Post by dave16mct »

Change the battery leads over and re-polarize the dynamo. See here: https://www.matchlessclueless.com/elect ... cas-dynamo.
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Groily
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Re: Dynamo Polarity Change

Post by Groily »

And (not mentioned in that part of the link as far as I could see - although might be going blind of course)
1. be prepared to reverse the wires on the back of the ammeter so you get your pluses and minuses the right way about!
2. if you've got a modern electronic regulator, be sure it's the right polarity. You'd need a Neg earth version - but doesn't apply if it's a Lucas mechanical one.
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dave16mct
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Re: Dynamo Polarity Change

Post by dave16mct »

Probabely have to re-wire the stoplamp switch (if fitted) as well?
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Re: Dynamo Polarity Change

Post by Groily »

If that - and not forgetting the dear old horn? - are taken directly from the battery, I think yes, so that current flows the preferred way.
If taken separately from the ammeter, then they'll need to be changed along with the regulator to battery leads swap, indeed they will, to keep the pluses and minuses in order.

And think about where and how fuses are situated, if there are any (very highly recommended, in fact high risk not to IMHO). I don't think it matters whether a main fuse is on the live or earth side frankly, as long as it covers everything including horn and stop light, which along with the tail light are the two of the commonest trouble spots.
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dave16mct
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Re: Dynamo Polarity Change

Post by dave16mct »

I think I'd leave it + earth! Why change it?
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Diana3899
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Re: Dynamo Polarity Change

Post by Diana3899 »

Thanks for info, the reason why I want to change it is because the old mechanical regulator has packed up and I happen to have a 6V negative electronic unit in my box of spares. If I had a 6v positive electronic unit I would have left it as positive earth.
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clive
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Re: Dynamo Polarity Change

Post by clive »

horn, stop light why would these need to be altered on a reverse of polarity?
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dave16mct
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Re: Dynamo Polarity Change

Post by dave16mct »

I think I was thinking about when I tried to fit a negative earth LED rear stop/tail bulb for which I just couldn't work out a way (and now you can get + earth ones easily.) My brain's hurting so perhaps Groily will help us out?
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Re: Dynamo Polarity Change

Post by Groily »

Diana3899 wrote: Fri Aug 12, 2022 8:37 pm Thanks for info, the reason why I want to change it is because the old mechanical regulator has packed up and I happen to have a 6V negative electronic unit in my box of spares. If I had a 6v positive electronic unit I would have left it as positive earth.
Thought that might be why - good plan then!

As to the brake and horn etc, it's just a Q of feeding them from the live side, which may be automatically sorted depending on how they are wired etc, but may need some rejigging so that when, eg, you push the horn button you're earthing it to the 'bars. For example, on my own bikes I usually power them directly from the battery, so would need to swap the feed from neg to pos if changing system polarity from pos to neg earth. Just a minor thing.

In cases where LEDs or indeed any gizmo with a particular polarity requirement (just like the electronic regulator itself) are used, then the change of earth has to be catered for too, as Dave implies.
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