Horn electrics

[Worty]

Folks, my horn stopped working.  The switch is a 'not too expensive' dip/main with horn switch on top (four wires).  The dip and main work from the switch, but the horn doesn't.  When I took the switch apart, it seemed a bit mucky in places but generally ok.  However I did the following:

I don't know how to use a mutimeter, so I use a couple of wires with a bulb in the middle.  When, I put the wire on the horn terminals in turn and the other end earthed on the horn body, the bulb lights up which shows power at both terminals.  As I understand it, this is normal and the horn operates when earthed at the switch, creating current.  When I touch the bit on the switch that's supposed to earth it,  then touch the terminals in turn, the light doesn't come on.  However, when I touch another metal bit on the switch, the light comes on and the horn works.

Am I to understand from this that my switch has had it?

Cheers.

[bsa-bill]

Maybe you need to check the horn push does actually go down enough to contact the handlebar, and the other notorious bit is that everything has to earth through the head bearings.
Could be you already thought of these, I haven't tinkered with A10 electrics for a couple of years

[Worty]

Maybe you need to check the horn push does actually go down enough to contact the handlebar, and the other notorious bit is that everything has to earth through the head bearings.
Could be you already thought of these, I haven't tinkered with A10 electrics for a couple of years

What does one do if it is the head bearings - things were working fine up until a took the motor out (but I didn't touch the electrics).

[Rex]

The 'bars will usually earth through the clutch cable, if nothing else.

[Worty]

I think it's an earthing problem, but I reckon it's a fault on the switch.  The dip switch works fine, there's power to the horn, all the connections in the headlamp seem fine, so I don't know what else to look at.  I have had issues with the electrics before, but they've been fine for a long time now.  I've ordered a new switch so that'll help diagnose the issue.

[Greybeard]

I put a new loom on my bike. It has earth/return wires included so doesn't use the frame as a conductor. You might be able to run an earth wire from the handlebar switch back to the earth connection on the frame.

I'm pretty sure there should be a permanent live to one of the horn terminals. If you disconnect the wires from the horn, (taking care not to allow the live one to touch the bike and blow a fuse) you can identify which wire has voltage by using a bulb with wires soldered to it, (or a meter). Put that wire back on the horn then short the other horn terminal to the frame. The horn should sound. If that is all OK

Sounds like you've proved that part already.

go to the horn button on the bars, identify which is the horn wire and touch that to the handlebar metal. If nothing happens try a wire from the frame earth point to the horn button wire. This will be taking the head bearings and loom out of the circuit. If still no horn the wire between the horn, (earthing terminal) and the horn button on the bars has a break.

[Worty]

Cheers GB, good suggestions!

Both wires to the horn are live, apparently the horn works by earthing one of them for current to flow.  I do get power to both wires, it's the earthing bit that is the question.  When I bypass the switch by connecting a wire from one of the terminals to the frame, the horn sounds.  I do think the frame earthing on these bikes can be a bit hit and miss at times.  I'll try the new switch and see what happens. 

I hate electrics - it's like ju ju magic to me.

[RDfella]

Both wires to the horn are live

When unplugged from the horn, only one should be live. The other is earth - but only when horn button is pressed. If you're getting voltage to both wires that's your problem (or are you saying both are live when attached to horn - that proves nothing as voltage will be flowing through the horn).

[Worty]

Both wires to the horn are live

When unplugged from the horn, only one should be live. The other is earth - but only when horn button is pressed. If you're getting voltage to both wires that's your problem (or are you saying both are live when attached to horn - that proves nothing as voltage will be flowing through the horn).

Ahhhh, I did wonder.  You're right, when they are unplugged, only one is live, when they're attached, both are live for the reasons you mention - I did wonder if that might be the case.

[Greybeard]

If you tested with the wires still connected to the horn they will both have voltage on them. This is correct. You need to disconnect the wires, (as I said) before deciding which is live.

[Worty]

I think I'm on the way to solving the horn issue.

New switch arrived today - waste of £16 because it ain't the switch   By playing around with the old switch, I discovered the black wire, when attached to the horn live terminal and earthed with the button pressed (on bars), the horn worked.  This led me to assume that the earth has failed somewhere in the loom.  I removed the black wire from the new switch to the loom in the headlamp shell, and ran a new wire from the horn live to the aforementioned black wire from the switch - horn works!! 

Question - Is this ok, or does the dodgy horn earth in the harness affect anything else (don't think it does 'cause everything else works).

Question - should I still run an extra earth from the headlamp shell to the frame earth, as suggested previously.

[RDfella]

If it's left lying around somewhere, just make sure the wire that used to go to the horn is not live. If it's black, one should be able to assume it's earth anyway.
An earth wire from headlamp shell to frame / batt earth is always a good idea, otherwise your earth is either through the headrace brgs or piggybacking on some other electrical stuff.

[Worty]

Deffo not live mate.  I'll sort that extra earth as well.  Not good with the sparks, but reckon I've sussed it here.