The brown cable

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Jan 10, 2006
There's a brown cable with a bigger terminal on the right hand side of the rear wheel. It comes out from behind the "Z" plate that holds the footpegs... it just won't stop touching the swingarm or any other part of the bike making the fuse blow! :?

Does anybody know what is it for?
That sounds like the wire that goes to the Zener diode (could it be Brown/Blue?) that should be fitted to the back of the R/H Z plate? The large spade terminal on the wire should have an insulated cover?

Try bending the tag of the diode away from the swingarm and then fix the wire to the frame tube with a cable tie as close to the diode as possible.

The insulation on the wire of course should be in good condition.

Or is the wire not connected to a diode?

Is there some other type of charge control box fitted? If so, the wire can be taped up out of the way somewhere?
It is the brown/blue wire you say, and it has a cover, but the very end still touches the bike. In my bike, the wire that's fitted to the z plate is the red one (earth). I've taped the terminal, but I think it must be there for some reason but the wire isn't connected to anything...
Thanks for helping!
Yes there should also be a Red earth/ground wire connected to the plate.

If the zener diode is not there then you should check that you have got some other type of charge control box fitted as otherwise there will be no voltage control which will result in blown bulbs and a boiled battery and may damage an electronic ignition system? I read from an earlier post that you have recently fitted a Boyer Bransden ignition?

Is there another 'electronic box' fitted anywhere?
No, there isn't any electronic box as long as I'm concerned... In case there is some other charge controller, Where should it supposed to be?
You're right, I've installed a boyer ignition... Does it do anything to this zener diode?
Bike's electrics seems to work and I've put a "warning light assimilator" substitute from "AO Services" which says there's no overcharging....

Gosh!, now I'm a mess again.

Thanks, L.A.B. for your help! :)
Sparkplug, your warning light assimilator or charge warning light controller will generally only tell you that either the charging system or the battery voltage (depending on which method is used) is less than the required amount not more than and the Zener Diode is supposed to 'Bleed off' any excess charge and turn it into heat which is why it is bolted to the footrest plate in the case of Nortons, or else in that aluminium 'sunburst' heat-sink generally mounted under the lower triple clamp on Triumphs.

The boyer ignition you have fitted will not affect this, there is however a boyer charging controller that can be fitted as an aftermarket device this does away with the necesity for a separate rectifier, Zener diode and (depending on the model purchased) warning light assimilator. The best way to check what charging control system you have is to see whether you have a rectifier fitted and wired up. It is generally mounted under the seat on the frame plate above the battery tray. It looks like a stack of black thin washers with male spade connectors for the appropriate wires to attach to. If you have a rectifier connected then you also need a Zener diode connected, failure to connect it can result in overcharging, boiling the battery acid or even in extreme cases a burst battery which plays havoc with chrome and other parts.

Since you note that the red wire is still attached to the footrest plate It may well be that the male spade-connector on your zener diode has broken off and is still attached to the wire in the female end or else lost.

The Zener diode on Nortons looks like a large dome-headed bolt with a connector tab sticking up out of it that the wire you are refering to is connected to. This diode is bolted through the footrest plate from the inside, it secures the red earth wire and is itself mounted with a small nut on the outside of the plate. A new zener diode can be purchased quite cheaply from any of the regular British bike suppliers mentioned on this forum. BSA, Triumph and norton all use the same system and part.

Good luck
I think I'll have to get a zener diode... I made the rewiring myself and just swapped old for new wires, so there was nothing there before. That's probably why the battery has gone so fast...
Thank you, Dave!
Anybody has a picture of the zener diode position on the bike? I've already ordered one, but I don't see any holes or anything where it could fit in near the brown /blue wire.
The Zener diode fits to the rear of the plate through hole shown by the tip of the screwdriver in the photo and is secured with a nut on the outside.

The retaining nut should only be torqued to 2 ft/lb.

The brown cable
Sorry for retaking this old thread, but the zener has arrived today and I've found out my right z plate has only 1 hole out of the two it was supposed to have... Is it for the zener or is it ground? Gosh, I'm a mess again...!
sparkplug said:
my right z plate has only 1 hole out of the two it was supposed to have...

Your footrest plate should only have one hole!

The plate I showed in the photo is a Mk3 plate which has a second hole (the one shown lower down) but that is for the Mk3 hydraulic junction mounting for the rear disc brake, so the ground wire should have a ring terminal that sits on the zener stud between the zener and the plate.
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