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Main fuse/Positive ground problems

Classic Norton Commando Motorcycles.

Main fuse/Positive ground problems

Postby Supercat » Wed Aug 23, 2017 4:09 pm

Hi Folks,

I'm a brand new old Norton owner, and I've got wiring troubles that I'm hoping to get some help with. I really don't know very much about electricity, so please be patient. I know there are lots of threads on here about blown main fuses, which I've read, but I have some basic questions that I haven't found answers to.

I bought a 1972 Norton Commando Roadster which I am dying to ride, but so far I can't even check to see if I can get sparks because the main fuse blows as soon as I connect the battery. The first time I tried connecting the battery I was not aware that this is a positive ground system, and when I did discover this critical piece of information I assumed that the fuse had blown because I had connected my regular negative ground battery the wrong way. So I tried connecting it the other way around, and the main fuse blew again.

So here's my first question: can you connect a modern negative ground battery to a positive ground system, and if so, do you just hook it up backwards? If not, do you think I've destroyed anything (besides a few fuses) by doing so?

I've tried disconnecting the battery altogether, pulling the plugs and kicking her over to see if there is any spark, but I'm getting nothing. Maybe because it has electronic ignition?

After reading the threads on here I started looking for a short, which I've spend a few hours doing but no luck so far. I tried disconnecting one component at a time and just touching the fuse quickly against it's home on the negative lead (just to save on fuses), but each time I still get a spark on the fuse which will blow if I let it touch for too long. I think I've tried pulling the connectors for all of the components (one at a time) except for the horn and the back light because they are hard to get to. During the process I did discover one of the three ground wires going to the top of the engine was burnt off, which I've since repaired, but that hasn't made any difference. At one point I thought I had made some progress when I disconnected the right ignition coil. For a few minutes I was able to touch the main fuse to it's slot without getting sparks, and I even managed to plug it right in without it blowing. I got excited for a minute and thought I might have been on the right track for finding a short, but a minute or so later the fuse popped again, and now the same thing happens with or without that coil connected. So my second question is: what the heck should I do next!?

Anyway, I'm totally stumped.... and I am dying to try the bike out! Any help would be hugely appreciated,

Supercat

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Re: Main fuse/Positive ground problems

Postby ashman » Wed Aug 23, 2017 4:39 pm

I say your earth wire has burnt out could be your problem happened to me on my old Triumph many years ago a power wire shorted out on the earth wire, burned the whole earth wiring, I fixed the short but had to replace the whole earth wiring, from that day on every rewiring jobs i do I run the earth wire out side the main power wiring harness.
Check out all the red wiring and see if its burnt out as a power wire touching a earth wire will burn it out very quickly.

Ashley

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Re: Main fuse/Positive ground problems

Postby Supercat » Wed Aug 23, 2017 5:17 pm

Thanks Ashman, I'll check all the red wiring. Is the way that I'm hooking the battery up ok? I'm just using a modern negative ground battery and I'm connecting the negative lead on the bike to the positive battery terminal.

Would a burnt earth wire cause the fuse to blow? Or maybe the same short that burnt out the red wire is causing the fuse to blow?

Thanks again!!

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Re: Main fuse/Positive ground problems

Postby L.A.B. » Wed Aug 23, 2017 5:18 pm

Supercat wrote:So here's my first question: can you connect a modern negative ground battery to a positive ground system, and if so, do you just hook it up backwards? If not, do you think I've destroyed anything (besides a few fuses) by doing so?


There's no such thing as a "negative ground" battery, so yes, you would normally connect the battery "backwards" as you put it, for a positive ground electrical system.

Connecting the battery incorrectly may have damaged an 'electronic' part of the electrical system, however, it's possible the fault was already there beforehand, nevertheless as the fuse blows immediately you try to (now, correctly) connect the battery it looks like there has to be a short circuit.

If you haven't already done so, to eliminate some potential causes from the incorrect battery polarity, try disconnecting the 2MC capacitor (red) wire, also the rectifier and Zener diode negative wires (brown/blue) or, if a one-piece regulator/rectifier has been fitted in place of the separate rectifier and Zener diode, disconnect the reg/rec positive and negative wires, also if electronic ignition has been fitted, disconnect the ignition negative wire. Then re-test, reconnecting one item at a time.

If the fuse blows with all those items disconnected then it's more likely to be a wiring fault.


The factory workshop manual is here if you don't have one (yet).
http://www.classicbike.biz/Norton/Repai ... mmando.pdf
Electrical: Section J

Edit: If you are using '35A' fuses, make sure they are 35A blow and not 35A continuous. Use 15A - 20A continuous fuses.
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Re: Main fuse/Positive ground problems

Postby Torontonian » Wed Aug 23, 2017 5:28 pm

So you want to hear it actually run ? Run a red wire from any motorcycle battery positive terminal (+) , to a frame ground . Simple so far ? Run a white wire (with a fuse wired in) to the white wire of the Boyer. This will be your hot wire from the negative battery terminal (-). The Boyer also needs a good solid ground by the way so check it ,(Boyer red wire). Pull the spark plugs (clean them or best new) out of the motor , put them back into the 2 high tension wires and rest or even tape them against the head near the spark plug holes. Kick over the motor and you should see spark plug electrode sparks each kickover. Now put the plugs and their wires back into place in the engine. There ya go that's the bypass way to get this girl started or get you home in emergencys. Fuel is a whole different ball of wax. Good luck.
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Re: Main fuse/Positive ground problems

Postby ashman » Wed Aug 23, 2017 5:32 pm

Supercat wrote:Thanks Ashman, I'll check all the red wiring. Is the way that I'm hooking the battery up ok? I'm just using a modern negative ground battery and I'm connecting the negative lead on the bike to the positive battery terminal.

