Anyone ever re made a full loom

Hi everyone

Well I have built a few bikes, restored a few, crashed a few and buggered up a few in my time, but I have always steered well away from the electrical bits as I just can’t me my head around all the black magic and evils.

So I have decided to ride myself of these demons and totally build all the harnesses new. I think a peg board approach so I can copy and match will be best. Can’t be that hard, can it :imp:

I’ve just been reading about this thin wall stuff and there is my first hurdle.

So two questions if I may please before I start:

  1. Does anyone know please what amperage thin wall cable matches the wiring in the standard loom?
  2. Has anyone tried this before and are there any handy hints and tips?

The harness is off a V50 ii (1980), and I will be using the Vehicle Wiring Products catalogue and products, unless of course the general recommendations are for some other supplier.



I have made a couple in the past. Best to make it on the bike and not to stretch wires tight so there is more length not too little. I’ve sent you a PM with my email address contact me if you would like links to connector blocks and other components. Last hint don’t use just black wires. My friend did that once it looked cool until it developed an electrical fault!

Vehicle wiring products tell you the size of cable to use on their website.
It would be worthwhile upgrading a few bits, like adding relays to the headlight supply and rerouting the starter relay wiring to avoid the usual Guzzi problems.


Chris, I have sent you the money via PayPal for the wiring diagram, so thank you very much. I will send you my email and yes please I would love the links to the blocks and other components. Love the idea of all black, but my feeble brain is going to struggle with the colours let alone all the same colour.

Don, thank you for the tips, I now have the catalogue and yes I can see they explain it very well so that is another potential cock up boxed off, he hopes.

I am intrigued about what you say about upgrades and the usual guzzi problems, if you ever get the time to expand that would be fab, please remember I am a simpleton.

My Cali 1100i ran like a dream, the only electrical thing I did was replace all the relays with some from some chap from America who was recommended on here years ago, never had a problem. Eager to learn more.

Once again thank you all so much for your kind advice and guidance



As Don has said, I would also strongly advise adding relays to a couple of circuits. I have one that switches the headlight on (small current in switch, large current direct to headlamp not through switch) I have also added one to the main/dip switch for the same reason. The horns could also benefit from one and the headlamp flash already has one.

The wiring is relatively simple, the circuit that the switch activates trips the relay and the supply for the headlight is direct from the fuse box and then from the relay to the headlamp, this is a simple on/off relay. The dip/main uses the same principal but uses the handle bar switch to change between the main and dip feed to the headlamp.

In all of these cases the main benefit is to get a stronger current to the electrical item it is opperating rather than the current flowing around the handlebar switches and loads of extra cable that that involves. It has a second benefit in that there are no high currents in the handlebar switches, on my bike with 750S/Le Mans I/early T3 switches that is a great advantage as the switches are fragile (and that is being kind)

Horn relay, the black wire from the switch is earthed at the switch when the horn button is pressed, the brown wire is the supply from the fusebox

Headlamp relays, as above the white wire from the switch is earthed at the switch when the light switch is turned on

The white and pink wire colours are on my bike because they matched the switch gear wires and I had those colours to hand. I advise against using all black as you will never find a fault in the system again, but otherwise any colour will do as long as you either match the colour from the relevant switch or record what you have done. My memory is reasonable but I would never be able to remember all of the colours and locations of wires on my bike.

Thanks Don, the starter relay mod is simple, instead of getting the supply (brown wire) from a tortuous route around the other electrical components, run a new wire from the fusebox to relay terminal 30/15. The black/white wire makes an earth at the handlebars when the button is pressed that then kicks in a large current to the starter solenoid.

This is a schematic I made for my V50 3. Three reasons:
To remove the load on the switches
To remove the load through the large connection block under the tank
To better protect the circuits

Ian, here is a link to the headlamp shell connector block I have used.

The standard wiring for the starter relay has the main power feed going from the battery up through the ignition switch and back. This introduces a lot of voltage losses. For the main power feed, run a wire from the batter, through a fuse and onto the relevant terminal of the relay. Obviously the switching side of the relay should go through the ignition.

Thanks everyone

Great stuff, I think this is going to be my nemesis as I am electrically challenged, but I will take it slow.



Good stuff but the supply to the starter motor relay that you have changed in red should be fused, either from the fuse box or if direct from the battery with an in-line fuse.

The starter relay when opperated sends power to the starter solenoid (a big round relay piggybacked on the starter motor) to my mind the supply to the starter motor relay should be via the ignition switch (fuses F1, F2 or F3) if it is taken directly from the battery it will be live even when the ignition is turned off. As long as there is enough current in the starter relay to acctivate the starter solenoid all will be well, as the starter solenoid switches a large current to the starter moter (fat cable direct to the batery) to turn the starter and engage the pinion in the starter ring.

Hi Chris,
I might have put a fuse in the starter relay supply, will have to check. If you take the solenoid relay feed through the ignition there is a good chance the voltage loss will be too great. I’ve already replaced one starter due to solenoid failure caused by low voltage. The supply to the start switch is through the ignition.
Keep up the good work with the laminated diagrams, (no I don’t need one) :wink:

Steve, I do not mean the strater solenoid supply should be from the fuse box (and so through the ignition switch) but I do suggest that the starter motor relay is fed from the fusebox. The trigger current to this relay is very low and it then just switches the the starter solenoid on. The solenoid needs a big current and is wired direct to the battery but the starter relay does not need so much umph.

Have made a full loom for a Spada and wired up a JZR 3-wheeler. With the Spada i removed the entire main loom and taped it to a board, then built up the new one in parallel with extra relays and modern waterproof connectors, was quite simple and worked well.

Basically what I did, and I didn’t even use a board! :smiley:

This is fine if you are making an exact copy of the original wiring loom, but if you want to make alterations as noted above it is better to make the loom on the frame.

Take it slow, always allow extra cable so you can cut it shorter later (measure twice, then once more before cutting) and build it up on the frame bike.
By the end you will know what you are doing and will posting advice on here.
If you can use 40 Amp relays throughout as they will then be interchangeable if you have a fault later on and cost little more.
Electrics are not difficult.
Even El Duffo understands them.


By the end you will know every wire colour and where it goes to and what it does, in your sleep :laughing:

I realise that you guys are giving the OP encouragement, but he asked for advice. Being told he will be an expert after he has finished the job does not help him as he sets out to start it. One unanswered question related to the wire capacity.

I use 0.75mm sq wire generally as I have a large supply of different colours of it that a lift erector threw in a skip when the installation was finished. Ive rewired a few bike with it and it works fine. Heavier wire is needed for battery and starter motor leads. has anyone else got a view on this?