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DamienE Offline OP
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Hello all,

My 1970 Thunderbolt has been running good and I’ve been enjoying it. Tonight however I went for a ride and came to a stop at a light. Suddenly I see a cloud of acrid smoke rolling out from under the front of the gas tank! My first thought is fire!

I walk it to the side of the road. It’s still running so I shut it down and inspect the bike. No gas leaking anywhere, no oil (haha any more than usual) leaking onto anything hot. OK, no fire. I thought maybe the carb belched something up or something. Starts right back up and I’m on my way back home.

As I’m riding I notice that my headlight high beam indicator isnt illuminated on my headlight. My gauge lights aren’t on either. None of my lights are working.

I got it home with no issues and take the side covers off. Ugh, I see melted wire harness cover. The small red wire that goes into the wiring harness from the battery is fried and severed. All the insulation from the battery to where the cloth on the harness starts is melted and gone. I jumped the two parts of the melted wire with an alligator lead and the light works fine. My alligator jumper didn’t melt or anything.

I assume a short caused this but why didn’t the jumper melt like the original wire? Just an old wire that let go? Any common places to look? Doesn’t seem to be much accessible due to the cloth insulation.

I could run a companion wire from the battery to where the red wire terminates in the headlight housing But if this wire went could others be next? What would you do? Keep in mind this is no concourse bike. It’s a rider. I’m guessing a new wiring harness is in order. Any recommendations?

As always, thanks for reading and I appreciate your feedback!

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Check the Zener mount and its connections , if it comes loose bad things happen.


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
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DamienE Offline OP
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Forgot to add that the zener is disconnected. I added a Pazon digital ignition module and a Podtronics regulator/rectifier. Rode it quite a bit after adding those things with no issues.

Last edited by DamienE; 09/14/20 12:56 am.
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Can the seat base (or anything metal) touch the live battery terminal?

Or is your fuse bigger than 20 A?


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Seat pan touching the negative battery terminal?
That would go away once you raise the seat to fit a jumper lead.
It would also bypass the normal fuse in the negative lead.


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Originally Posted by triton thrasher
Can the seat base (or anything metal) touch the live battery terminal?

Or is your fuse bigger than 20 A?


If its fitted with the correct panels, the panels are metal. they should have a pad on the oil tank or the side panel (depending on which way you have fitted the battery) to stop the terminals touching the metal to ground.


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DamienE Offline OP
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I appreciate the replies!

I have a strip of rubber covering the negative terminal so I don’t think anything touched it. Just checked and my fuse is 30amp. Too high and something somewhere shorted and fried the wire but not the fuse?

Edit: yeah, looks like I should have had a 15A in there Or a 20A max. I think I’ll run a fused wire to the headlight for now and plan on a new harness install this winter.

Last edited by DamienE; 09/14/20 1:46 pm.
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The manual is misleading on fuse ratings if you’re not fluent in 1960s British automotive industry lingo.


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Do consider replacing the harness. 50+ year old wiring on any motorcycle shouldn't be trusted.


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DamienE Offline OP
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New wiring harness ordered. It will be a fun weekend project! Thanks again for the help everyone!

I’ll be sure to fit a 15A fuse this time so my wiring harness doesn’t become the disable link again should something happen!

Last edited by DamienE; 09/15/20 4:40 pm.
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Hello everyone! Just wanted to report back, I got the wiring harness installed today. About 5 of the connectors on the harness weren’t correct so I spliced the old ones in.

I also replaced the 35A glass vial in-line fuse that came on the harness with a 15A blade style fuse. I figure those will be more easily sourced if need be. I even threw a couple it my tank bag.

It took 7.5 hours start to finish with a few breaks thrown in but I got it done. Just got back from a test ride and everything worked great.

That’s one more BSA back on the road again!

Thanks for the help and have a great weekend!

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Did you identify what caused the melt-down in the first place? (I hope so).

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DamienE Offline OP
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In the course of my replacement I didn’t see anything obvious. I pulled out the entire headlight harness and went over everything. I didn’t see any damaged wire anywhere. With the entire main harness replaced I’m hoping (fingers crossed, knock on wood) that I’ve replaced whatever caused the issue.

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When you have a spare moment, un-peel the harness from that melted red wire and see how far the damage goes.
There was clearly a short happening.
It is better to put the fuse in the ground wire of the battery, than the live wire. This has been documented many times.
That your fuse apparently was twice the rating that it should have been obviously allowed more damage to occur than should have, but it doesn't change the fact that there was a short.
Hopefully the fresh installation will have eliminated that short.
I would have to understand the original short, for my own comfort.

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DamienE Offline OP
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The factory harness had the fuse going to the neg battery terminal. I kept it that way.
Are you suggesting I move the fuse to the positive battery terminal ground wire?

I should have a chance to tear into the old harness this week and hopefully see where the short occurred.

Thanks!
Damien

Last edited by DamienE; 09/29/20 2:38 am.
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I don't know that it's any different electrically speaking, but I find it more convenient to have the fuse on the positive (ground) side of the battery. For one thing, the inline fuse holder that I bought (blade type) has 14-gauge red wire. So without any fiddling except cutting and applying ring terminals, I can run the fuse holder from the battery to the ground post - done. Since I don't have an ammeter on my BSA, nor a terminal block, I have three wires on the negative battery terminal: Wire to power switch, wire to rectifier/regulator, and "accessory" wire. It would be inconvenient to fuse all of these paths.


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It doesn’t matter all that much, but a fuse in the return lead, near to the battery, prevents the sort of fireworks you get when you inadvertently connect the battery live terminal to the frame with some hand tool, or the seat base or toolbox lid touches the terminal.


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Originally Posted by triton thrasher
It doesn’t matter all that much, but a fuse in the return lead, near to the battery, prevents the sort of fireworks you get when you inadvertently connect the battery live terminal to the frame with some hand tool, or the seat base or toolbox lid touches the terminal.

Yes, that too!


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.

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