I have often used the color wiring diagram from Bill Turnbull – i find it much easier to read than the diagrams in the factory manual because it is color-coded rather than black and white.
Plus, Bill’s rendition is a faithful reproduction of the factory drawing, less the additional wiring found on the Interpol, which makes it simpler still.
You can find Bill’s website with his original diagram right here.
I often help people with their motorcycle wiring issues – both in ‘real life’ as well as on the Facebook groups and various forums.
I’m pretty good at wiring, and fault finding so am not scared to roll my sleeves up and get stuck in.
They say a picture paints of thousand words, so I often refer to pics when I’m trying to help someone, and Bill’s circuit diagram has proved really helpful on numerous occasions.
Over the next few days, I will be posting diagrams that have been modified to accommodate common upgrades – in particular electronic ignition and modern regulator/rectifier units.
In order to make it easy for me to overlay Bill’s superb diagram with different components, I have made a few small updates.
The wiring in these pics is as per the original, just reordered slightly, and made even easier to follow and with a few edits:
L.A.B from the Access Norton forum pointed out some inaccuracies of the factory diagram, so these have been changed:
- indicator switch wires (GR and GW) should be routed back to the main connector block
- there was no color shown for the points wires – these should be BW and BY
- the wire from the ballast resistor and linking the two coils should be WP
- the pin numbering on the master switch was wrong – with think this was an error in Bill’s diagram.
Pre-1971 (ammeter in the headlight shell and Wipac Tricon type handlebar switch) PNG 3066×1841
1971 (three wires to the master switch) PNG 3066×1841
1972 onwards (four wires to the master switch) PNG 3066×1841
On the MK3 diagrams, you will see a grey wire that goes from PIN 2 of the ignition switch (master switch) up to the right side handlebar switch. It is a hot feed to the starter button.
This was present well into MK3 production, but as Norton and Triumph models started sharing the parts bins the switch gear became common with the Triumph T140E. As a result of this, the grey wire disappeared, and the starter button was subsequently fed by the white wire that also feeds the kill switch (off/run switch)
1974 MK3 Early Bikes – there were around 2,000 bikes that were built around the December 1974 timeframe that have three additional fuses that can be found in the headlamp bucket.
These bikes are also wired with the old Lucas 3AW 3 wire ‘silver can’ assimilator.
1974 MK3 (Early) PNG 3066×1841
1975 MK3 this is the most common configuration, and takes us through to the final Commando that rolls off the production line.
1975 MK3 (Original) PNG 3066×1841
1975 MK3 Canadian Market – there were legal requirement in Canada around the headlamp being on while the engine was running, so the wiring diagram includes changes needed (swapping out the Warning Light Assimilator 06-6393 for the Headlamp Warning Unit 06-6392). Note that a different Master Switch is also required.
This is covered in the Factory Wiring Diagram, by notes.
1975 MK3 (Canadian Market) PNG 3066×1841
A couple of points about the way these diagrams have been drawn:
- 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.
- Wherever the earth or ground side of a component goes back to the battery, the drawing shows a red earth symbol:
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!
Hopefully these will help someone – don’t be scared of electrics.
The trick is to not be overwhelmed, print out the diagram, and follow each line.
There is no hurry, take your time.
And if you get stuck, reach out – there are lot’s of people willing to help!