***UPDATED*** Custom Norton Commando JPN Wiring Diagram (Kelvin Borrow)

Grant Tiller
  • Q) What could be better than a JPN?
  • A) Two JPNs!!!

Kelvin first reached out via the website in 2022 – he was working on building/restoring a JPN and wanted to rewire it with a custom harness to eliminate unused wiring, get rid of the mass of connectors – especially the big block under the tank as well as accommodate an electronic ignition, a combined regulator/rectifier and generally tidy things up.

However, Kelvin is also building a second JPN, which will be his dream bike and will accommodate many more extras, including negative earth, digital instruments, electric starter and two USB chargers.

Again this will have a custom harness.

Bike 1 – the ‘Standard’ JPN

We will start with the most standard bike.

This is wired with standard positive earth, and has the mild upgrades of electronic ignition, and a combined regulator/rectifier.

Wiring Choices

Kelvin had several choices available to him when considering rewiring his bike – the JPN is built on a MK2a Commando, and only has a few minor differences.

There are a multitude of harnesses available ‘off the shelf’:

These are all made by the same company – Autosparks – the one and the same company that built the wiring harnesses originally for our bikes!

However, each company that sells them has specified different criteria/materials/price points to Autosparks, so although they are made on the same fixture, they are not all identical in terms of quality.

Also remember that the piece of paper that comes in the box with the Lucas harness is STILL WRONG!!!
I have written about the errors in an article here.

(I have now given up trying to tell them this)

Kelvin wants to implement several non-standard parts to update the bike for modern motoring, plus on the basis he has four Japanese bikes in his garage, he would like to wire one of his Commandos as negative earth, so it’s all the same as the others (one of the only decent reasons to go ahead and do this, in my opinion)

So, with that in mind he has decided to build his own harnesses for both Commandos.

This is not as scary or as daunting as it may seem (many find it fun and therapeutic)

When we are ‘starting from scratch’ like this, we can take out all the unused bits (like the Interpol wiring) as well as the superseded bits (like ballast resistor, condensers, assimilator etc…) from the very outset, which means we can make things a lot neater!

Positive ‘Bus’

One thing I like to do, and highly recommend that others do when making your own harness is use this as an opportunity to sort out earths once and for all.

With the Commando, Norton (and Lucas) were innovative in that there was no reliance on the frame as a ground for the majority of the components on the bike – for example, although the original zener diode sunk it’s heat to ground on the nice, chunky heat dissipating aluminium z-plate (two on the JPN, as it has an uprated alternator stator as standard), and the component operated by electrically connecting via it’s mounting stud, there was still a ring terminal and a red wire on the back of it – I have gone into more detail about that in an article here.

The downside in the way Norton did it is that in many of the cases, each component has a loop – so there are two wires going to it – look at it as an ‘in’ and an ‘out’ – if the wire breaks at the connector, or one of the connectors becomes unplugged, it can interrupt the earth to the rest of the bike.

If I am building a harness from scratch, I like to run a cable from the front to back of the bike, then tap in for an earth for each component as required – there is a massive advantage in doing this, and making it part of the loom, as it means you rule out any unreliability associated with bad earths, or trying to get power through rust, paint, powder coating, paper gaskets, loctite, clutch cables, steering bearings and speedo/tacho drives.

For the sake of spending an hour running this cable now, it means you rule out a massive variable, and make troubleshooting really easy in the future. So well worth doing in my opinion!

I have drawn a simple diagram that shows the positive cable with the splices for the individual components – I usually have a 12-gauge cable to handle this – a 2mm² cable will handle 25 amps which is more than enough.

When you splice into the cable to tee off a positive to an individual component, you can use a lighter gauge cable that is rated for the maximum current that particular part will draw.

For example, the turn signals can have an 18-gauge cable – 1mm² will handle 8.75 amps and be more than enough.

Those old 21-watt lamps used in the turn signals will draw no more than 3.5 amps.

