>can anyone hazard a guess as to the cause.
>There was substantial melting of the insulation wire on the br/bl at the battery end, the fuse was also melted on the red earth wire.
>I opened the headlamp and again the brown/blue had melted as well as the white/brown wire.
>Would a fault in the ammeter cause this.
No. A short within the Ammeter would have to be to the headlamp shell and then other metal parts of the bike back to the battery. So, while the Brown/Blue and Red wires might've been affected, none of the others would've been.
Quick query; the "white/brown wire" affected is the one between the ignition switch and the lighting switch? Not the Brown/White (mainly Brown with White tracer(s)) from the rectifier?
>Something has shorted out but the fuse should have stopped it well before the wires melted.
'Fraid not necessarily. An educated guess says the wires are all 14-strand, even the Brown/Blue and the main Red. Depending whether the wire's Imperial or metric, it's only rated for 7.5 Amps or 8.75 Amps. Even if the fuse is 'correct' 25A, that's still 12.5A 'continuous'. While there is a safety factor in the wire ratings, it isn't that great.
Also that assumes the correct type of tubular fuse. The ratings on original British fuses are 'blow', the 'continuous' rating is half that (hence 25A and 12.5A in the previous sentence. However, unfortunately, the US rates exactly those fuses differently - "25A" there is the 'continuous' rating, which is 50A 'blow'. Why is this of interest? Because Japanese bikes use US-rated tubular fuses and some parts sellers are ignorant and/or careless ...
>Assuming the bike is positive earth, the fuse should be in the blue/brown wire
Nope, it absolutely should not. The metal structure of the bike is connected directly to one of the battery terminals - battery +ve on a "positive earth" bike. If something metal (e.g. loose tool?) touches both the other battery terminal (battery -ve on a "positive earth" bike) and any part of the structure of the bike, a single fuse in a wire attached to battery -ve won't blow, because the short-circuit is not through the fuse in that position.
If the vehicle has only one fuse, the correct place is in the one single wire between the vehicle structure and one of the battery terminals - battery +ve on a "positive earth" bike (irrespective of where Lucas put it). Then the only short-circuit not protected against is within the battery itself.
The only times you can't position the fuse so is when the vehicle has an electric starter, because there's a thick cable between engine/frame and the 'earth' battery terminal.
>The electronic ignition will almost certainly have required a reversal of the earth polarity thingy.
As it stands, this is not correct, although I appreciate it might not have been meant in the way it was written.
Because DC electrics all work exactly the same way (I'm assuming no-one'll actually admit to asking for a "negative earth battery" or a "positive earth battery" ), any electronic ignition connections must always be to the same battery terminals, irrespective of 'earth'. The only difference is which e.i. connection is through the switch(es) to the relevant battery terminal. If anyone doesn't believe this, be invited to actually study some e.i. makers' wiring diagrams before posting?
>The real fix is to check all connections and make them fail safe. This means the male part of the bullet connector is on the earth side and the female part, the bit with the insulation round it, is on the 12V side.
I'm guessing Pete is completely unfamiliar with standard British bike wiring, because it is impossible to do as he suggests without changing every terminal, because they're all male metal bullets on wires, pushed into insulated female connectors.
>if polarity has been reversed to fit electronic ignition, then all your connections will be in non failsafe mode which explains it even more
Which doesn't explain anything at all, because it's nonsense. As I've posted already, any electronic ignition's connections are always to the same battery terminals, the only difference is which connection is through the bike's switch(es). Any e.i. maker's wiring diagrams shows this.