BSA Owners' Club Workshop Discussions

Hi,
I put a wiring loom on bike ( see attached) . The bike had no electrics whatsoever when I got it.
I converted to 12v, bulbs, rectifier, mag, bulbs etc. with negative earth and negative earth rectifier.
Everything is working: Main beam, high beam, light switches, pilot light, rear light, break light and light for speedo.
However can't get the horn to work ( 12v) nor the ammeter.
If i put a battery straight across the ammeter it works.

I have a couple of questions;
Is the wiring loom pictured for negative earth ? or does it mater?
Also do i still have to earth the main beam light fitting and pilot light with the head lamp casing?

I am surprised that you didn't,t fry the ammeter putting it across the battery. The headlamp reflector needs a earth return back to the shell . Normally there is two wires for the horn from the loom , one live and one back to the horn push. The feed for the lights has to come off the ammeter the opposite side to the feed from the battery. If not although the light will work the ammeter will show nothing. I take it the you are referring to a voltage regulator and not a rectifier which were not fitted on a B31 until 1959.











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Hi Idie,

Thanks for advice, luckily  did not fry ammeter!!
Sorry , I was referring to the voltage regulator .
I will try your advice tonight and let you know how it goes.
As you can see from the photos, there are 3 wires connected to the horn spades ( came this way when I bought the loom) and then another separate black wire coming from the same area ( see wiring diagram) which is assume is the horn push. The horn push wire is black ( but yellow and red on diagram) as stated and is long enough to connect to to all one light /dipswitch/ horn push on handle bar.  I assume the second photo is the horn push (black button)

I believe for a 1954 B31, the wiring should be +ve earth but changing to -ve earth isn't a big issue. The ammeter should be connected between the output from the regulator output (A terminal) and the battery +ve terminal - in the Lucas wiring diagram, it is the black/white from terminal A to the lighting switch T3 and from there on to the ammeter, and the yellow/black wire from the ammeter to the battery +ve terminal. The ammeter will then display the current from regulator to the battery.

The horn should be connected to the battery +ve terminal and then to the horn push and then to ground. The other yellow/black wire from the battery should go to the horn (feed) and then via the yellow/red wire from the horn to the horn press (button) with another wire from the press to ground.

As in most BSA wiring systems, the return from all the lights is through the tinware and frame. However, I always use a separate earth return wire back to the battery earth and never use the tinware or frame as you can never be sure there aren't any voltage drops due corroded connections of one sort or another.

Up to 1952 they were -ve earth but from 1953 were +ve earth. The original voltage control unit would work with either but a modern electronic regulator is made for either -ve OR +ve earth, so the bike will have to be wired to suit.
Attached are both the +ve and -ve earth circuit diagrams of it helps.

Thanks Dereck,
I have a negative earth electronic regulator.

I think your negative earth diagram follows the wiring loom diagram that i posted?


Cheers

Brian

Hi all,

I have squiggled on the wiring diagram which i have attached
The horn press wire  yellow/red is actually black  ( black squiggle) on the loom and is in the same place. There is no other horn press wire.
There are 2 yellow/red wires attached to the horn switch spade and I received the loom with the spades connected. I have marked this with a blue squiggle.

Please excuse my ignorance , as I am new to this .

Getting a little confused.
I followed the diagram exactly .

Many thanks to all

Brian

The horn feed (yellow/brown) should be live from the battery (or the battery side of the ammeter) and should be connected to one of the horn terminals. The yellow/red wire should go to the other horn terminal and the other end of it should go to the horn switch, although apparently it is black at that end in your loom. If the horn is connected and the battery fitted dabbing the back wire to earth should sound the horn. If so connect it to the horn switch and it should work.

Normally, the horn press will earth against the handlebars and so a separate wire is not normally needed. However, you would need to test that this happens when you press the button.

There should be 2 x yellow/red wires emerging from the loop which are two ends of the same wire. The yellow/black wire for the horn feed comes from the battery +ve terminal. One yellow/black wire goes to the horn and the other goes to the horn press button. As  said, pressing the horn button should earth its yellow/black wire to ground causing the horn to sound.

Thanks all

Fantastic wealth of information!

Once again Thanks

Brian

Does this dynamo need converting from pos earth to neg earth at all?
Might need flashing to convert

Thanks, I've already flashed the mag

Brian

Hi All,

Just to let you know I got everything working fine.

Thanks so much to you all for the wonderful advice .

Brian

Brian

In case you haven't noticed, in your first photo it looks like the insulation on the yellow/red wire is broken, just by your fingers; I can see the copper.

Roger

Hi Roger
Thanks I spotted that and sorted it .
Cheers Brian

Hi,
Just rewired my bike using new loom, ( using loom diagram  that i posted earlier in thread)  everything is fine except tail and break light..
converted bike to 12 v .

The issue is break light works ( engine off) fine.
However when I turn tail lights on the break light does not work.
I had a 12 led tail/brake light and it suddenly smoked and stopped working , I have put in an ordinary 12 light, this time the break light is quite dim but  when used with tail light you can't see it, but gets extremely hot. . Changed wiring around in brake switch with no joy also changed around wiring from light fitment to loom and again no joy. The bulb will only fit into bulb holder one way.
I bought a new tail light /brake fitting but no difference.
I stuck to wiring diagram which i posted earlier in this thread.

Thanks

Brian

I'm not sure what you did to the LED unit that burnt it out, it shouldn't be possible as an LED will only conduct in one direction. So if you connect it with the polarity reversed it just doesn't light up.
With the bulb though you have the brake light circuit connected to the rear light filament connection and the tail light wired to the stop light connection. Swapping the wires at the light unit should fix that.

Hi thanks,
I did reverse the wires but no difference. There is only one earthing point on the loom. Should I connect an earth wire from the light fitment to the bike frame?

Thanks for your help

Brian

If only as a test, it would be worth running a separate earth wire back from the rear light unit to the earth point or battery -ve terminal (I believe is a -ve earth system). I personally never rely on earth returns through the bike's tinware because a corroded bolt here or there will generate a voltage drop that will degrade the voltage reaching the lights, although less so on a 12V system. I always use separate earth return wires.

Ok will try that.
Will let you know how it goes.
Thanks so much
Brian

Hi Guys ,  tail/stop now working correctly with separate earth installed.

Thanks alot

Brian