Here is a diagram which covers the design for the positive ‘bus’ that runs from front to back of Kelvin’s bike, with the splices to feed the individual components.

Grant Tiller

This is also downloadable as a PDF:

Grant Tiller

So, the correct routing for the positive feed is:

  • A 12-gauge cable running from front to back of the bike
  • Smaller ‘taps’ to feed positive supply to each individual power consumer
  • No looping from components, take each positive feed back to the main cable that runs along the spine of the bike

It is important to make the ‘head steady’ connection on the engine side of the head steady… Isolastics are a good electrical insulator as well as a good vibration isolator!

Don’t forget, we have no need to earth the frame on these bikes.

Note that in the above diagram, I have drawn in a positive feed to each of the pilot lights (don’t forget there are two on the JPN)

Sometimes these have a proper, wired positive connection, other times they earth out through the body of the lampholder.

I don’t understand the rhyme or reason to these lampholder types – I have seen two different types on two bikes of exactly the same year. But it is something to watch out for when you are running wires for your positive ‘BUS’

The other factor to note is the turn signal indicator ‘stalks’ – I have mentioned in other articles, that this is one to watch out for.

Originally parts of these were chromed plastic, and relied on conductivity through the chromed plastic for an earth – which was not reliable!

It’s a really poor design, and I don’t like it – there is room inside the tube of the ‘stalk’ to run an extra wire and do a proper job!

Wiring Technique

When I am making up looms and harness from scratch, I like to minimize the number of connectors I use – ideally using them only at the point the cables plug in to the components themselves.

This feels contrary to what they did back in the 60s on bikes, where you seem to run into connectors for the sake of it – these are potential points for moisture ingress, terminal corrosion (the dreaded verdigris) and ultimately failure.

If I am splicing like in the case of the positive ‘bus’ (covered above) that I like to run from front to back of a bike, I like to bare the cable using wire strippers where I want my splice to be, I then twist the junction wire around the bared section, solder it, and use an adhesive lined heatshrink sleeve over the top which will protect the joint from moisture ingress and provide decent strain relief.

I feel that mechanically twisting the cables as I do, and then strain relieving them so well means there is zero risk of a soldered joint becoming dry or failing – I have certainly never had a failure in many years.

Here are some pics of my splicing procedure:

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For the connectors themselves, I do not like soldering – I much prefer to see a quality crimped connector – made utterly reliable using a decent and correct crimp tool.

When I have finished making the harness, and I wrap the whole thing in cloth tape, you’d never know there is a joint there!

The tape I like to use is Tessa 51608 fabric tape (it is also known as fleece tape)

It’s nice and furry, sticks well to itself and doesn’t come unraveled. It gives a great OEM look.

Grant Tiller


The reg/rec will be on it’s own dedicated fuse which is wired directly to the battery.

The beauty of doing this is that if, for whatever reason there is an issue with the charging system, you can pull the fuse and still get safely home on battery power alone.

Also, if there is a failure of the reg/rec, there is no risk of overcharging and boiling the battery dry – it’s a smart move.

I have found issues with the glass-style fuseholders in the past – the springs become weak over time, and eventually the circuit becomes intermittent.

As the fuse disconnects and reconnects to the contacts, a small amount of arcing occurs – over time a layer of ‘soot’ will build up over the contact patch, which in itself acts as an electrical insulator.

This can impact all sorts of things, not least the smooth running of your engine!

The symptoms of this feel very much like fuel starvation, so most assume there is a carb problem before they even start looking at the electrics!

I would recommend using automotive blade type fuses all round instead of the original glass type used on these bikes.

These are great, as blade fuses are available in every garage and petrol station, and are very resilient to vibration.

My rule of thumb is usually a 15-amp fuse for standard bikes, or a 20-amp fuse for MK3s or if you have fitted high output alternator etc…

Grant Tiller

The other thing to watch with the old style fuses is the value.

Our workshop manual specifies a 35-amp fuse:

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This was written in the 70s for a 70s british bike

It did not take into consideration that a US fuse is rated in a different way!

