Norton Commando Wiring Diagram with Boyer reg/rec

Grant Tiller

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, 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.

Grant Tiller

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

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 and will join to the red wire if you are using existing wiring (it goes straight to the ground/earth of the frame)
Blackthis 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)
Whitethis is the positive 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 in to the Red ground lead

Here is the factory wiring diagram with the Boyer unit added into the circuit.

Pre-1971 (ammeter in the headlight shell and Wipac Tricon type handlebar switch) PNG 3066×1841

Grant Tiller

1971 (three wires to the master switch) PNG 3066×1841

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1972 onwards (four wires to the master switch) PNG 3066×1841

Grant Tiller

1974 MK3 (l fuses in the headlamp shellPNG 3066×1841

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1975 MK3 (Original) PNG 3066×1841

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1975 MK3 (Canadian Market) PNG 3066×1841

Grant Tiller


A couple of points about the way these diagrams have been drawn:

  1. Where the same colour wire goes in to and out of a single connector, that connector has usually been omitted from the drawing.
    It’s obvious on the bike, is easy to spot and easy to troubleshoot.
    Leaving them off the diagrams makes them a LOT easier to read, and considerably less cluttered.
  2. Wherever the earth or ground side of a component goes back to the battery, the drawing shows a red earth symbol:
    Grant Tiller
    In reality, this could be connected either to a red wire in the bike’s wiring harness (loom) OR it could be attached to the frame or engine of the bike.

    I have shown the red earth symbol each time in order to massively simplify the diagram, and make it a lot easier to understand for everyone.

    I have also coloured them red as a gentle reminder that these bikes are wired positive earth!

This article is from a series of three covering Boyer Bransden products:

As always don’t hesitate to reach out if you need any help or advice.

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