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MG Midget and Sprite Technical - horn push- how does the earth work?

Im refitting a horn on my midget after a life of racing.It has an aftermarket steering wheel with a central horn push. The horn push has a spade terminal on the back to purple wire that goes down the centre of the column. Is the idea that when you push the horn push that it will earth to the boss? There is a purple wire from a little prong that rubs against a ring on the column that ends in a bullet connector, do I attach the earth from the horn to that? Any help appreciated. Working with your head under a Midget dashboard is no fun!
m fairclough

there should only be one wire for the horns under the dash and that is the purple one(white stripe?) for the horns. In stock configuration, there was a mystirious looking object that was placed inside the steering wheel hub. Most parts catalogues called it a "horn brush" It looks like plastic tube with an unisulated wire running from one end to the other with brass bits sticking out either end and a spring along the inside wall. I have no clue how this makes or breakes any electrical connection, but It works. A nail has been used sometimes with some success, its just there to earth the wire when you press the horn pus at the center of the wheel. I think your steering column should be earthed already. Check it with a test light.
S.A. Jones

M,
Yes. The idea is that the horn button in the steering wheel effectively grounds out that purple/black wire. In fact, you should be able to take that bullet connector and ground (earth) it, making the horns sound off. It does, however, assume that it is getting an adequate ground through the steering column... not always the case. Since the ground has to be adequate to operate the horns, many folks have opted to use it to actuate a relay instead and hook the horns to the relay.

Seth, the pencil connector merely serves as a "brush" from the commutator ring to the horn button which contains the switch to ground.
David "earth? ground? whatever!" Lieb
David Lieb

Thanks David, on closer inspection it is a purple/ black wire under the dash. I have tested the column for an earth and it is pretty poor. I can't see an earth strap from the rack to the cross member that I remember from my other midget, probably taken off in its racing days so I might have to replace that.
m fairclough

Yep - earth is provided by a small braided strap from column (pinion housing cover screw?) to rack securing bracket / x-member
Steve Clark

david, I'm just mystified as to how that brush acts as a switch
S.A. Jones

SA - it doesn't -
"....the horn button which contains the switch to ground"
David Smith

There is a brass slip ring attached to the steering column cowl under the steering wheel, this is connected to the horn. The horn brush runs on this and passes though the wheel boss (from which it is insulated) and presses against a contact pad on the horn button. This gives a rotary contact to pass the earth through from the rotating steering wheel to the fixed cowl.

The horn button is basically a large push button switch with normally open contacts. It has two connections, one is the pad mentioned above and the other is a sprung loaded connection to the wheel boss from where it picks up a fixed earth.

The circuit consists of a permanent earth to the steering column and thus to the wheel boss, this is picked up by the horn buttons side contact. When the button is pressed, the earth is passed on to the contact pad which is connected to the horn brush. The brush passes the earth through the boss and down to the slip ring and thus to the horn.

That is the cct, which is far easier to show than explain. I hope that it makes sense!!!!
Graham.
Graham P 1330 Frogeye

Thanks chaps, very comprehensive answer Graham, the brass slip ring thing now makes sense.
m fairclough

The "brass slip ring"

actually copper plated mild steel, but we know what we mean.

I thought it was copper until it picked up a magnet!



Bill sdgpm

Bill, I think it depends what model you have, that looks nothing like the "slip ring" on my frogeye and I'm pretty sure that mine is brass, but as I haven't put a magnet on it I can't guarantee that.
Graham.
Graham P 1330 Frogeye

that's not the slip ring! that's the horn push retaining plate... the slip ring is behind / below the steering wheel boss.
David Smith

Ah that might explain it that was a part of the skeletal leather rimmed wheel that disintegrated while my car slowly sank into the sunset, I'm sure large parts of everything steeringy went west with it...

It seems to have a bayonet retaining feel about it, was it push in and twist or screwed in?

As you can tell, the firt thing I did was fit a pushbutton indicator arm/ combined horn doohdah

I never did try to reassemble that bit, presumably there're some more missing bits

as well as the leather and foam padding off the wheel


Still

it is plated steel whatever it is :-)
Bill sdgpm

What's certain is that the earth route is tenuous, and the load is high.

It's a job for a little relay.
Nick

One thing I have noticed is that the horn on my car doesnt work sometimes, usually its in colder weather. Would a relay help this?
S.A. Jones

After struggling to get a decent earth to the boss today I took the easy option and put an old off/momentarily on toggle swith I had lying around in a spare hole in the dash and it works!. Lets hope MOT test man likes it!
m fairclough

This thread was discussed between 07/03/2009 and 08/03/2009

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