The British standard was to show the blow value on the fuse, and in the manual – not it’s continuous rating.

The US standard (which was subsequently adopted internationally) is to show the continuous rating value on the fuse, and in the manual.

Some fuses, back in the day showed BOTH their continuous rating value AND their blow value, but this was certainly not always the case.

Grant Tiller
Grant Tiller

To this end, I see MANY bikes fitted with the wrong value fuse – a 35-amp continuous rated fuse will blow at 70 amps… a long time after every cable on the bike has melted.

Not good.

Modern blade type fuses are labelled and referred to by their continuous rating. Everywhere. Worldwide.

So, you know where you are, and there are no nasty surprises.

Electronic Ignition

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A really popular Norton Commando upgrade is to move from the old points-based ignition system over to Electronic Ignition.

One of the most common units of the time is Boyer Bransden, who have been around since 1969.

Grant Tiller

Boyer Bransden are still going today, and their website can be found here.

Moving from points to Boyer electronic ignition is a pretty simple upgrade.

From a wiring perspective, the most important thing to note is that you will be moving from a pair of coils that are wired in parallel to series.

Originally, the points make and break the positive (earth) side of each coil in turn.

The Boyer electronic ignition system uses a concept called “wasted spark” – with the two coils wired in series, they are energized together on every rotation of the camshaft.

You’ll note in the wiring diagrams below that the Ballast Resistor and Condensers have been removed as part of the conversion to Electronic Ignition.

The color coding of the wiring is simple:

Wire ColourDescription
Redthis is the positive feed to the Boyer – it joins straight in to the ‘positive feed bus’
Blackthis is the negative supply FROM the Boyer TO the coils
Whitethis is the negative feed to the Boyer – it joins in to the White/Yellow cable from the handlebars, and is the kill switch (push-to-break)
Black/Yellow and Black/Whitethese go from the Boyer black box (they call it the Transistor Box) down to the Stator Plate that sits behind the points cover.

The Boyer Bransden Micro MKIV is the current iteration of their Electronic Ignition system, and offers a few improvements over the MKIII – including a slightly optimized timing curve which improves stability at engine start, plus the ability for the electronics to run at lower voltages (10 volts)

The old Boyer MKIII liked a good, strong 12 volts, and this was the cause of backfiring, particularly on the Electric Start Commando MK3

The Instructions specific to the Norton Commando that come with the Boyer Bransden Micro MarkIV can be found here:

Grant Tiller

Here is the Boyer Bransden Micro MKIII General Information Sheet

Here is the Boyer Bransden Micro MKIV General Information Sheet


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One of the most common upgrades or modifications for a classic british bike is to add a combined regulator/rectifier unit.

Our Commandos use a blue can capacitor, 2 zener diodes on the JPN (which can be found mounted on the back of each of the z-plate) and rectifier unit (half wave for the JPN).

A combined regulator/rectifier replaces all of these components with one package.

Grant Tiller

Boyer Bransden are also popular manufacturer of aftermarket regulator/rectifiers with their Power Box units which are available in single phase, three phase and high power versions.

Bike owners that have gone up the route of Boyer electronic ignition often choose to install the Boyer reg/rec too, as many like them to match.

However, one very important thing to note, which often gets overlooked is that the Boyer Bransden Power Box is not compatible with the Warning Light Assimilator used on the Norton Commando.

You can see in the note below:

If your Norton has a charging warning light simulator it must not be used with the
power box, remove it. If the light control is required fit the power box type
PBOX00166 with charging light control.

While the wording is poorly put, and the grammar is bad, the message is very clear.

You can see the original instructions here:

Grant Tiller

As you can see in the instructions, Boyer Bransden sell a different Power-Box unit, if you want to continue and use the red warning light to show charge. It is the PBOX00166 model.

