Custom Norton Commando Wiring Diagram (Grant Hepburn)

Grant Hepburn got in touch via the Norton Commando facebook page, as he is in the process of working on his MK3 Commando

Grant Tiller

Electronic Ignition

Grant Tiller

The new kid on the block for Electronic Ignitions is Tri-Spark.

Well, I say new kid – they have been around since about 2009.

You can find the Tri-Spark website here.

Grant Tiller

Tri-Spark get a bad press for reasons I have gone into in an article here – I have never, ever had an issue with them – always reliable, great customer service, and fully of some great features.

They are my personal preference for electronic ignitions, and I recommend them to anyone thinking of moving based on my own great experience.

The Tri-Spark unit is a one box solution – all the gubbins are mounted inside the points cover – no additional black box to try and hide under the tank, and very, very simple to connect up.

The wiring is as follows:

  • Red wire – this is the positive feed to the Tri-Spark unit. Most people attach this wire to one of the two fixing posts inside the points cover. I would personally recommend running an additional red wire alongside the other two and splicing in to the red at the head steady or better still directly into the coil – I have drawn the wiring diagrams this way.
  • Black/Yellow – this is the negative feed to the Tri-Spark unit. As standard, this joins in to the White/Yellow wire that used to feed the Ballast Resistor (which gets removed).
  • Black/White – this is the negative supply FROM the Tri-Spark TO the coil.

Grant has a dual output single tower coil from cNw – so there is no worry with moving the pair of single 6-volt coils to a series setup for wasted spark ignition as with a normal points conversion.

You’ll note in the wiring diagram below that the Ballast Resistor and Condensers have been removed as part of the conversion to Electronic Ignition – they are no longer required.

Two major benefits of the Tri-Spark:

  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 (aka sprag clutches)

One point worthy of note with dual output single tower coil conversions is sparkplug choice.

Most people don’t realise that with this type of coil, the sparks at the sparkplugs jump the opposite way on one lead.

So for one sparkplug, the spark jumps from the centre electrode to the ground electrode (as you’d normally expect)

However, for the other sparkplug, the spark jumps from the ground electrode to the centre electrode (which is opposite to what you’d think)

Therefore, when choosing your sparkplugs, you need to bear this factor in mind!

Many people go for precious metal plugs – like platinum or iridium.

The benefit of these plugs is longevity – they simply last longer.

With the spark jumping from the ground electrode to the centre electrode, this benefit is totally lost.

The answer is to run double platinum or similar plugs, where a precious metal is used at BOTH the centre electrode AND the ground electrode.

My personal preference is the Denso VW22 plug – this has a platinum pad on the ground electrode and an iridium centre electrode.

I like Densos as they just seem to last forever – I use the IW22 on twin coil bikes, and their VW22 is the ‘twin precious metal’ equivalent.


In lieu of the twin 6 volt coils, Grant has gone with the dual output single tower coil from Colorado Norton Works (it’s the brilliant Crane Cams unit) and looks much neater.

Grant Tiller

3 phase Alternator

Grant will also be fitting a 3 phase Lucas alternator – a nice choice for superior charging in modern traffic conditions.

Grant Tiller


I have recommended that Grant considers fitting the superb Shindengen SH775 regulator/rectifier – it’s the only one I will recommend, and have never had one go wrong on me.

Grant Tiller


I recently did an article on upgrading turn signal flashers to a more modern type that will allow for LED lamps in the future, or different wattage bulbs without effecting flashing speed. You can find the article here.

On the grounds that Grant is rewiring his bike from scratch, it makes absolute sense to run an extra wire in the harness from the flasher relay up to the warning lamp in the headlight bucket.

This gives you lots of options for the future, and means that if you go up the LED route, you will not have to worry about extra resistors or diodes (which some people refer to as tweakers) to get things working.

There are several options available, but I recommend Classic Car LEDs as one worthy of consideration.

Grant Tiller

They have a solid-state unit available, and the option of Positive Earth – so perfect for our bikes.

The reason I like these units is that the same unit will support incandescent lamps or LEDs, so there is a good degree of future proofing.

Grant Tiller

The unit is a simple canister design, and can easily be hidden away on the bike.

Here follows a copy of the wiring diagram as per their website:

Grant Tiller

You will note that this has a separate line to the warning lamp, which means no requirement for additional diodes (or tweakers).

For the sake of running one extra wire, it’s a worthy addition in my opinion!

I have added this change to the new wiring diagram.

Charge Warning Light

The final thing worth noting is the Charge Warning Light.

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.

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 Wiring Diagram for Grant’s bike.

Custom Norton Commando Wiring Diagram – Grant Hepburn PNG 3066×1841

Grant Tiller

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