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MG MGB Technical - Horn Headache

I have a 1965 mgb roadster that I recently purchased. I need to get my vehicle inspected so I have been going through the vehicle systems to get it inspect-able. The horn was working before I changed the turn signal switch for a new one. Since I have changed the part I have working turn signals but a non operating horn. I have checked the voltages to the horn (12.4v) and the continuity of all wires plus the resistance of the circuit. At the column when I depress the horn push the voltage only goes to 6.8 - 7.2 volts. I have cleaned all grounds checked and cleaned all the wiring. If I ground the horn it works but nothing with the horn push. As I was checking everything again i touched the inner steering column spring loaded push and it was very hot. I bought a new horn and it responds the same. I know a relay is a possible solution but I'm OCD and I want to know why it's not working . Any help would greatly be appreciated
JF Fiori

The inner column shouldn't be "hot".

Try a jumper wire between the purple/black wire at the column switch snap connector and the inner column, the horn should then sound, if it doesn't the inner shaft is poorly earthed.

All the push does is connect the horn to earth.
SR Smith 1

see if any of this helps from Paul Hunt's excellent web site and note which way the horn pencil fits -

general info -
Nigel Atkins

JF - The top of the steering column should have a copper collar with a nylon insulation between it and the steering column. The copper collar is attached to a button that is grounded when the center horn button is pushed. On the turn signal assembly there is an arm that rides on the copper collar. The arm is connected to the horns. When the horn button is pushed, it applies a ground to the horn via the copper collar and the arm on the turn signal assembly.
D W DuBois

SR Smith I think my inner shaft is poorly earthed but why and how do I find the problem??
Nigel I have gone over Paul's excellent web site and I think I can recite the horn posting by heart but still to no avail,
DW I have tried the grounding procedure but not sure what you mean by the top of the steering column. Is that the horn push?? I have one but it looks like a black insulator with the button coming out of the end. This button was the part that got hot trying to make the horn sound.
Is there any resistance values that I can measure??
I have decided to try the relay but would still like to know where I'm getting the current draw to only make the horn push around 6-7 volts and not near 0.
JF Fiori

"This button was the part that got hot trying to make the horn sound.
Is there any resistance values that I can measure??"

That button is what makes contact (via a wire) to the copper sleeve and the arm from the turn signal switch assembly that goes to the horns. The ground should come from the horn push in the center of the steering wheel.
D W DuBois

Ok, quick test for a good inner column earth.

Get a decent test lamp with a 21 watt bulb in it.

Connect one end to a live battery supply, then touch the other end to the inner column, the bulb should light brightly. Do a back to back test against one of the column mounting bolts to compare.

If it appears dim, the centre shaft is poorly earthed.

Do the same test running down the steering shaft in the engine bay, it may have high resistance thro the u/j for instance or the rack may be poorly grounded to the x-member.

Had to fix a springy piece of wire that touched the shaft once to a Porsche 911 once to get a good earth!

A cruder test would be to make up a length of wire from the purple/black terminal at one horn and try earthing that to various points up the column, which is what the circuit does in effect.

If the earthing is ok, you problem is in the arrangement at the top of the wheel I'd say.

Good luck.

Hope that is some help
SR Smith 1

When you say "At the column when I depress the horn push the voltage only goes to 6.8 - 7.2 volts." what part of the column do you mean? The steel shaft? Or the brass collar that is attached to the top part but is insulated from it?

The steel shaft should always be at 0v i.e. earth. If that goes up when you push the horn button then the earth is bad through the shaft, UJ, rack and front cross-member, which is common.

If that stays at 0v but the brass collar only drops to 7v from 12v when the horn button is pushed then there is a bad connection from that collar, up through the insulated central contact to one side of the horn button, through the horn button to the wheel hub, and from there to the steel shaft.

If the brass collar drops to 0v from 12v but the purple/black wire connected to the brush on the indicator switch doesn't, then the brush is making a poor contact top the brass cylinder, which will make the brush hot.

With iffy connections like this, and the current a horn takes in comparison to even a 21w test lamp bulb, a meter is the only way to get a true picture of what is happening.
Paul Hunt

This thread was discussed between 21/06/2014 and 24/06/2014

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