An anonymous Commando owner has reached out for some assistance – he has his MK3 Commando on the bench and is in the process of rewiring it.

Wiring Choices

They have bought a new harness from our friends at Andover, but will be making lots of changes to accommodate some aftermarket parts. So with this in mind, he has decided to make a new loom and incorporate some of the optimisations I recommend including the positive earth ‘bus’.

Battery Connection

As you read through this article, and start to look at the diagrams, be sure to notice that there is only one connection to the battery positive – this is the heavy 6-gauge cable that goes to the primary case for the factory electric start.

I usually use welding wire for this (as I had a large reel of it) – lot’s of strands, it remains nice and flexible, and it can carry a lot of current.

I also have great experience with Auto Electric Supplies Limited – AES

Their flexible battery cable (pictured above is lovely to work with.

It’s really important that these very heavy cables are crimped and not soldered – the cable is too large, and there are too many strands to get a decent, reliable solder joint.

I have done some extended testing with an upgraded Prestolite (4 brush/4 field coil) and this cable can handle the load with no problems – it barely gets warm.

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 gauge 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.

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!!!

Positive Feed

One thing I like to do, and highly recommend that others do when making your own harness is use this as an opportunity to sort out earths (in our case the positive feed) once and for all.

With the Commando, Norton (and Lucas) were innovative in that there was no reliance on the frame as a ground for the majority of the components on the bike – for example, although the original zener diode (or two in the case of the MK3) sunk their heat to ground on the nice, chunky heat dissipating aluminium z-plates, and the component operated by electrically connecting via it’s mounting stud, there was still a ring terminal and a red wire right next to it (connected via the footpeg fastening) – I have gone into more detail about earthing in an article here.

The downside in the way Norton did it is that in many of the cases, each component has a positive loop – so there are two red wires going to it – look at it as an ‘in’ and an ‘out’ – if the wire breaks at the connector, or one of the connectors becomes unplugged, it can interrupt the positive feed to the rest of the bike.

If I am building a harness from scratch, I like to run a positive wire from the front to back of the bike, then tap in for a feed to each component as required – there is a massive advantage in doing this, and making it part of the loom, as it means you rule out any unreliability associated with bad earths, or trying to get power through rust, paint, powder coating, paper gaskets, loctite, clutch cables, steering bearings and speedo/tacho drives.

For the sake of spending an hour running this wire now, it means you rule out a massive variable, and make troubleshooting really easy in the future. So well worth doing in my opinion!

I have drawn a simple diagram that shows the negative wire with the splices for the individual components – I usually have a 12-gauge cable to handle this – a 2mm² cable will handle 25 amps which is more than enough.

When you splice into the cable to feed the positive to an individual component, you can use a lighter gauge cable that is rated for the maximum current that particular part will draw.

For example, the turn signals can have an 18-gauge cable – 1mm² will handle 8.75 amps and be more than enough.

Those old 21-watt lamps used in the turn signals will draw no more than 3.5 amps.

Here is a diagram which covers the design for the positive ‘BUS’ that runs from front to back of this bike, with the splices to feed the individual components.

The above diagram can be downloaded as a PDF by clicking here:

So, the correct routing for the positive feed is:

• A single 6-gauge cable from the battery positive terminal to the MK3 primary case
• A connection at the head steady into the rest of the wiring harness (I like 12-gauge for this)

It is important to make the ‘head steady’ connection on the engine side of the head steady… Isolastics are a good electrical insulator as well as a good vibration isolator!

Don’t forget, we have no need to earth the frame on these bikes.

Wiring Technique

When I am making up looms and harnesses from scratch, I like to minimize the number of connectors I use – ideally using them only at the point the cables plug in to the components themselves.

This feels contrary to what they did back in the 60s on bikes, where you seem to run into connectors for the sake of it – these are potential points for moisture ingress, terminal corrosion (the dreaded verdigris) and ultimately failure.

If I am splicing like in the case of the positive ‘BUS’ (covered above) that I like to run from front to back of a bike, I like to bare the cable using wire strippers where I want my splice to be, I then twist the junction wire around the bared section, solder it, and use an adhesive lined heatshrink sleeve over the top which will protect the joint from moisture ingress and provide decent strain relief.

