***UPDATED*** Pre-MK3 Commando Custom Wiring Diagram (Greg Marsh Enterprises)

Grant Tiller

*** UPDATED APRIL 2024 ***

Updated to include a few tweaks that Greg now makes to his wiring harnesses.

Greg Marsh from the Access Norton forum reached out to ask for some assistance with a wiring diagram.

Greg has already modified the original Bill Turnbull diagram (which you can find right here to accommodate some of the changes he has made. However, I have since made several changes and edits to Bill’s original (as well as covering the older models)

Greg has actually done all the work already, designed and built a superb, bespoke wiring loom.

He has documented it all in great detail over on his website, which you can find here.

The purpose of building a Wiring Diagram after the event is to help others that want to follow Greg’s superb example!


Greg has done the usual thing and swapped in a modern regulator/rectifier in lieu of zener diode and lucas rectifier (Greg has chosen to retain the capacitor).

Our Commandos use a lucas full wave rectifier and separate zener diode (which can be found mounted on the back of the z-plate) and rectifier unit.

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

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

Grant Tiller

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

There are four wires to connect:

  • Two Yellows – these are the AC input 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)
  • The Red – this is the Positive output and will join to the red wire if you are using existing wiring (it goes straight to the ground/earth of the frame)
  • The Black – this is the Negative output (known as the hot wire) – it will pick up on the Brown/Blue wire (which goes via a fuse straight to the battery negative terminal)

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.

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

Grant Tiller

Electronic Ignition

In addition, he has chosen to go up the electronic ignition route in lieu of points, condenser pack and ballast resistor.

His choice is the excellent Tri-Spark Classic Twin.

You can find the Tri-Spark website here.

Grant Tiller

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.
  • 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/White – this is the negative supply FROM the Tri-Spark TO the coils.

As with the Boyer, 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 diagrams 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. circuity 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 the delicate Commando sprag clutch)

Simplified Wiring

However, it is some of the other upgrades and optimisations here that Greg has made in order to give simplicity and reliability that has really piqued my interest!

For example, he has removed the main connector block found under the fuel tank – a popular home for many faults and unreliability.

Wherever possible Greg has removed the bullet connectors, choosing to wire from the connector of the component itself straight in to the wiring loom.

He has run a 14 gauge red wire from front to back, splicing in to it wherever a component needs a ground. This negates the unreliability of using the frame as a ground, and relying on painted/rusted surfaces for electrical conductivity.

All of his joints are soldered and heat shrinked, and I am very impressed with the quality of his work. He has also soldered and heat shrinked a pair of diodes in the turn signal warning light leads – which means he can use LEDs for the warning lamps instead of filament lamps.

It’s well worth popping over to his website to see how he’s done it!

Wiring Diagram

So without further ado, here is the Wiring Diagram!

Greg Marsh Enterprises Pre-MK3 Commando Custom Wiring Diagram PNG 3066×1841

Grant Tiller

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

Grant Tiller

Wiring Diagram (Negative Earth)

Greg also mentioned that some of his customer specify a bike that is wired for Negative Earth – in line with more modern machinery.

So here is a modified wiring diagram that covers negative earth.

Greg Marsh Enterprises Pre-MK3 Commando Custom Wiring Diagram (Negative Earth) PNG 3066×1841

Grant Tiller

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

Grant Tiller

Greg says on his website “It might sound strange, but I like wiring Vintage British Motorcycles!”

It might sound really, really strange, but I really enjoyed helping out by putting this wiring diagram together!

7 replies

  1. Grant,

    In your recent post that included the custom diagram you made for me, you mentioned PODtronics. The drawing now uses the Tri-Spark MOSFET regulator.

    The drawing shows two diodes that allow the turn signal indicator to be LEDs. The write-up is in: https://www.gregmarsh.com/MC/Norton/Norton_1974_Wiring.aspx, near the bottom and the drawing that explains it is here: https://www.gregmarsh.com/MC/Norton/images/1974%20Norton%20Wiring/Turn%20Signal%20Wiring.jpg

    Finally, since the Tri-Spark regulator can be used single- or three-phase, you might show a third yellow wire from it.

    Thanks! Even though I could wire a Norton or Triumph from memory, when doing Nortons I print one and mark off the wires as I run them.


    • Hi Greg – great to hear from you!

      I am not sure I follow what you are saying in your note?

      The diagram here on my site was modified back on 26th August 2020 to show the Tri-Spark MOSFET reg/rec instead of the PODtronics one, as I noted that you were promoting them quite heavily on the Access Norton site.
      I also amended the article at the same time to replace the PODtronics overview with some Tri-Spark info.
      I don’t think there is any reference to PODtronics in the article?
      I was still on the Access Forum at that point (I left just after Christmas 2020)

  2. Hi Grant,
    I’ve been studying your diagrams trying to figure it all out and finding them invaluable in making sense of it. I notice you don’t show many fuses? I’d prefer to have a fuse box in the battery compartment. Do you have a diagram showing such an arrangement, preferably with Trispark, sh775, warning light assimilator and the three pole flasher?
    Harry McMillan

    • Hi Harry!

      To be honest, I am not a big fan at all of using loads of fuses on a bike as simple ours.

      Norton tried it themselves with the first 2,000 or so MK3s which were built at the end of 1974.
      There were three additional fuses in the headlight bucket which were added to the original one at the battery.
      It was a nightmare to troubleshoot, and was very quickly replaced!

      The other idea of using a fuse box at the battery also unnecessarily complicates things – much more bulk in the wiring harness, with additional heavier gauge wires running the length of the bike.
      The other challenge with using a fuse box is that you cannot easily use a common feed from the battery, as actually you want the majority of the hot negative feeds to be coming from the ignition switch, not the battery, so that everything is shut down when you take your key out.
      …in my opinion it makes for a messy install.

      That said, I have done a few diagrams for people that are insistent on having this.

      My recommendation would be to have one additional fuse.
      Use that between the reg/rec and the battery.
      It means if there is ever an issue with the charging system or the battery, you have a good chance of getting home on the alternator alone, and the rest of your bike will be well protected.

      I did a diagram like this a while ago, see what you think:

      Extra Fuse

  3. can you help me with a query with a mosfet 20 fitted,do I need the warning light assimilator relay to control the charging light on the dash.I wish to leave the dash light operating cheers

    • Hi Keith,

      Tri-Spark say that the factory standard warning light assimilator is not compatible with their MOSFET reg/rec.

      I believe that continuing to use it will invalidate the warranty.

      I as personally a big fan of the Improving Classic Motorcycles Charge Warning Lights.

      Their ‘standard’ model is about the size of a postage stamp, can easily be hidden away and connected up inside the headlamp bucket, and can use the existing incandescent warning lamp.

      I prefer this to the various LED options available on the market, because to the outside world, everything looks totally standard.

      Here is a link to their website.

      You can buy via their site, or through their eBay store – they ship all over the world.

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