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MG MGB Technical - Horn wiring.

Hello Folks.

New here. A transplanted American now in Essex.

Recently bought a 1972 Roadster. It's our first MG but I have had dozens of minis. Very clean car with no visible bodges

My other classics are Mustangs

I have a question about horns

They don't work from the horn switch. I have bench tested them and they work. If I remove one wire on each horn and ground that tab they work.

The car has dual horns. Both have the same wiring. A purple and purple w black stripe wire to each

I find this strange though. Both wires to each horn are live. I would figure one should be ground activated by the horn switch?

Under the dash is a solid purple wire run to the turn signal switch. This wire is live. Doing a continuity test this solid purple leads to the purple with black at the horn side.

Grounding this solid purple wire on the dash side blows the fuse

I know just enough about wiring to be dangerous

Any suggestions most welcome

My next test is to remove one wire off the horn assemblies then run a wire from the horn switch wire to the horn contact and see if that works.

Cheers. Peter
P Cavallo

Peter,

Not an expert myself but as always, Advance Auto's wiring diagrams are a godsend.

http://www.advanceautowire.com/mgb.pdf
A Riddett

Peter,
Paul Hunt's excellent site will help you, including what many get wrong - the correct way round for the horn pencil

Paul's site is mgb-stuff, you want the 'Spanners' link, then 'Electrics' link, then 'Horns' link

the site also has a link to the colour wiring diagrams

here's direct for you - http://www.mgb-stuff.org.uk/electricstext1.htm#horns

forget Haynes the best book of the car you can have is the relevant Driver's Handbook (Ref: 0052) - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html
Nigel Atkins

Thanks so much. I did find the diagrams you mentioned. They are brilliant

They appear to confirm what I expected to find. One live and one switched ground at the horns.

Thx for the suggestion

Peter
P Cavallo

also see my post in case you missed it
Nigel Atkins

The solid purple wire going to the turn signal switch is providing a live feed to the headlamp 'flash' function. It should not be connected to the purple/black.
D O'Neill

you also have to allow for errors and omissions in wiring diagrams
Nigel Atkins

The horns are both fed directly via a purple (fused always live) circuit. This returns to the stalk wiring via the purple/black and is earthed via the column directly, or later via a black wire inside the column switch.

As mentioned, the purple wire going to the stalk is for a live feed to flash the high beams.

Check the purple with black will honk the horns when earthed. If not, see why.
If the above works, check it connects to the copper contact properly. Check this connects to the copper ring on the reverse of the wheel hub properly. Check the hub connects to the horn push assembly: Assuming a standard wheel you should have the 'sprung biro' assembly connecting the contact ring to the metal area of the sprung wheel centre. This should have continuity to the metal base/tabs (only when depressed!) which in turn should have earth continuity down through the hub and the column. If any of the above doesn't work, the horns don't.
Roadwarrior

Talking of continuity down the column reminds me of an issue on my brother's BGT. The horn wouldn't work, but everything was fine up to and through the button.

I found there was no continuity down the column and had to add some solder to the lower collapsible joint to get it to work.
D O'Neill

Peter, have you got an original steering wheel or an after market one ? I've found that the horn switch on a couple of AM wheels has left a lot to be desired.I got around the problem on 2 occasions by changing the indicator switch with ones that had the horn switch on the end of the stalk. If it's a 46 year old original wheel, it's spring loaded contact or the slip ring could be worn out or not making contact. Barrie E
B Egerton

Oh wow. Thanks folks. Really appreciate the replies

B. Egerton: Car is a 72 with an aftermarket wheel. Everything new/rebuilt circa 2001. Turn signal switch wiring looks like new. Switch is identical to a mini one. You push the switch on the very end and the whole stalk moves inward to activate the horn.

Nigel, i am a US expat. You lost me at Horn pencil, pretty familiar with Brit terms but never heard that one.

