I did I diagram for Peter Taylor back in February 2022 and he reached out to me here at the site yesterday to say he is helping out a mate of his Leigh Giles with a 1971 Norton Commando 750 Roadster rewire, and that they’d both find it useful to have one of my custom diagrams.

The guys are taking the route of getting a new wiring harness, and modifying it to accommodate the upgraded components, and also using this as an opportunity to strip out unused cables (like the interpol wiring).
It is well worth noting that if you are going for Lucas aftermarket harnesses, you need to be careful with the wiring diagram that comes in the box. While the quality of the harnesses are excellent, the diagrams are wrong!
I have two articles here on my site that provide the corrections.
- Lucas/Wassell 1970-74 Norton Commando Main Harness
- Lucas/Wassell 1970-74 Norton Commando Headlamp Harness
Electronic Ignition

Leigh is using the Tri-Spark electronic ignition – they have been around since about 2009.
You can find the Tri-Spark website here.
Tri-Spark get a bad press for reasons I have gone into in an article here – personally I have never, ever had an issue with them – always reliable, great customer service, and full 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 Colour | Description |
---|---|
Red | 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 wire up to the coils, I always draw my wiring diagrams in this way to cover this recommendation |
Black/Yellow | this is the negative feed to the Tri-Spark unit. This joins in to the White/Blue wire that used to feed the Ballast Resistor that you are removing. As standard, this goes up to the big connector block under the tank, where it’s joined to the White/Yellow that is the kill switch on your left side handlebar switch cluster |
Black | this 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:
- 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
- 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.
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 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.
Leigh has gone for the relatively new to market Tri-Spark MOSFET unit.
It is certainly easy to spot in it’s blue anodised heatsink!
There are five wires to connect:
Wire Colour | Description |
---|---|
Yellow (x 3) | these are the AC input and pick up on the three wires coming out of the three phase alternator stator (connection can be any way round, as this is the AC side of the circuit) |
Red | this is the Positive output and will join to one of the red wires in the harness |
Black | this is the Negative output (known as the hot wire) – it will be wired to the NU (brown/blue) that goes back to the battery negative terminal via a 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 behaviour which you can find here
Here are the wiring instructions for the Tri-Spark VR-0030 MOSFET regulator/rectifier.
The Tri-Spark MOSFET reg/rec is available at many stockists including from our good friends over at Andover Norton who are carrying decent stock levels of this unit in their inventory!
Their part number is 13.1801 and you can find it here:
Warning Light Assimilators
It is important to note that MOST aftermarket reg/rec manufacturers do not support either the pre-MK3 Lucas 3AW silver can warning light assimilator OR the MK3 one.
Boyer Bransden are very explicit in their instructions for the Power Bow – they have even made an alternative Power Box model that includes a charge warning light.
Other manufacturers (PODtronics, SPARX and the Tri-Spark MOSFET units) either mention it in their smallprint/FAQs or neglect to mention it at all.
As such, I would recommend NOT using the original assimilator and consider your alternatives.
You could replace your assimilator with a solid state equivalent like the CoolCat Express (warning: you have to buy positive SS3AW-P or negative SS3AW earth)
Alternatively, you could follow my recommendation and buy a Charge Warning Light instead.
A warning light assimilator tells you that the alternator stator is producing an AC output, whereas a Charge Warning Light tells you that the battery is charging, the reg/rec is working and it gives you much more useful information about performance of the charging system and the state of charge.
I personally recommend the Improving Classic Motorcycles “Standard” Charge Warning Light. This model wires in to the standard incandescent lamp, so it looks better than a modern LED. It matches all your other warning lamps, which is particularly important on the MK3 with it’s instrument ‘console’
The ‘brainbox’ is about the size of a postage stamp, and can easily live inside the headlight bucket or under the MK3 ‘console’
Wiring Diagram
Here is the Custom Wiring Diagram for Leigh’s Commando.
Custom Norton Commando Wiring Diagram – Leigh Giles PNG 3066×1841
This is available as a PDF too – it can be downloaded here.
Categories: Custom Wiring Diagrams