Klaus has a 1973 Norton Commando.
He is in the process of doing some work to the bike, including fitting some electrical upgrades.
One of the things that Klaus has asked me to help him with is wiring his bike for Negative Earth.
My usual recommendation is to keep it factory and refrain from doing this.
Changing to Negative Earth means that people like me can no longer look at the factory wiring diagrams and help with troubleshooting and fault finding.
Also, I have seen on many occasions boiled batteries and melted wires when someone helpful (or even a professional from roadside assistance or recovery) is at the roadside and has gone in to lend a hand and inadvertently caused untold damage.
When swapping to negative earth, from the factory positive, every red wire on your bike becomes a negative wire – which is confusing to most.
Plus every book, article, manual and drawing becomes wrong for your bike.
Most people think they want negative earth so that they can run LED lights and USB chargers.
However, your USB charger can still work fine and you can easily get LEDs that run on positive earth bikes.
Of course it’s the owner’s choice, and I’m just here to help.
This article assumes that Klaus is making the leap and rewiring his bike for Negative Earth.
Going for negative earth is not a simple case of swapping around the battery terminals.
Klaus has Electronic Ignition, a combined regulator/rectifier and a charge warning light – all of which must be rewired in order to accommodate the change.
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, so we need to make some changes here anyway.
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 to one of the coils, it is picked up from the WU (white/blue) which becomes WY (white/yellow) at the big connector block under the tank. This is the wire that is linked to the kill switch (push to break) on the handlebars.
- The Black – this is the negative supply FROM the Boyer TO the coil. It is important to note that even though this is feeding the negative terminal of the second coil, it is NOT earthed to the frame. It is still the black wire from the Boyer that pulses on and off as it receives a signal from the Stator Plate (trigger)
- The White – in a negative earth scenario, this becomes the connection to the earth.
- 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.
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 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.
The most common manufacturer of these is Podtronics and Klaus has gone for the three phase version.
There are five wires to connect:
- Three Yellows – these are the AC input and pick up on the three wires that come from the alternator stator. The connection can be any way round, as this is the AC side of the circuit, so polarity is not important.
- The Red – this is the Positive output and will join to the positive battery terminal via it’s own dedicated fuse.
- The Black – this is the Negative output which goes to the battery negative terminal or to the frame earthing point if you would prefer.
3 Phase Alternator
Klaus has fitted a 3 phase Lucas alternator – a great choice for superior charging in modern traffic conditions – typically you’ll see a positive charge at around 1,000rpm compared to the equivalent single phase alternator that will only swing to positive between 2,000 and 3,000rpm.
Charge Warning Light
A worthwhile upgrade that is worth it’s weight in gold is adding a Charge Warning Light in lieu of the standard warning light assimilator. This is another component whose wiring must be altered for a negative earth bike.
The nice thing about the Improving Classic Motorcycles unit is that you can retain the original warning light – so it looks totally factory.
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.
Here is the Negative Earth wiring diagram for Klaus’s bike – incorporating the upgraded components
- Boyer Bransden Electronic Ignition
- Podtronics regulator/rectifier
- 3 phase alternator
- Charge Warning Light
Custom Norton Commando Wiring Diagram – Klaus Völmle (negative earth)
Positive Earth Alternative
I don’t feel that I would be doing a proper job here unless I also produced the same drawing but retaining the standard Positive Earth.
So here are the same upgrades, but keeping the wiring factory standard.
Custom Norton Commando Wiring Diagram – Klaus Völmle (positive earth)