Tom Parham reached out for us help and advice regarding his rewire.

He has a 1972 750 Commando, and after 50 years and several previous owners, the wiring is in a bit of a mess.

Electronic Ignition

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.

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:

• The Red – this is the positive feed to the Boyer, and is usually picked up from the red wire that goes to the Coil positive terminal.
• The Black – this is the negative supply FROM the Boyer TO the coils.
• The White – this is the negative feed to the Boyer. This joins in to the White/Yellow wire that is the kill switch on your left side handlebar switch cluster.
• Black/Yellow and Black/White – these go from the Boyer black box (they call it the Transistor Box) down to the Stator Plate that sits behind the points cover.

Here is the factory instruction sheet for the Boyer Bransden Micro MKIV:

Regulator/Rectifier

Tom has gone for the A REG 1 combine regulator/rectifier.

This is designed and sold by Alan Osborn of AO Services

Alan is famous in Norton circles – being the Norton Owner’s Club Electrical Guru.

Here is the instruction sheet which comes with the A REG 1 combined regulator rectifier unit:

Light Switch

Tom has fitted a three position light switch instead of a two position one.

I was puzzled with what to do with the extra position at first, while still retaining the full functionality of all the positions of the master switch (ignition key switch)

So I decided to keep the ignition switch as-is – but not use pin 4.

It means that in “Parking with Lights” and “Ignition and Lights” key positions, the pilot light and tail light are illuminated (as per the 1971 bike)

Tom has used the Lucas LU31788 switch.

The terminals on the LU31788 switch are wired as follows:

So it functions, as follows:

I feel this is an accurate representation of what the factory would have done if they had used a three position switch on this bike.

Tidy Up

Of course a few things went in lieu of the above upgrades.

The ballast resistor and condensers went when the new Electronic Ignition was added.

The zener diode, full wave silicon rectifier, and blue-can capacitor were discarded to make way for the new A REG 1 combined regulator/rectifier.

Tom has also chosen to removed the warning light assimilator and the power socket.

The only other work done to Tom’s bike is to remove the superfluous Interpol wiring and minimise the number of connectors under the tank where possible.

This has made for a nice, simple and uncluttered wiring harness!

Wiring Diagram

Here is the Wiring Diagram for Tom’s bike.

Custom Norton Commando Wiring Diagram – Tom Parham PNG 3066×1841

This is also available for download as a PDF