D O'Neil. I need to confirm what color wire is coming from the TS switch to solid purple under dash. When did disconnected and checked continuity from the purple wire leading from this connection towards engine bay, the purple wire became purple with black at the horn assembly (I was checking at the RH facing horn) I probably should be checking at the LH horn as that's the first in the series.

I need to digest the replies while looking at the car as I've just woken up...:-)

Cheers. Peter
P Cavallo

After-market wheel sound like it could be the cause.

The 12v supply from the fusebox goes to the horns on the purple wire, and with that connected you should have 12v on both horn terminals, regardless of whether the purple/black is connected or not.

To sound the horns you connect an earth to the purple/black.

On cars with the three alloy spoked steering wheels the purple/black goes back to a spring contact attached to the bracket that secures the indicator switch to the column outer.

That sprung contact should bear on a slip-ring on the back of the wheel hub.

That slip-ring should be connected to one side of the horn button, either by a sprung 'pencil' or rod in the case of the original wheel, but it can just be a wire on after-market wheels. Earthing this wire, the slip-ring, or sprung finger should sound the horns. If at some point it doesn't then there is an open-circuit between the the point you have just tested and the last point at which it worked.

The other side of the horn button needs an earth, which it gets from the wheel hub where it is bolted to the steering column. This has to come through both halves of the column, the column UJ, the rack shaft, the rack body, the cross-member, and the cross-member fixing bolts, before it gets to a guaranteed earth at the chassis rails. I've had to add relays to both my cars as this earth was less than ideal and resulted in weak or non-operational horns.
Paul Hunt

Hi Peter,
if you follow the link in my first reply you'll soon learn about the horn pencil, a name for the part rather than a fender/wing thing

also follow the other link for much info info in one small book
Nigel Atkins

Guys, I don't want you to think I popped in just to get some info and disappeared

Thanks so much for the replies.

I have four classics, a huge backyard project going on and several shows to attend.

I promise to come back and give my results and chat some more

thanks very much.

Peter
P Cavallo

This is my other passion :-)


P Cavallo

May as well show the rest of the fleet

Let me know if anyone is interested in this, 5.0 5 speed state troopers car.


P Cavallo

This one is just being completed.

After importing a fully restored (sound familiar) car it turned out several main areas needed re-doing.

cheers. Peter


P Cavallo

thanks for sharing

love the 60s design of bodies

but that State Trooper cars shows where the US cars started to loose flair on design

being over here now gives you the opportunity to see interesting dash designs too including the MG ones, TVRs can be pretty unique
Nigel Atkins

Whey Ey!

Excellent

Found the purple w blk stripe. Under the dash. When grounded the horns honk!

I would agree with the above that the column is not grounded properly.

Thanks so much folks.

Peter.

PS. Also put retractor securon a in the car, lurvely.
P Cavallo

Sorry me again

My car is a bit of a bits a. From looking at the net it appears I don't have a 72 TS switch. It looks to me like an 80s mini item with the shiny metal fixed button on the end.

I have an unconnected wire from the TS switch. It is black. I would assume that grounds the TS switch but grounding it does not allow the horns to work.

Let me post a photo....


P Cavallo

Here are the wires

To the right the red and blacks were not connected.

If you ground the purple with BLK on the right the horns sound

To the left is a purple w/blk and a blk.



Edit...just sorted

Purple/blk to purple/blk

Blk to ground. Job done.

:-)


P Cavallo

by TS switch did you mean column/stalk switch
Nigel Atkins

'turn signal'
Dave O'Neill 2

Haha. I'm an american in the US. My ocean on cars is now mixed between the two
peter
P Cavallo

ah, yes, obvious now I've been told
Nigel Atkins

The 71 to 76 indicator switch on the MGB had a fixed shiny metal 'button' on the end. Which doesn't mean to say the 80 Mini isn't the same.
Paul Hunt

and rusty buttons came later :)
Nigel Atkins

This thread was discussed between 01/06/2014 and 16/06/2014

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