I feel that mechanically twisting the cables as I do, and then strain relieving them so well means there is zero risk of a soldered joint becoming dry or failing – I have certainly never had a failure in many years.

Here are some pics of my splicing procedure:

For the connectors themselves, I do not like soldering – I much prefer to see a quality crimped connector – made utterly reliable using a decent and correct crimp tool.

When I have finished making the harness, I wrap the whole thing in cloth tape, you’d never know there is a joint there!

The tape I like to use is Tessa 51608 fabric tape (it is also known as fleece tape)

It’s nice and furry, sticks well to itself and doesn’t come unraveled. It gives a great OEM look.

Fuses

I have found issues with the glass-style fuseholders in the past – the springs become weak over time, and eventually the circuit becomes intermittent.

As the fuse disconnects and reconnects to the contacts, a small amount of arcing occurs – over time a layer of ‘soot’ will build up over the contact patch, which in itself acts as an electrical insulator.

This can impact all sorts of things, not least the smooth running of your engine!

The symptoms of this feel very much like fuel starvation, so most assume there is a carb problem before they even start looking at the electrics!

I would recommend using automotive blade type fuses all round instead of the original glass type used on these bikes.

These are great, as blade fuses are available in every garage and petrol station, and are very resilient to vibration.

My rule of thumb is usually a 15-amp fuse for standard bikes, or a 20-amp fuse for MK3s and electric start conversions or if you have fitted high output alternator etc…

The other thing to watch with the old style fuses is the value.

Our workshop manual specifies a 35-amp fuse:

This was written in the 70s for a 70s british bike

It did not take into consideration that a US fuse is rated in a different way!

The British standard was to show the blow value on the fuse, and in the manual – not it’s continuous rating.

The US standard (which was subsequently adopted internationally) is to show the continuous rating value on the fuse, and in the manual.

Some fuses, back in the day showed BOTH their continuous rating value AND their blow value, but this was certainly not always the case.

To this end, I see MANY bikes fitted with the wrong value fuse – a 35-amp continuous rated fuse will blow at 70 amps… a long time after every cable on the bike has melted.

Not good.

Modern blade type fuses are labelled and referred to by their continuous rating. Everywhere. Worldwide.

So, you know where you are, and there are no nasty surprises.

You will notice that on this schematic, I have taken the fused negative NU (Brown/Blue) that feeds the bike from the solenoid terminal instead of the battery – this makes it so much easier to get the battery in and out – a single heavy gauge black cable to the negative terminal, and a single heavy gauge red cable to the positive terminal.

This really works well!

Starter Solenoid

It is important to remember that the “I” Ignition terminal on the starter solenoid is no longer used – and neither is the WP (white/purple) cable.

It was there originally to bypass the ballast resistor when the starter motor was cranking.

With the two 6 volt coils wired in series and acting as a single load in wasted spark configuration, the ballast resistor has been dispensed with.

The solenoid gets it’s positive supply via the metal mounting plate.

Rather than rely on the frame (we don’t do that on the Commando!!!) don’t forget to add a ring terminal and wire back into the earth bus.

The solenoid has been shown as a node on the positive routing diagram shown above.

Warning Light Assimilator

The factory warning light assimilator is not compatible with a modern regulator/rectifier so has been removed from the schematic.

Plus, there is the matter of what the WLA is actually doing, and how much use that is.

The WLA is looking purely for some AC output from the alternator stator (about 6 ½ volts AC) it is not designed for a three phase stator either.

It gives you no information about the charging (i.e., the regulator (zeners) and rectifier)

It gives you no information about the state of the battery.

Charge Warning Light

I have taken the liberty of adding an ICM Charge Warning Light to the schematic – I can certainly recommend the Improving Classic Motorcycles charge warning light as a brilliant alternative.

I use them myself, and have had a good experience with them.

The nice thing about the Improving Classic Motorcycles unit is that you can retain the original warning light – so it looks totally factory (this for me is an important factor with the MK3 with it’s quirky little instrument panel).

Stick with the Standard, and avoid the deluxe – as the Lucas type LED will not fit the MK3 lamp holders.

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.