The Instructions that come with the PBOX00166 Boyer Bransden Power_box regulator/rectifier with charging light control can be found here:

Grant Tiller

There are six wires to connect:

Wire ColourDescription
Yellow (x 2)these are the AC inputs and pick up on the Green/Yellow and Green/White (connection can be any way round, as this is the AC side of the circuit)
Redthis is the Positive output from the Power Box – it joins straight in to the ‘positive feed bus’
Blackthis is the Negative output – it will go via a dedicated fuse straight to the battery negative terminal
Whitethis is the feed to the warning lamp – it is switched by the reg/rec based on the presence of an AC output from the stator
Red/Whitethis is linked to the ‘positive feed bus’

Turn Signals

I previously did an article on upgrading turn signal flashers to a more modern type that will allow for LED lamps in the future, or different wattage bulbs without affecting flashing speed. You can find the article here.

On the grounds that Kelvin is rewiring his bike from scratch, it makes absolute sense to run an extra wire in the harness from the flasher relay up to the warning lamp in the headlight shell.

This gives you lots of options for the future, and means that if you go up the LED route, you will not have to worry about extra resistors or diodes (which some people refer to as tweakers) to get things working.

There are several options available, but I recommend Classic Car LEDs as one worthy of consideration.

Grant Tiller

They have a solid-state unit available, and purchase options for both Positive Earth and Negative Earth bikes – so perfect for our bikes.

Just make sure you order the correct one, as they are not interchangeable!

The reason I like these units is that the same unit will support incandescent lamps or LEDs, so there is a good degree of future proofing.

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The unit is a simple canister design, and can easily be hidden away on the bike.

Here follows a copy of the wiring diagram for positive earth as per their website:

Grant Tiller

You will note that this has a separate line to the warning lamp, which means no requirement for additional diodes (or tweakers).

For the sake of running one extra wire, it’s a worthy addition in my opinion!

I have added this change to the new wiring diagram.

General Tidy Up

I have taken this opportunity to remove the superfluous Interpol wiring, as it is of course not required on this bike.

The diagram also contains a lot of additional tidying – as I mentioned already eliminating connectors that are there for the sake of it, as these are typically areas of potential failure through vibration or water ingress.

Wiring Diagram

Here is the Wiring Diagram for Kelvin’s ‘standard’ Norton Commando JPN.

Kelvin Borrow ‘Custom ‘Standard’ Norton Commando JPN Wiring Diagram PNG 3066×1841

Grant Tiller

This is available as a PDF too – it can be downloaded here.

Grant Tiller

Bike 2 – the Custom JPN

This bike will be wired negative earth, and will have an electric starter kit fitted.

So let’s start by talking about the battery, and how it is wired into the bike, as there are some important considerations.

Battery Connection

As you read through the remainder of this article, and start to look at the diagrams, be sure to notice that there is only one connection to the battery negative – this is the heavy 6-gauge cable that goes to the primary case for the Alton Electric start.

It is important not to take any other connection to the battery on this side, as if for any reason you have disconnected the heavy gauge cable (maybe you are doing some maintenance work on the primary), and you inadvertently touch the starter button, you can quite easily pull 200 amps of unfused power through the other cables in the wiring harness that are rated at 20 amps.

The wires simply melt!

NVT had the same problem with the Triumph T160 back in the day.

They sent a Service Bulletin out to the dealers and distributors instructing them to cut the light gauge wire from the battery, leaving only the heavy one. The same thing should be done on the MK3 Commando too (there are a lot of commonalities with the electrical system on the MK3 Commando and the T160 Triumph, as they were under the same ownership by that point)

All of the MK3 diagrams here on my site have that cable deleted for this very reason.

Grant Tiller

You often see MK3’s with this cable cut. I think it is common for a new owner of a bike to wrongly reinstate this wire when they open the side cover for the first time and discover that the wire has been cut. Wrong – it’s been cut for a very good reason!!!

Negative ‘Bus’

The same methodology as described above with the Positive ‘Bus’ but incorporating the best practice recommended for the correct and safe connection of the battery.

