Custom Norton Commando Wiring Diagram (Steve Constable)

Steve Constable is a member of the Facebook Norton Commando Group as well as the Norton Owners Club, and reached out for some advice on rewiring his 1972 750 Norton Commando Fastback, which he is currently in the process of restoring.

It’s a great project, and I’m following along his posts with interest!

Grant Tiller

Wiring Choices

There are several choices available when considering a rewire.

This is a 1972 model, and although it is an unusual Fastback model, the harnesses are all identical.

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

These are all made by the same company – Autosparks – 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)

Steve is making a few changes to his bike as part of the restoration project.

Subtle but unobtrusive modifications that will aid riding in today’s modern stop/start traffic, so he has decided to go the other route, and build his own harness instead.

This is a fun process – bewildering for some, but if you take your time and go one step at a time, it’s actually quite a simple thing to do.

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’

Building your own means that you can take out all the unused bits (like the Interpol wiring) as well as the superseded bits (like ballast resistor, condensers, assimilator etc…

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 as a pair (front and rear).

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

Grant Tiller

If you are grouping the Tri-Spark and coil connections at the cylinder head, it is important to do this at the engine side, and not the frame side.

Remember that the isolastics are an electrical insulator as well as a vibration insulator!

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


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:

Grant Tiller

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.

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.

Grant Tiller

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:

Wire ColourDescription
Redthis 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 wire up to the coils, I always draw my wiring diagrams in this way to cover this recommendation
Black/Yellowthis is the negative feed to the Tri-Spark unit. This joins in to the White/Yellow that is the kill switch on your left side handlebar switch cluster
Blackthis is the negative supply FROM the Tri-Spark TO the coils

As with most electronic ignitions, 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 Tri-Spark 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 diagram below that the Ballast Resistor and Condensers have been removed as part of the conversion to Electronic Ignition.

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.


Steve has very sensibly opted for a three phase alternator stator.

This is, in my opinion, a great option to go for and is far superior to the factory original single phase unit.

The spec is as follows:

  • Lucas RM24
  • 3 Phase
  • 10.5 amp
  • Part Number LU47252
  • Also found under Part Number WW10193L
Grant Tiller

This stator has roughly the same output as the standard RM21 that came on the bike – however, the rated output is produced at a much lower RPM making this an ideal solution for around town and in stop start traffic conditions.

As always, I would recommend that you ride with your headlight on, as this will always help with the longevity of the components that make up your charging system.

The factory standard zener diode can handle this with no problem at all, so no need to change that.


Of course, the factory single phase alternator stator ‘talked’ to a single phase rectifier, so it is necessary to upgrade this component to a three phase one.

Fortunately, these are available at a very low cost indeed (around 10 dollars) – plus you get the advantage of something that is encapsulated in resin so is shake-proof (something that the original Lucas single phase rectifier suffered with.

A good three phase rectifier would be something like the 36MT60 – it is a very common component, and there are plenty of places online that sell them (you are not limited to classic Norton dealers)

The component looks like this:

Grant Tiller

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.

Oil Pressure/Charge Warning Light

A worthwhile upgrade that is worth it’s weight in gold is adding a Charge Warning Light in lieu of the standard warning light assimilator.

Grant Tiller

Steve has gone for a charge warning light from Improving Classic Motorcycles

These units give you a lot more useful information about the state of the battery and charging system compared to the standard assimilator unit, which looks for AC output from the alternator stator only.

Grant Tiller

Steve has, in addition gone one stage further, and got the version which includes and extra wire for an oil pressure switch.

I think this is a superb idea, again giving you peace of mind that the oil pump is working.

Steve makes and sells oil pressure switch kits, which are fully compatible with the ICM charge warning light.

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Steve will be using LEDs for his turn signals and warning lights, so it is important to change the facotry original flasher unit for a solid state one that can handler lower current lamps.

I have written about 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.

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

Grant Tiller

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.

Grant Tiller

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:

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 Steve’s wiring diagram.

General Tidy Up

Of course, when you are building a harness from scratch, it means there is no Interpol wiring to remove.

You’ll also notice that Steve has chosen to remove his Power Socket too.

Also gone is the factory blue can capacitor. This is not needed with a decent, modern battery in my opinion. Plus with the smoother, less choppy output of a three phase alternator, there is less benefit anyway.

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 Custom Wiring Diagram for Steve’s 1972 Norton Commando Fastback.

Custom Norton Commando Wiring Diagram – Steve Constable PNG 3066×1841

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

Grant Tiller

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