Horn issue

[Tea]

Hi all,
Any ideas why our horn works when the car is off, but not when the car is switched on? The m.o.t is on Friday, so last minutes checks and this has come up.

[ManyMinors]

Probably a coincidence and just a loose connection somewhere - maybe in the contact at the top of the steering column.
The horn should work at any time because the live supply to it is not fed through the ignition switch but through the same fuse as the interior light. It is worth checking that the fuse and fusebox are clean. When the horn fails to work, have a look at the interior light and see if that has also failed. If it has then the fuse connection is probably dirty.

[geoberni]

You've not said what variant your car is, so perhaps you don't even have an interior light, but as ManyMinors said, it's a permanent live feed from A2.

How many times have you tried the

Any ideas why our horn works when the car is off, but not when the car is switched on?

Once, Thrice, or many times?
As I've got one, I'd stick a multimeter on the supply to the horn and see if it changes when the ignition is switched on.
Electricity will always take the easiest (lowest resistance) path to complete the circuit.
If your connection at A2 is dirty it will be a 'high' resistance, but with the ignition off (so everything that direction is infinitely higher) it's still a viable circuit so the horn will work.
Turn the ignition on and that electricity effectively says 'Stuff going that Horn Route, all these other ways back are much easier.....'

The connections around A2 are the most likely problem.

[Tea]

Thank you for your replies. It's a standard '71 traveller.

It happened on 3 occasions over the course of the afternoon and into the evening and the same result each time. I went to check your suggestions today and before starting tried the horn with car off, ignition on and with the car fully on and it worked each time and so must be a loose or generally bad connection.

Thanks again