Here is a diagram which covers the design for the negative ‘bus’ that runs from front to back of Kelvin’s bike, with the splices to feed the individual components.

Grant Tiller

This is also downloadable as a PDF:

Grant Tiller

So, the correct routing for the negative return is:

  • A single 6-gauge cable from the battery negative terminal to the Alton primary case
  • A connection at the head steady into the rest of the wiring harness (I like 12-gauge for this)

It is important to make the ‘head steady’ connection on the engine side of the head steady… Isolastics are a good electrical insulator as well as a good vibration isolator!

Don’t forget, we have no need to earth the frame on these bikes.

Electric Start

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Kelvin is planning to fit the Alton Electric Start kit.

These are superb quality, well-engineered and made in France.

They allow you to keep the triplex primary chain and the factory standard ‘ham can’ air filter.

They come with their own bespoke alternator stator and rotor as part of the kit.

The Alton alternator puts out around 90 watts at ‘cruising speed’ which is just enough for a standard bike – it maybe a bit of a struggle on a JPN with that extra headlight, so keep an eye on it!

The kit comes with a very well written installation guide, which you can access here:

Grant Tiller

There are lots of circuit diagrams in the installation guide, which hand-hold you through adding the Electric Starter kit to your bike – this is really helpful and shows superb attention to detail from the Alton guys – so kudos to them!

I am also pleased to see ‘alternative’ wiring diagrams for negative earth installs.

Do remember though, that you need to order the negative earth electric start kit from Alton, as the motor will spin the wrong way otherwise!

Grant Tiller

Another point worthy of mention is the negative return to the battery – I mentioned it before, and I’ll say it again – there should be one heavy gauge cable from the battery negative to the back of the Alton primary case and nothing else.

The harness (i.e., the rest of the bike) receives it’s negative connection from the ring terminal on the engine side of the head steady.

The Alton diagrams suggests that the battery should have an additional lighter gauge negative return. This is not correct.

Grant Tiller


Of course the largest battery you can squeeze in should be fitted.

Alton recommend the Yuasa YTX 20L-BS in their installation guide.

I personally much prefer the Motobatt AGM batteries – they are much better through a cold winter, are totally mess free, and have slightly more cold cranking amps than the valve regulated lead acid Yuasa equivalent.

There is less room on the ‘a’ bikes and the MK3, as the battery sits the other way and if you plan on fitting the Alton relay on top of the battery using their metal bracket (which personally I’m not a fan of) you will need to consider this when it comes to the height of the battery you go for.

If you do go up the Motobatt route, it is worth going for the “HD” version – these are black in colour instead of bright yellow. You can see it if you plan on removing the big plastic airbox, so it’s worth remembering!

Digital Instruments

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Grant Tiller
Grant Tiller

Kelvin has chosen to fit the aftermarket digital Speedo and Tacho units that are made by Speedhut and sold by Legendary Motorcycles and RGM.

Grant Tiller
Grant Tiller

These are superb – especially the Speedo, as it is a GPS unit, which makes it really easy to setup – no need to play with sensors on the back wheel!

The wiring for these is not complicated, but it is a little messy – due to the inverter box for the gauge lighting.

Because the same gauge internals can be used in a car, you can control the dial back lighting and pointer lighting independently – again this means there are more cables to play with than is ideal, but it’s not complicated.

***UPDATE*** the wiring diagrams and instructions have been updated for these units, as it would seem that Speedhut have recently deprecated the inverter unit used for the gauge lighting.

Here is the installation manual for the Speedhut GPS Digital Speedo

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Here is the installation manual for the Speedhut Digital Tacho

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Wiring Diagram

So, without further ado – enough rambling from me already – here is the Custom Wiring Diagram for Kelvin’s second Norton Commando JPN.

Kelvin Borrow Custom Norton Commando JPN Wiring Diagram (with Negative Earth) PNG 3066×1841

Grant Tiller

This is available as a PDF too – it can be downloaded here.

Grant Tiller

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