Custom Norton Commando Wiring Diagram – Jem Nicholls

Jem Nicholls reached out, as he has decided after many years that now is the right time to rewire his bike.

He has owned his 750 Interstate for the last thirty-five years, and during this time he has made several tweaks and upgrades to improve things on the bike.

Jem has ridden all over Britain and Europe on his Commando, covering over 60,000 miles!

Grant Tiller

He is an active member of the Norton Owners Club, and has attended many rallies over the years. He also edited the NOC Roadholder Magazine for around ten years, so many of you may recognize his name!

Wiring Choices

Jem had several choices available to him when considering rewiring his bike – it is a 1973 model year (built in November 1972).

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 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)

Jem has made a lot of changes to his bike over the years (some of which I’ll cover here) so he has decided to go the other route, and build his own harness instead.

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

It helps that you have the original wiring for reference, and with the components already fitted to the bike, it is just a case of ‘stringing them all together’

This means that you can take out all the unused bits (like the Interpol wiring) as well as the superseded bits (like ballast resistor, condensers, assimilator etc…

Battery Connection

As you read through this article, and start to look at the diagrams, be sure to notice that there is only one connection to the battery positive – 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!!!

Positive Feed

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 (in our case the positive feed) 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, 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 positive loop – so there are two red 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 positive feed to the rest of the bike.

If I am building a harness from scratch, I like to run a positive wire from the front to back of the bike, then tap in for a feed to 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 wire 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 negative wire 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 feed the 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 Jem’s bike, with the splices to feed the individual components.

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So, the correct routing for the positive feed is:

  • A single 6-gauge cable from the battery positive 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.

Note that in the above diagram, I have drawn in a positive feed to the pilot light and both instrument backlights.

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 the stalks were chromed plastic, and relied on this for the positive feed to the lamp.

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 red 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

Fuses

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.

Jem has also requested that we have an additional fuse for the headlight – this is the UY (blue/yellow) wire that comes out of Pin 4 on the ignition switch. A 5-amp fuse will work fine for this.

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…

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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
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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.

Warning Light Assimilator

The Lucas 3AW 3 wire ‘silver can’ assimilator is the most unreliable part of the bike in my opinion.

Think of the old-fashioned mechanical bi-metallic strip that is part of the thermostat on an old central- heating system – it’s basically the same sort of technology used here. Warming up and expansion/contraction of different metals to open and close contacts.

Couple that to a rattly, vibrating motorcycle, and you can suddenly understand why they were not wholly reliable.

Plus, there is the matter of what they are actually doing, and how much use that is.

The 3AW is looking for about 6 ½ volts coming out of the alternator stator.

It gives you no information about the charging (i.e., the regulator (zener) and rectifier)

It gives you no information about the state of the battery.

To this end, Jem has turned to fellow Norton Owners Club member (and Electrical subject matter expert) Al Osborn from A.O. Services.

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Al has a battery status monitor available, which replaces the standard assimilator.

It has two wires:

  • positive, which will tie in on the positive feed ‘BUS’
  • negative, which is picked up as the W (white) from pin 2 of the ignition switch
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This provides you with a lot of far more useful information than the standard 3AW provides, plus it is solid state so is far more reliable.

Handlebar Switches

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A common modification – in fact one that was wholly supported by Norton back in the day is to swap left and right handlebar switches.

It makes sense to most to have the turn signal switch on the left side.

Norton sent out a Service Bulletin to the distributors and dealers showing their support of this – and instructed that the change be implemented by the dealer according to the customer’s wishes.

Grant Tiller

In Jem’s case we are mixing it up a little more though, and to me his preference makes perfect sense!

He wants the indicators and the horn on the left side.

So, to this end, the wiring diagram has been modified:

  • the PB (purple/black) is now for the starter button (in use, as he has an Alton electric start)
  • the WR (white/red) is now for the horn

The engine cut-out button WY (white/yellow) will remain alongside the indicator switch on the left. It is more difficult to relocate this, as it is the only push-to-break switch out of the four push buttons. Pressing it momentarily cuts the hot feed to the ignition system.

The main beam flasher button of course sits alongside the dip beam/main beam dip switch.

Electronic Ignition

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The new kid on the block for Electronic Ignitions is Tri-Spark.

Well, I say new kid – they have been around since about 2009.

You can find the Tri-Spark website here.

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Tri-Spark get a bad press for reasons I have gone into in an article here – I have never, ever had an issue with them – always reliable, great customer service, and fully of some great features.

They are my personal preference for electronic ignitions, and I recommend them to anyone thinking of moving based on my own great experience.

The Tri-Spark unit is a one box solution – all the gubbins are mounted inside the points cover – no additional black box to try and hide under the tank, and very, very simple to connect up.

The wiring is as follows:

  • Red wire – this is the positive feed to the Tri-Spark unit. Most people attach this wire to one of the two fixing posts inside the points cover. I would personally recommend running an additional red wire alongside the other two and splicing in to the positive ‘BUS’ – I have drawn the wiring diagrams this way.
  • Black/Yellow – this is the negative feed to the Tri-Spark unit. As standard, this joins in to the White/Blue wire that used to feed the Ballast Resistor (which gets removed). In Jem’s case, this will go to the White/Yellow that is the kill switch on your left side handlebar switch cluster.
  • Black/White – this is the negative supply FROM the Tri-Spark TO the coil.

