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Romano Offline OP
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Yes you’ve read correctly I’ve put negative to earth on my 1970 BSA lightning.
I have been given the bike as a gift, it’s in great condition and working order. The only thing was the missing battery, so out of my experience I mounted negative to earth since there was no colours on the cables,
As soon as I did, smoke started coming from the front light so I immediately unscrewed the cable.
Now the bike starts and incredibly everything seems to work, the only thing I think it’s not charging the battery since I’ve put a voltmeter on the battery and the voltage doesn’t vary when I rev the engine, and the amp meter is always negative.
My question is this:
What have I probably fried ?
And where to buy ?
Thanks in advance for any suggestions

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Start by testing, not buying random parts.

What is burnt in the front light?

May we assume you have no Manual?

Last edited by triton thrasher; 04/22/21 6:41 pm.

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If it has the stock rectifier, it will be dead, smoke from the headlight may have been the zener diode finned thing releasing its working fluid.
A good time to upgrade to a new power box/ pod sort of thing. This all assumes the electrics were stock to begin with.


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Romano Offline OP
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I have a Manual but thought better not to read it.
I think the smoke came from the amp meter, the rest in the front light looks ok.

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Romano Offline OP
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Yes probably the rectifier Is damaged. And yes it’s ana incredibly original bike with what I think was an original rectifier.
Had another look in the front light and the wires leading to the amp meter are melted, that’s where the smoke came from.
Do you think just the rectifier has shorted or considering that the bike starts the rest should be ok?

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I hate to say this , your loom may be fried, at least certainly the ammeter wires will be, theres every chance the zener went dead short circuit this would pass a lot of fault current from the battery lead to the ammeter( which will also be cooked) then out through the brown and white lead which stops off at the hot side of the ignition switch on its way to the zener diode, the rectifier might be OK. Check the battery to frame wire, its probably cooked as well.
if it runs , put a volt meter across the battery terminals and see if volts rise with engine revs. be sure you tape off any exposed conductors if insulation has melted,


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Did it not have an original fuse?


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Originally Posted by Romano
I have a Manual but thought better not to read it.
I think the smoke came from the amp meter, the rest in the front light looks ok.

The ammeter can usually handle more current than any other electrical component, except the battery.

Last edited by triton thrasher; 04/22/21 8:06 pm.

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was there a fuse ?

sounds like you fried the zener . ( not the first , American anyway, to do this )
it lives lived under and infront of the gas tank ( under the ignition key )
... between the frame down tubes and front forks
iside the finned aluminum piece )
[Linked Image from cdn1.mecum.com]

the zener will flow , when hooked up backwards , as almost a dead short .
( like a normal diode ) (with only about 0.6 volt drop ) ... this would have melted the insulation
on the wires to the zener ( the good news is this wire will not be used when upgrading to a
new style voltage regulator ) ... but check the harness , this is part of determining what failed .
( it fits with smoke and smell at headlight area ... the wire to the zener woulda been white

you may have fried the rectifier too [Linked Image from i.ebayimg.com]
5his piece will also be replaced with a R/R ... but check that
that any wire insulation attached here are not melted .
( no smoke or smell from under the seat is a good sign ... but check )

to fix ;
20 bucks will get you a good , but generic Chinese regulator/ rectifier
60 bucks will get you a 'branded' regulator/rectifier with a warranty and customer service
60 bucks will get a new zener .. if you can find one .

as part of one step back and 2 steps forward
it would make sense to check the AC output from the stator
before buying any new regulator rectifier

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Hi Romano,
With the battery installed the wrong way round, both the Zener diode and the rectifier will have become freely conducting from battery +ve to the –ve ground in this wrong system.
Both would have been carrying their proportional share of the high short circuit current.

I wonder why the fuse didn’t blow?

The Zener is 1 diode, so has 1 diode’s voltage drop (or resistance if you like).

The rectifier involves 2 diodes in series, so twice the voltage drop (or resistance if you like).

So the Zener would have been carrying about twice the current that the rectifier was carrying.
Assuming the battery was well-charged, both currents would be dangerously large.

Both the Zener and rectifier are possibly burnt out. Only testing will tell.
However the smoke you observed is more likely due to wires seriously over-heating.

