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Hi all,

I start this thread with some trepidation as I have seen a number of threads that discuss fuses and where to put them and these can get quite heated (the threads not the fuses)! But here goes anyway...

Without going into too much detail, on a ride at the weekend I had the main (only) fuse blow on me. Fortunately I had a spare and after fitting that the bike ran fine for the rest of the ride (hour or so and 20 + miles).

The issue I have is now trying to diagnose an intermittent electrical fault with no indication as to where the fault might lie. It got me thinking that if I had more than one fuse then it would help to narrow things down.

I have rewired the bike with a custom (homemade) loom to remove surplus wires and to introduce an SPG. I have not been complete with the SPG and still have the horn and the tail / stop lights earthed through the frame. However, my thoughts now are to run a ground wire to these from the SPG and start to introduce some fuses into the ground wires by function. Probably one for tail / stop lights, one for headlight / pilot and speedo, etc.

I am favouring fusing the the ground side as this should make it easier to group items and the SPG is under the seat so there is room for a small fusebox or a number of inline fuses. I will still have the frame grounded and I will retain a main fuse but may shift this to the +ve terminal on the battery.

Some things I am unsure about at the moment.

Do I need to / should I fuse the Regulator / Recifier? Fuse the -ve or +ve?
Do I need to / should I fuse the coil? I have a PowerSpark EI, so I have a switched live (-ve) to the EI, EI to coil -ve and coil +ve to SPG. Would I fuse the Coil ground?

I would welcome thoughts and comments. Should I leave well alone or will this approach make my bike safer / more reliable?

Many thanks
Peter

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A main “everything” fuse at the battery is good enough for the rectifier. If the rectifier DC (somehow) goes short-circuit, that fuse will blow. The AC circuits don’t need to be fused.

The main fuse is enough for the headlight, I reckon. Circuits that take a substantially lower current than the headlight can beneficially be individually fused.


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It is tempting to look at the array of fuses in a car and think that the simple fusing arrangement in our old bikes could be vastly improved by similarly installing separate fuses for each separate circuit.

A car has a vast amount of wiring, most of which is hidden in very inaccessible places.
All of those locations are protected from the elements and experience negligible vibration.
The fuse board is usually in a dry location (normally under the bonnet or in the dashboard).

A motorcycle is a very different environment, in terms of exposure to moisture and vibration.
Each extra connection adds more vulnerability to both of those factors.

To use a sophisticated fuse system as an aid for diagnosis of an intermittent (or any other) electrical fault seems to me the wrong way round, not to mention the amount of effort.
First of all, how do you know that you have an intermittent fault? According to your post, the main (only) fuse blew once, but hasn’t blown again.
A fuse can fail as can any other electrical component, unless fuses keep blowing intermittently, it doesn’t necessarily mean anything.
Hopefully that fuse was of 15-20A continuous rating?

As you have pretty much made an SPG system, just complete it with the tail and horn grounds, AND a wire from the engine (from a head steady bolt maybe).
Then a single wire from the SPG to the battery +ve (assuming +ve ground) with the main fuse in it.
This will protect from any direct short in the entire system, from runaway current from the battery (this is the only source of runaway current), thus avoiding a fire/total loom meltdown.
The usually extremely brief period of high current won’t normally damage any of the wires in the harness, it’s more of a very brief pulse that doesn’t last long enough to seriously heat the wires.

I agree with TT that the generating system doesn’t need fusing, it is simply not capable of enough current to be a problem. This is where some others will disagree, fair dinkum.

If you do choose to install additional fuses, I think your idea of using the ground wires before the SPG is a good one. Right next to the SPG, I’d keep it simple, 1 headlamp area, 1 tail area, 1 ignition.
Food for thought, trace out the diagram I’d suggest.

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Thanks Triton Thrasher and Koan58 for the feedback.

I have realised that I haven't got the speedo light on the SPG as this is grounded through the speedo case. That seems like a reasonable effort to isolate from the frame. Also the horn will need an isolated mounting.

As the consensus seems to be that the single fuse is probably adequate I think I am going to pretty much leave things alone and monitor the situation. I am confident that my loom is good with all new wires and proper terminals. It was a useful exercise making and fitting a loom as I know exactly where it runs and what is connected to what.

That said, I am going to swap out the glass fuse for a blade as these are more readily available should I get stuck without a spare. I am also going to move this fuse to the +ve (ground). I don't have a specific engine ground so I will add that to the SPG as well.

If "intermittent" becomes "one off" then I shall leave well alone. Otherwise some logical / methodical fault finding will be needed rather than adding fuses.

I have created a schematic of my simplified wiring if anyone is interested. [Linked Image from i.postimg.cc]

What I don't have and would be useful is a schematic for the internals of Lighting & Ignition Switch. I would quite like to understand exactly what is going on inside that if possible. Does anyone have this?

Many thanks
Peter

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Originally Posted by Peter Williams
I have created a schematic of my simplified wiring if anyone is interested.

This is prior to changes I mention above.

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Those old fashioned glass fuses are prone to failure not caused by an electrical fault, ie vibration etc, blade fuses are much more reliable, hopefully this is so in your case.



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I change the fuse holder to a blade type and then use these for fuses, 15A is good as its below the original 17.5A and it does not blow.

https://www.almsolutions.co.uk/index.php?main_page=product_info&products_id=354

If it does blow then you just press a button on top. If it rapidly blows again then switch everything off and start troubleshooting what is causing it to blow.

No need to rewire the bike for a fusebox.


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