Bad Earth

I often hear and read about Norton Commando owners discussing their poor earth and how terrible the earth is on these old bikes of ours.

Personally, I disagree – I feel the Commando is actually one of the best bikes from the time from an electrical standpoint.

Hopefully I can clarify some points in this article.

That was then

Back in the olden days, cars and bikes were wired like this:

Grant Tiller

The positive terminal of the battery was wired directly to the bodywork or chassis (for a car) or to the frame (in the case of a bike)

Then only the other side of the battery (the negative side) was wired up to switches and loads (lamps, horns, etc)

The other side of the load was then wired to a suitable point on the frame, to complete the circuit.

This meant simpler wiring, and less runs of cable in the main harness.

However, you’d typically see poor connectivity to the headlamp bucket, and earthing out through the speedo and tacho cable, or just as bad the steering bearings or the clutch cable – not good, and hence the bad reputation!

Fantastic Isolastic

When the Commando came along with it’s innovative isolastic engine mounting system – a way to seperate the vibrations of the engine from the rider, the engineers panicked slightly about wiring.

You see, the isolastic mountings are made of rubber, which is a superb insulator of electricity as well as vibration.

So it was decided that they would not rely on using the frame as the main conductor of the positive feed from the battery, but instead run a separate red wire from the battery inside the main harness to connect to all the key points on the bike.

Building on my example above, the Norton Commando is instead wired more like this:

Grant Tiller


You will see that these symbols are often used in electrical drawings (including mine that you will find on this site):

Grant Tiller
Grant Tiller

These symbols are interchangeable and both refer to earth or ground which is red/positive as standard on our bikes (and most brit bikes from the sixties and seventies era)

In the case of the Norton Commando, the symbols are referring to a physical connection to a red wire which is part of the wiring harness. The symbols are still used, because it simplifies the wiring diagrams and drawings.

Head Steady

One thing you should consider is moving the positive (red) ring terminal at the head steady from the frame side, to the engine side if it hasn’t been done already.

This becomes even more important if you have reinstated the third isolastic mounting by upgrading the head steady to the type sold by our friend Norman White.

This is what the factory had originally designed, but was dropped just prior to the Commando’s production on the grounds of cost savings in favour of the inferior exhaust mounting rubbers – the racer boys put a proper isolastic mount back on for the Proddy Racers.

The same too goes for the “Dave Taylor” head steady which relies on clamping the painted frame rail.

One Thing to Watch For

Of course, nothing in this life is simple – and there is one thing you need to watch out for…

Grant Tiller

Over the years various parts of these have been made out of plastic, and chromed – relying on the chrome to take the positive feed from the mounting nut to the lamp….
…certainly not the most effective solution!!!

So if you have issues with your turn signals not working properly, or one of them is flashing erratically or at the wrong speed, check here first!!!

My personal preference is to run a dedicated positive feed cable up to the lamp holder (there is just enough room inside the stalk) and then pick up on the red cable that feeds the tail lights – it is a much more reliable solution.

Norton Commando Wiring Harness

Here are some pictures of the three main types of Norton Commando wiring harnesses.

On them, I have circled the positive connections that are included in the loom, hopefully showing you clearly that the positive supply from the battery on these bikes is excellent, and there is no requirement around making sure the frame is well earthed.


Grant Tiller

1972 onwards

Grant Tiller

1975 MK3

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Electric Start

You will note that on the MK3 harness picture, there is NO red + wire to the battery.

This is because there is a heavy gauge cable that from the battery positive terminal directly to the engine crankcase right next to the Prestolite electric starter motor.

Grant Tiller

It is important not to take any other connection to the battery on this side, as if for any reason you have disconnected the heavy gauge cable (maybe you are doing some maintenance work on the primary), and you inadvertently touch the starter button, you can quite easily pull 200 amps of unfused power through the other cables in the wiring harness that are rated at 20 amps.

The wires simply melt!

NVT had the same problem with the Triumph T160 back in the day.

They sent a Service Bulletin out to the dealers and distributors instructing them to cut the light guage wire from the battery, leaving only the heavy one. The same thing should be done on the MK3 Commando too (there are a lot of commonalities with the electrical system on the MK3 Commando and the T160 Triumph, as they were under the same ownership by that point)

All of the MK3 diagrams here on my site have that cable deleted for this very reason.

Grant Tiller

You often see MK3’s with this cable cut. I think it is common for a new owner of a bike to wrongly reinstate this wire when they open the side cover for the first time and discover that the wire has been cut. Wrong – it’s been cut for a very good reason!!!

So the correct routing for the positive battery feed is:

  • A single 6 gauge cable from the battery to the crankcase on the MK3
  • A connection at the head steady into the rest of the wiring harness as detailed in the standard factory harness)
Grant Tiller

It is important to note that the aftermarket Colorado Norton Works and Alton electric starter kits should be wired in exactly the same manner.

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