Apart from those 2 components, the wires that are likely to have suffered serious overheating are:

Brown/Blue from battery to ammeter

the ammeter itself

Brown/White from ammeter via ignition switch to rectifier

Brown/White to Zener

The Red return wires from the Zener and rectifier.

As most of the length of these wires run inside the harness, alongside other wires, where heat dissipation is much less than where wires are open, if you find significant damage to any open wires, I would be seriously concerned about condition inside the harness.

I don’t like to be the bearer of bad news, but a new harness may be wise.

Best of.

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Romano Offline OP
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I haven’t had a chance to check all the wiring but for the moment the brown and white cables leading to the amp neater are fried.
When I rev the engine there is no voltage rise on the volt eater attached to the battery.
Hopefully it’s just the zener and a bit of wiring.
Thanks for the reply

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Romano Offline OP
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No
No fuse unfortunately

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Smoke came also from under the seat, so I assume it was the rectifier.
I’m not sure I what advantages are to upgrade to a new style regulator.
Please let me know if it’s worth doing.
And thanks for such a detailed reply.

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Romano Offline OP
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Hello koan
Best to have bad news all at once.

So realistically I have to change or give a good check to:
The wiring loom
The zener
The rectifier
The amp meter

Missing anything ?
Thanks so much for taking your time to reply

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Firstly, fit a fuse. Let’s not assume the loom is good, It may have fried the ends that you can see but there will also be a lot of insulation that you cannot see that will most likely have turned to powder. The book will say use a 35amp fuse..... don’t do this. Fuses 35 amp was the blow rating. A 15 amp new fuse will have a blow rating of around 30 amp and it will be sufficient.

A modern reg rec will be a good move. Fit a fuse from here also.

I doubt you will have fried the ammeter. Which the bike not running, switch main beam on and see if the needle moves, it should swing to about 4amps left marker on the meter.


Now let’s all have a beer beerchug

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71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
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Romano Offline OP
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Hello Allan
Certainly I will fit a fuse and put red and black cables to the battery (presently they are both black)
Yes the ammeter needle moves, only to negative amp, but it moves.
At this point don’t think it’s damaged, but can’t tell for certain.
I will take your advice to put a 15amp fuse
thanks

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did the "smoke happen" with the ignition off ... or on ?
if it happened with the ignition off it simplifies the parts and wires involved .

what color is the wire to the zener ?

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Any loose open wires that show heat damage will have suffered far worse inside the harness, as heat loss is more restricted there.
Inside the harness, the melting of the insulation of the same wires will be greater than is evident on the free wires, and it will have impacted upon adjacent wires.The integrity of the insulation between those wires will have suffered. and as Allan described it will be more brittle and flaky.
Especially in places where flexing occurs (eg the headstock area) problems of shorting between damaged wires are quite possible as time goes by.

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Romano Offline OP
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Originally Posted by quinten
did the "smoke happen" with the ignition off ... or on ?
if it happened with the ignition off it simplifies the parts and wires involved .

what color is the wire to the zener ?

Can’t tell you for sure, but I think ignition off.
I will tell you tomorrow what color wire leads to the diode, it’s one o clock in the morning here wink

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Romano Offline OP
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Originally Posted by koan58
Any loose open wires that show heat damage will have suffered far worse inside the harness, as heat loss is more restricted there.
Inside the harness, the melting of the insulation of the same wires will be greater than is evident on the free wires, and it will have impacted upon adjacent wires.The integrity of the insulation between those wires will have suffered. and as Allan described it will be more brittle and flaky.
Especially in places where flexing occurs (eg the headstock area) problems of shorting between damaged wires are quite possible as time goes by.

I will probably change the entire wiring, as soon as I have a bit of time I will check better the wires in the harness and if I spot damage I would feel safer just changing it.
Hope it’s not to time consuming.
Thanks for the tip on where to look

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Reversing the polarity on the battery causes a dead short through the rectifier, heating up the wires between and frying the rectifier. This should have blown the fuse if the battery was properly fused. I find a fuse between battery (+) and the ground post removes all confusion; that is, you know nothing can flow to or from the battery without going through that fuse.


Mark Z

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

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