Jem has a dual output single tower coil from Boyer Bransden – so there is no worry with moving the pair of single 6-volt coils to a series setup for wasted spark ignition as with a normal points conversion.

You’ll note in the wiring diagram below that the Ballast Resistor and Condensers have been removed as part of the conversion to Electronic Ignition – they are no longer required.

Two major benefits of the Tri-Spark:

  1. a very low operating voltage – as low as 8 volts means your bike will still run with a less than optimal battery and charging system
  2. circuitry performs the electronic equivalent of advance and retard to make the bike easier to start and stop the possibility of kick-back. This makes it gentler on your knees, and kinder to electric start systems (aka sprag clutches and in the case of the Alton electric start kit tiny plastic shear pins)

One point worthy of note with dual output single tower coil conversions is sparkplug choice.

Most people don’t realise that with this type of coil, the sparks at the sparkplugs jump the opposite way on one lead.

So for one sparkplug, the spark jumps from the centre electrode to the ground electrode (as you’d normally expect)

However, for the other sparkplug, the spark jumps from the ground electrode to the centre electrode (which is opposite to what you’d think)

Therefore, when choosing your sparkplugs, you need to bear this factor in mind!

Many people go for precious metal plugs – like platinum or iridium.

The benefit of these plugs is longevity – they simply last longer.

With the spark jumping from the ground electrode to the centre electrode, this benefit is totally lost.

The answer is to run double platinum or similar plugs, where a precious metal is used at BOTH the centre electrode AND the ground electrode.

My personal preference is the Denso VW22 plug – this has a platinum pad on the ground electrode and an iridium centre electrode.

I like Densos as they just seem to last forever – I use the IW22 on twin coil bikes, and their VW22 is the ‘twin precious metal’ equivalent.

Electric Start

Jem has chosen the Alton Electric Start kit.

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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. And while it is ‘only’ single phase, it can easily work with the Tri-Spark MOSFET regulator/rectifier – you just use any of two of the three AC inputs instead.

The Alton alternator puts out around 90 watts at ‘cruising speed’ which is plenty.

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!

A couple of changes in Jem’s case – for example, he wanted his turn signals and horn button on the left side so the button with the white/red wires is no longer for the starter. This is now the job of the button with the purple/black wires instead.

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

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

The Alton diagrams suggest that the battery should have an additional lighter gauge positive feed. This is not correct.

Grant Tiller

Battery

Jem has followed the Alton advice and uprated his battery.

He has gone for the Motobatt MBTX20UHD

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This is a great choice – I really like the Motobatt AGM batteries, they seem to get through a winter layup with no problems at all.

No need to leave these on a tender over the winter, just a charge the night before your first springtime ride of the year, and you are good to go!

AGM is a sealed battery, so no worries about venting or spillage.

They are not overly expensive, and their Cold Cranking ability is good, making them a nice upgrade for electric start bikes!

Regulator/Rectifier

Another 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, zener diode (which can be found mounted on the back of the z-plate) and rectifier unit.

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

Jem has gone for the relatively new to market Tri-Spark MOSFET unit.

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It is certainly easy to spot in it’s blue anodised heatsink!

There are four wires to connect:

  • Two of the three Yellows – these are the AC input and pick up on the two wires coming out of the Alton alternator stator (connection can be any way round, as this is the AC side of the circuit)
    Note that the reg/rec is three-phase capable, but Jem will only be using it as a single-phase unit.
  • The Red – this is the Positive output and will join to the positive ‘BUS’
  • The Black – this is the Negative output (known as the hot wire) – it will be wired directly to the battery negative terminal and will have it’s own dedicated fuse.

The spec on paper is very good, being able to handle up to 20 amps.

And the benefit of MOSFET is much more precise control of the charge voltage. I have done a deep dive into reg/rec types and behavior which you can find here

Here are the wiring instructions for the Tri-Spark VR-0030 MOSFET regulator/rectifier.

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The other piece of great news is that our friends over at Andover Norton are carrying this unit in their inventory!

Their part number is 13.1801 and you can find it here:

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It is also now available from RGM and Rex’s Speedshop – my other two favorite suppliers!

Flashers

I recently 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 effecting flashing speed. You can find the article here.

On the grounds that Jem 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 bucket.

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.

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They have a solid-state unit available, and the option of Positive Earth – so perfect for our bikes.

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 as per their website:

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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.

Jem has elected to remove his Power Socket but install a USB charger up on the handlebars for charging his phone and powering his stanav.

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

So, without further ado – enough rambling from me already – here is the Custom Wiring Diagram for Jem’s 1972 Norton Commando Interstate.

Custom Norton Commando Wiring Diagram – Jem Nicholls PNG 3066×1841

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

Grant Tiller

2 replies

  1. I’ve just finished re-wiring the bike using all the above information and lots more help from Grant, it’s so good to have these knowledgable chaps around to help us keep our Nortons running well!

    Thanks Grant!

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