The owner of this bike has gone for the Charge Warning Light that includes an additional input for an oil pressure warning light, which he has sourced from the excellent Donald Pender – while Don is based in the Philippines, I am always blown away by how reasonable his postage pricing is, and how fast his parcels arrive!

MK3 Handlebar Switches

During the manufacture of the MK3, Norton and Triumph were coming together, and they were often feeding from the same parts bins.

We have noted some anomalies between the handlebar switches while the MK3 was in production, as they frequently used the Triumph T140E switches, which look the same, but have a couple of small wiring differences.

Left handlebar switch the U (blue) used by Norton (and illustrated in the factory workshop manual) has been replaced with a UY (blue/yellow) cable. This connects to the U (blue) of the right handlebar switch inside the headlamp bucket.

Right handlebar switch there is no S (slate grey) instead, the single “hot” negative from the pin 2 of the Master Switch (ignition key switch) is jumpered for both engine run/kill switch and the starter button.

For this particular bike, we have noted that a Triumph handlebar switch is fitted to the left side (you will note the Blue/Yellow cable in lieu of the plain blue one) The schematic has been updated accordingly.

Electronic Ignition

A popular upgrade in the late 70s and early 80s is the Lucas RITA electronic ignition system.

The owner has sourced a RITA unit from Norton electrical guru Al Osborn.

Al is the Norton Owners Club resident Electrical expert, and you can find him over at http://www.aoservices.co.uk/

Personally, a RITA would not be my choice or recommendation for a MK3 Commando – it only takes one kickback to wreck the sprag clutch in the electric start mechanism, and with the RITA’s insatiable hunger for power, it makes it a very high risk indeed (in my personal opinion)

Furthermore, there are newer, simpler ignition systems on the market that not only draw less power (and operate down to 8 volts) but also have an improved ignition map for easy starting, and idle stabilisation circuitry too.

Anyway, back to the RITA!

Supplied in a nice kit of parts, it is important to note that you need to run a ring terminal to one of the mounting lugs for earthing.

The RITA is easily spotted on the Norton Commando, as it needed a larger points cover to house the large “reluctor” plate.

You can find a copy of the original Lucas RITA AB11 fitting instructions here:

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.

The two 6 volt coils are wires in series, and act in a wasted spark configuration.

Alternator

The owner has very sensibly opted for a three phase alternator stator.

This is, in my opinion, a great option to go for and is far superior to the factory original single phase unit.

The spec is as follows:

• Lucas RM24
• 3 Phase
• 14.5 amp
• Part Number LU47244
• Also found under Part Number WW10192L

This stator is the high output version, putting out around 14.5 amps – this is a sound choice on an electric start bike, as the duration of most rides will be spent recharging a depleted battery.

In addition, because it is a three-phase unit, output is produced at a much lower RPM making this an ideal solution for around town and in stop start traffic conditions.

As always, I recommend that you ride with your headlight on, as this will always help with the longevity of the components that make up your charging system.

Regulator/Rectifier

Our MK3 Commandos use a blue can capacitor, two zener diodes (which can be found mounted on the back of each the z-plates) and a half-wave rectifier unit.

A combined regulator/rectifier replaces all of these components with one package.

The owner will be fitting the superb Shindengen SH775 regulator/rectifier – a very sensible choice in terms of longevity and looking after your stator.

The Shindengen is wired in the same way as most aftermarket reg/rec units:

Identifying the pins/terminals on the Shindengen is as follows:

General Tidy Up

I have taken this opportunity to remove all superfluous wiring, as it is of course not required on this bike.

The diagram also contains a lot of additional tidying – as I mentioned already eliminating connectors that are there for the sake of it, as these are typically areas of potential failure through vibration or water ingress.

The owner has chosen to fit a pair of horns – with this in mind, it’s a wise move to wire these in via a relay in order to keep the increased current away from the handlebar switch.

This modification has been added to the wiring schematic (and the earth bus map pictured above)

I have also included permanent wiring for the Beeline Moto II SatNav unit

Wiring Diagram

So, without further ado – here is the Custom Wiring Diagram for the MK3 Commando with Lucas RITA

Custom MK3 Wiring Diagram – with Lucas RITA PNG 3066×1841

This is available as a PDF too – it can be downloaded here: