Motorcycle Wiring

You should spend a bit of time thinking about wiring looms on old bikes. They need to be properly designed to be functional, reliable & look the part, but above all they need to be safe. 

This page is dedicated to all matters relating to wiring harnesses for the SQ4 and the W/NG. in each case, when building the harness, we are going to start with the original wiring loom and attempt to follow it for appearance, routing and cable clips.

There's also a few items of general use.

Ariel Square Four

For the SQ4, I will make an original looking wiring loom but we will make some sensible modifications as we go along. These might include:
  1. Indicators
  2. Fuses
  3. A modern control box
  4. Split charging circuit
  5. A brake light
  6. Front brake light switch
  7. Additional earths
Here is the original diagram, from Mr. Waller's book:

1953 Wiring Diagram

Now, I know it says 1953 and Amelia is a 1951 bike, but... There are some anomalies. Mr. Waller's book shows a diagram pertaining to be for 1949, 1951 and 1952 (but not 1950, which was the year of the tank mounted switch with fork mounted speedo):

What year is this for?
The problems I have with this diagram is that we know the ignition switch moved under the seat for 1951.

So this is what we will use to wire up:

I'll add some additional diagrams for the earth system and the indicators.

Fuse calculations:

Loads Power (W) Current (A)
Horn 80 13
Stop light 21 4
Ignition 15 3
Lights (total) 63 11
Indicators 42 7
Main Fuse 37


I fitted indicators to the SQ4 since its my go-to commuting bike and gets used in traffic more than the others.

You can use this diagram as a stand-alone circuit or in conjunction with other systems on the bike.

Brake Light

Here's a useful circuit diagram I made showing how to wire up a brake light.

Ariel W/NG

This is the wiring diagram I will use for the W/NG. You will note that it is the civilian one from the same period; I don't ride in many convoys, I need a dip beam and my repro headlamp does not have the correct switch anyway:

I've made a cable list showing destinations, colours, lengths & ratings:

We'll buy some cotton braided cable from This is the wire we will use:

We start by removing the lighting switch/ammeter panel in the headlamp:

We'll need it out to get at the connections to the lighting switch, since we will use the most complex area as the starting point for the harness.

Start by laying in the wires that run the length of the bike - that is wires 1, 10 and 11, running from the lighting switch to the tail lamp and the area under the seat. At this point, you will want to add an earth wire to run from the headlamp to the common earth point under the seat.

You might consider using this plastic mesh to sheat the cables, which looks a little like the cloth braid that was used originally, and is very easy to modify. You can break wires out though the walls, or add cables along the length.

The W/NG used rubber sleeving over this main harness run. Today this is hard to find, but some pvc sheathing looks suitable. Try to shop around for sleeving that is slightly sticky and less shiny. When using PVC sheathing, put all the wires you need in at the same time, choose the sleeve size carefully and consider pulling the wire through with another wire - there is a lot of friction in the sleeving and you are unlikely to be able to add a cable later, by threading it through.

When you have the main trunk of the harness on the bike, roughly retain it with a couple of loose tie-wraps. You can then run wire #1 all the way to the tail lamp, and add an earth wire from the central frame earth point under the seat to the rear lamp.

You can terminate this earth cable, and the one to the headlamp in a large 1/4" ring terminal - you can use the same terminal for both wires, to save having too many rings on the earth screw. Its critical to crimp and solder the terminals correctly - use a 40W soldering iron and do not overheat the joint. Use a cored solder designed for electrical work - this is not the place for solder paste or Baker's fluid.

When complete, the under-seat earth terminal will have several cables attached - the additional headlamp/tail lamp earths, the original regulator earth and the original battery negative.

At the headlamp end, terminate the earth wire with a British-style bullet into a double socket; leave some length in the headlamp:

This will later be used for the headlamp BPF connector earth and the pilot lamp earth; there is an earth connection on the headlamp shell as well, but you can ignore this - we are going to earth all the lighting equipment on dedicated wires, back to a common point. The frame is effectively excluded from the earthing system.

We can lay in wires 7, 8 and 9 to the dip switch, in a piece of PVC sleeving. At the main lighting switch, these wires and the main harness are clamped in the instrument panel using this clamp and a 4 BA screw. 

These wires will be terminated into a Lucas 31482 dip switch on most machines. This W/NG however, with it's Italian heritage, is using this Italian aftermarket switch:

This is cabled up with 1.5 mm four core cable, since it is so small. I'm using LED lamps in the headlight, which will reduce the current consumed to a very small number, so this small cable will be acceptable. It's terminated with weatherproof japanese style bullets:

These are very effective and take up little space - I have them on the SQ4 in the indicator circuit where they are discrete enough not to be too noticeable around the rear mudguard stays. Also in this picture is cable #2, the lighting switch to side lamp, terminated into the main lighting switch:

Cloth covered sleeving needs to be trimmed back with a sharp knife. Regular strippers skid on the sheathing and leave the braid an untidy mess. Twist the conductors together at the end, to avoid them separating when we terminate.

Finish the sheathing with a small length of matching heat shrink sleeving. Hardly original, especially during wartime, but neat and tidy. Solder the end to stop it unravelling.

A short link completed. Note the ring terminal, selected to fit the ammeter terminals, crimped with the correct tool, soldered and insulated with heat shrink.

Here it is in place.

Wiring up the regulator is much easier with the seat lifted out of the way. Wiring the regulator is also easier than getting the nuts back inside the seat springs!

Ariel W/NG with the bonnet up
The split bullet on the regulator earth wire. We showed how to terminate them here.

The MCR1 regulator terminals finished:

Smiths Speedometer Lamps

W/NGs don't normally carry a lamp to illuminate the speedo dial but mine has a civilian speedo as I would like to see the speedo at night.

The wiring to these is interesting and worth recording for those of you arriving from more modern equipment. The bulb holder is nickel plated brass and is retained in the speedo using a nickel plated brass ring and a spring. The cable terminal lives within the bulb holder and is unusually made of plastic; it too has a spring to keep it in contact with the bulb:

A brass screw in the top of this connector makes contact with the bulb.

Removing the brass screw reveals the end of the wire, whose conductors are splayed out; a cone formed on the screw contacts these wires and retains them in the plastic enclosure

1940's technology. Neat, reliable and easily fixed at the side of the road!

WD Battery Connector

These connectors do not appear on the early W/NG wiring diagram but are shown on the later diagrams in several places as the 'L.J.W.' connector; they also appear in the workshop manual:

This is an extract from the later wiring diagram, with the lighting switch under the seat. The earlier wiring diagrams do not use this connector:

They were made by Lucas under part number 571395. Here are the main components, provided very generously by Ray Tolman:

Terminate the cable by passing through the female sleeve, and into the little brass doughnut. Then splay the individual conductors:

Do the same on the male side. The conductors need to be short enough not to foul the thread:

Screw the two halves together:

Slip on the sleeve:

Here's the connector hanging in place next to the battery:


  1. You have shared very precious information.

  2. This is a great technical site. I've made a couple of 571395 Lucas connectors but cannot find the 5/16" x 7/16". or even 8mm. x 11mm. rubber sleeving anywhere in Australia. Can Ray Tolman or anyone else offer any clues, please?

  3. 2 pictures in last section "WD Battery Connector" appear as a triangle with ! in it (i.e. picture not found) on this page:
    Thank you.

  4. Thank you so much for such an informative piece of information :)
    If anyone interested similar one's have a look here


  5. Thanks for your great Tips
    I introduce your system in My ariel wng.
    fortunato from Malta

  6. Hi, stop light ‘fuse calculation’ is incorrect, 6 volts x 4 amps = 24 watts not 21, same for the ignition, should be 3 amps x 6 volts = 18 watts

    1. What the table is trying to say is that the load for the stop light is 21 watts, and that a 4 amp fuse is adequate.

  7. Just going by the maths.
    Kind regards

  8. Thanks for the indicator wiring diagram, I am going to use it to fit indicators to my 2970 Triumph T100S

  9. I was wondering--where does the wiring to the tail-stop lamp go? under the fender-on top or along the side...?

  10. Hi==so the "earth" for the speedo bulb is through the speedo frame to the mounting--there is no "earth" wire to the speedo correct?

    1. Yes that’s right. You’ll have to decide how to earth whatever the speedo is mounted to!

  11. beatifull and verry helpfull thanks