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I would be tempted to say the relay is wired up wrong and instead of switching the horn on, it's cutting power when the ignition is on. 

Have you done anything to the wires before this problem occurred? If not it could still be a fault in the relay. 

Let's see what others can come up with. 

Gareth

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22 minutes ago, Steve Walker said:

My 1972 TR6 has a horn relay. A new problem has occured. When the ignition is OFF the horn works but when the ignition is ON the horn will not work. Where should I start looking?

Hi Steve,

here is a basic wiring diagram with the relay  http://www.advanceautowire.com/tr2506.pdf

The horn button earths the winding of the relay W2.

12V is supplied direct from the battery.

With the button pushed the relay should energise, the contacts C2 & C1 will close and the horn will sound.

So, with the ignition OFF have you got 12V n C1 and W1 (purple wires) - it sunds like oyu have.

With the ignition ON have you got 12V at C1 & W1.  If not is there 12V at the purple wred fuse  If not trace the wire back and see where it is connected.

It should be coming of the starter wiring somewhere.

Roger

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Two questions Steve.  Is the wiring standard and have you been carrying out any work on the car immediately prior to the fault occurring? 

The reason for asking is that it is very difficult to understand how this fault could happen spontaneously with the standard wiring set-up. The horn wiring is separate from the ignition circuit and any feasible relay failure cannot cause this.  

I'm not too familiar with the TR6 steering column arrangement but the only thing I can suggest is to check the steering column is properly earthed, as I think that is the return path for the horn-push. There is an off-chance that the earth is somehow making through the metalwork of the ignition switch body, and only when the lock is in the off position. 

 

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Gentlemen, thank you all for your replies and suggestions, much appreciated. I took the advice of people who know what they are talking about and reconnected the relay correctly.

It WAS W2 bridged to C2 from the fuse and W1 to the horn push. It is NOW W2 to the horn push and W1 bridged to C2. The horn works now irrespective of the ignition position. Hoorah!  It now begs the questions:-

I have done nothing to the wiring in 15 or so years, so why have I not noticed the problem before?

Is it just a fluke that every MOT in that time has tested the horn with the ignition off?

I am not going to worry about answers to these points, and once again, thanks for your input.

Steve.

 

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