Simplifying wire harness

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Mar 2, 2008
About 25 years ago I owned a MK3 that had been rewired by the PO to a simpler harness ( with negative ground)..I am thinking of updating my Mark2A similarly...anyone have any diagrams they can share or ideas on the matter? TIA .Cheers
I think it's a great idea. Probably the easiest way is to start with a wiring diagram and a "full" wiring harness, figure out what you don't need (and that's likely to be a LOT, beginning with the Interpol stuff, indicator stuff if you don't run them, capacitor, ammeter, you name it.

I just put a new harness on, and I was gonna go this route but was discouraged by the thought of having to unwrap miles of electrical tape to get to the wiring (I got one of the tape-wrapped harnesses) so I just put it on as is. Wish I'd taken the time to stick to my original plan.

I would also recommend using plastic split-loom conduit for as much of the harness as possible. Protects the wiring and looks great too. Get the 1/4" inch diameter stuff in a 25' foot roll, maybe $10.

Good luck. - B
The harness could be tidied up, and changed to negative ground to make it more "normal". The problem would be to get a new zener diode with reverse polarity; if existing diode was re-used, a positive on the isolated side would see a direct short to ground. Would need to change to a podronics or similar regulator, assuming that they will work with negative ground. Good luck.
I added more circuit breakers.. isolating the various circuits ..To me one fuse DOES NOT cut the cheese !!

At least now I can keep going if one circuit gets into trouble ~

For example.. ignition circuits on one and lights and permanent feed on another ~ Involves a little tinkering but well worth it I feel !

(*))*&(&^&* off that multiple junction under the fuel tank..

( Did the MKIII have that ? )

I have had so much dramas with that thing over the years and have replaced it !

As Parker suggests.. the polarity can be changed but the rectifer has to be changed or severely modified !

Some of these replacement units are fully insulated and can be (fairly) easily reversed~ and~ some come with instructions on how to do that !

Oh and don't forget to reverse the polarity on your ignition coils as well !

Obviously the electronic ignition has to changed as well if you have it !
A positive ground Zener can be easily wired for negative gound by bolting on the hot lead via a ring terminal to the stud, and mounting it to a non-conductive tab of some sort; then ground it via a spade terminal to ground.
Negative ground is the way to go

I do wiring for Nortons and other brits when my friend has too much work, it's not what I do for a living, but I do a good job (well I think so )

Keep ip as simple as possible, draw it out on paper with what you need and where they will go. You can usually stuff most of the wiring connections and the flasher relay in the headlight shell. If you want something that work and don't mind not being original, a jap bike headlight switch on the left will control most anything, lights on-off, high beam, flashers and horn. There are aftermarket switches that are just as complete. If you have an electric start, a right hand control with engine cutoff and starter button will be needed. When you are satisfied of your wire placement, use a heat gun to shrink it in place (I don't use a torch like the guys on OCC)

I use heat shrinkable tubing instead of split tubing, first it will be smaller and it looks neater (black of course). Run one or two extra wires from the headlight shell to the battery compartment, they could be used if a wire breaks or you want to add something later on. With heat shrink tube, you can cut slits and take wires out where they will be needed (horn, points...).

As suggested, multiple fuses is a good idea, especially to keep the ignition circuit separate from the rest. Crimps are good, but make sure to use the proper sized connectors for the wires. Larger caliber wires will help reduce voltage drops, #14 for headlight and ignition as well as going to and from the ammeter, the rest (flashers, taillights, horn) can be done with #16 or #18. Use LEDs if you can for taillights and flashers, the stock alternator is not a powerhouse. Use #14 from the alternator to the rectifier and from there to the battery. Some suggest using a relay to give power to the coils, but if you use new components, I don't feel it is necessary.

You can do without the capacitor as long as you always have a battery. You can also remove the charge indicator and the relay that controls it. I would also take off the ampmeter, running wires from the front to the back of the bike withh all the current going through it is asking for trouble, leave it in place for the looks only. There are modern rectifier / regulators available like the Podtronics, use those, not only will they perform better, they keep the wiring simpler. You can get a good main swith with a key from just about any auto parts store. You can do all your wiring with black wires (did that on an Indian Scout, they didn't have any colored wires back then), but it is better to use different colors to avoid frying something.


Oh, even a motorcycle battery will fry a #14 wire in seconds, so be carefull where everything get connected.
Good thread.

I'll be embarking down the custom harness route myself soon, for my MKIII.

Not looking forward to it.
I have simplified wiring diagrams for Commando, Bonneville, BSA twins & thumpers, but they are all in AutoCAD and don't convert well to other image types.

Old Britts ( has simplified wiring diagrams posted in their 'Technical Articles' section. I modified my existing harness (1973 850 with Boyer and Podtronics rectifier instead of zener diode) using Old Britt's (Fred Eatons) diagrams as a go-by.

British Wiring ( sells the harness tape, bullet connectors, correct color and gauge wire, etc. to make a decent "like-original" job of a harness rebuild.

Hope this helps.
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