Custom Norton Commando Wiring Diagram – Mik Crispin

Grant Tiller

Mik Crispin got in touch via the contact button here on the website.

He has an August 1973 Commando with a 920cc engine, and has reported that due to a ‘knackered knee’ will be fitting an Alton Starter kit to his bike.

Mik does not have turn signals on his bike, so a lot of wiring can be removed for those (including the flasher unit of course).

With this in mind, I have gone ahead and stripped out as much from the standard wiring harness as I possibly could – making it as simple and streamlined as possible,

Using a standard harness as a donor, without too much work, a lot of the bulk can easily be gotten rid of.

Electronic Ignition

Grant Tiller

A really popular Norton Commando upgrade is to move from the old points-based ignition system over to Electronic Ignition.

One of the most common units of the time is Boyer Bransden, who have been around since 1969.

Grant Tiller

They are still going today, and their website can be found here.

Moving from points to Boyer electronic ignition is a pretty simple upgrade.

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 Boyer 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.

The color coding of the wiring is simple:

Wire ColourDescription
Redthis is the positive feed to the Boyer, and is usually picked up from the red wire that goes to the Coil positive terminal
Blackthis is the negative supply FROM the Boyer TO the coils
Whitethis is the negative feed to the Boyer – it 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. If the white/yellow is long enough, you can connect the white wire of the Boyer directly to it!
Black/Yellow and Black/Whitethese go from the Boyer black box (they call it the Transistor Box) down to the Stator Plate that sits behind the points cover.
  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.

Regulator/Rectifier

Mik currently has a Boyer Bransden Power Box regulator/rectifier fitted, which is fed from a three phase Lucas alternator.

When he fits the Alton kit, it comes with it’s own single phase alternator stator, so Mik can retain the Boyer Power Box, and simple use two of the three yellow AC wires (carefully taping up the third, unused one)

The Power Box does not support the Lucas silver can assimilator, so it is necessary to remove it.

There are five wires to connect:

Wire ColourDescription
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)
Redthis is the Positive output and will join to one of the red wires in the harness
Blackthis 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
Grant Tiller

Electric Start

Grant Tiller

Mik has chosen the Alton Electric Start kit.

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 three phase Boyer Bransden Power Box – 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!

Grant Tiller

Another point worthy of mention is the positive feed from the battery – I have mentioned it several times on this site, 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

Charge Warning Light

As I mentioned, the Boyer Bransden Power Box is one of the manufacturers that has detailed that the factory warning light assimilator is NOT supported with their reg/rec unit.

I can certainly recommend the Improving Classic Motorcycles charge warning light as a brilliant alternative.

I use them myself, and was pleased that Mik said he is using one on his bike.

Grant Tiller

The nice thing about the Improving Classic Motorcycles unit is that you can retain the original warning light – so it looks totally factory (this for me is an important factory with the MK3 with it’s quirky little instrument panel.

It gives 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

Wiring Diagram

Here is the Custom Wiring Diagram for Mik’s Commando.

Custom Norton Commando Wiring Diagram – Mik Crispin PNG 3066×1841

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

Grant Tiller

1 reply

  1. Grant, many thanks for your time and assistance, your help greatly simplifies my loom which I shall make from scratch & I hope to have the Alton by April’s end, in time for the summer and the £2 litre petrol prices! Best to you , Mik

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