Would a burnt earth wire cause the fuse to blow? Or maybe the same short that burnt out the red wire is causing the fuse to blow?

Thanks again!!


If the short is still touching the earth wire it will blow, but it has to be touching a bare wire, you will know if the earth wire is shot as all the wire coating will be burnt, as for your battery + to earth - to power and do as LAB has said to trouble shoot your problems as you could have damaged you electrics, but hopefully the fuse might have saved it by blowing straight away. Its hard to put the smoke back in your wiring.

Ashley

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Re: Main fuse/Positive ground problems

Postby Supercat » Wed Aug 23, 2017 5:41 pm

Thanks L.A.B. I had each of those components disconnected but only one at a time, not all at once. I'll try this out. You mean to disconnect the black box negative wire, or the negative wires at the coils?

So if the fuse doesn't blow with everything you listed disconnected, but then it does when I start reconnecting things, does this indicate that the problem is with the last component I connected?

Thanks also for the manual link!! I had an owners manual and a service manual, but this looks different - and longer - than the service manual that I had.

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Re: Main fuse/Positive ground problems

Postby Supercat » Wed Aug 23, 2017 5:49 pm

Thanks Torontonian!!

This is an exciting option :). Because I'm using a modern battery and this is a positive ground system, should I connect my negative battery terminal to ground, and the positive terminal to the Boyer white wire? Sorry I'm so slow to grasp this positive ground situation! Hearing her run would be amazing.

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Re: Main fuse/Positive ground problems

Postby L.A.B. » Wed Aug 23, 2017 5:56 pm

Supercat wrote:I had each of those components disconnected but only one at a time, not all at once.


That's an alternative way of doing it.

Supercat wrote:You mean to disconnect the black box negative wire, or the negative wires at the coils?


Which make and type is the "black box" (I will assume it's a Boyer Micro MkIII ignition unless you say otherwise)?

Yes, disconnect the negative wire (white/yellow?), from the (Boyer?) box white wire. The box feeds the coils so there shouldn't be any power (negative) feed wire connected directly to the coils.

Boyer instructions and wiring diagrams (assuming it is a 'black box' Boyer Micro MkIII):
http://www.boyerbransden.com/pdf/KIT00053.pdf
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Re: Main fuse/Positive ground problems

Postby L.A.B. » Wed Aug 23, 2017 6:01 pm

Supercat wrote: Because I'm using a modern battery and this is a positive ground system, should I connect my negative battery terminal to ground,



NO, do not connect the negative side of the battery to ground.

Forget about the battery being "modern" or "a negative ground battery" as that's nothing to do with it.
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Re: Main fuse/Positive ground problems

Postby MikeM » Wed Aug 23, 2017 6:12 pm

Supercat wrote:Thanks Torontonian!!

This is an exciting option :). Because I'm using a modern battery and this is a positive ground system, should I connect my negative battery terminal to ground, and the positive terminal to the Boyer white wire? Sorry I'm so slow to grasp this positive ground situation! Hearing her run would be amazing.



There are times that you have to realize your limitations. I find this myself many times, especially electrically. It sounds like you have reached yours and its time to find someone that has the experience to move you forward. Seek someone with the experience that you need before you have something bad happen. JMHO.
Learning all the time.
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Re: Main fuse/Positive ground problems

Postby texasSlick » Wed Aug 23, 2017 6:19 pm

You can save fuses by using a Volt-Ohm-Meter. If you do not have one, you will find one a valuable investment.
A good meter for amateur use is less than $30.00

On the meter, select the most sensitive Ohm setting. Then with lighting switches Off, connect one meter lead to frame ground (earth), the other to the battery Neg terminal (pos ground system). You have a short if the meter reads zero or near zero ohms. Disconnect things one at a time untill the meter reads infinity or goes off scale.

FYI, a lamp filament will read near zero, so be sure to have all lamps Off or disconnected.

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The Second Law (of thermodynamics) rules.
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Re: Main fuse/Positive ground problems

Postby Supercat » Wed Aug 23, 2017 6:34 pm

Thanks MikeM and Slick. Yes, I definitely realize that I'm trying to sort something out that is beyond what I should be trying to sort out :). Unfortunately I live in a very small, very remote community that literally has nobody with any experience with old motorcycles. I've asked the folks here who do electrical work, and they were as confused as I am when I mentioned 'positive ground' and that 'the fuse is on the negative wire'. I would love to have someone who knows what they're doing help me, but unfortunately it's not an option where I live.

I've got a volt-ohm meter - and maybe by the end of this project I'll know how to use it properly. The method you're suggesting sounds like a good one. I'm definitely up for saving fuses.

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Re: Main fuse/Positive ground problems

Postby Supercat » Wed Aug 23, 2017 6:36 pm

Thanks folks! Ok, I think I am finally understanding this. When I said I was connecting the battery backwards I was connecting the negative bike wire (with the fuse) to the positive battery terminal. I gather that this is wrong, and that I should be connecting the negative bike wire to the negative battery terminal, and the positive bike wire to the positive battery terminal. This is very helpful :).

It is a Boyer Micro MkIII ignition.

I'll try hooking the battery up correctly and disconnecting all the components L.A.B. listed to try to find the short, and if I get too excited I'll try the Torontonian bypass option :). I'll let you know what happens when I try. This forum is great! Hopefully the blown fuses have kept me from doing any damage!!

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Re: Main fuse/Positive ground problems

Postby texasSlick » Wed Aug 23, 2017 6:53 pm

Where are you Located?

Many Forum members would be happy to ride over and give you some assistance, if you were local to them.

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