The horn starts to make a noise but chokes off each time the button is pressed.
Is it completely dead or just mostly dead?

There is almost no reference to the horn in any of my workshop manuals.
It is the original Lucas horn on a 1966 car, so it would be nice to have it work, but can understand why it wants to retire.

Any one had experience pulling one of these apart? I assume there is a set of points that need cleaning.

My car does this if it has been sitting for a month or 2 and the battery voltage drops a bit. As soon as I start the car and bring the revs up to charge the battery it works fine.

I have never pulled apart a Lucas 9H horn but some horns have a tone adjustment and playing with that can loosen up the internals and get it going again. There is a large flat blade screw in the centre that may be the tone adjustment.

In your case the first thing I would do is a "bench test" by applying jumper wires directly to the 2 terminals. If the horn still doesn't work then it will have to come off and be looked at closer.

If it worked by applying 2 jumper wires then you will need to workout which part of the circuit is faulty. There may be a voltage drop when the horn button is pressed. If so then check the condition of all the connections in the circuit that can cause a high resistance. Clean the fuse connections as well. Clean the horn button connections and the rubbing block on the indicator arm.

On car testing
- Leaving the power wire connected - use an earth jumper wire to work out if the problem is down the line at the horn button earthing.
- Or leave the earth wire on and apply a 12v jumper wire directly from the starter solenoid to the horn. This will discount the fuse box and supply power wiring.

If all else fails wire it up through a relay with Good power supplied to realy. End of Voltage drop.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

Thanks for the ideas. Will keep working on it.

I dismantled one once, inside was full of surface rust. The steel diaphragm in particular. I cleaned it all up and it worked afterwards.

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67 Deluxe
Dry suspension
1293cc, 11:1 comp, RE 282 camshaft, 45 Weber, extractors etc
4 synchro close ratio gbox
Cooper S brakes, no booster
Stealth box, as Doc says "goes harder and idles!"
RH tank cos I needed it crossing the Nullarbor etc..

Tried bench testing on a good battery but still just clicking.
Found a youtube video showing the innards. That will be the way forward, except I need something honking for registration.

You can get cheap new ones at the usual parts shops for under $20 that'll get you through rego until you can get your original one working.

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Yes. That will be the plan now.
Thanks Tim and all.

They often come good after a good whack.
Fiddling with the adjusters also helps.

gotogreen1969 was correct. In my 1965 Mini the horn has a permanent positive connection that is always on. The negative lead goes back to the steering column and is earthed to the metal of the steering column when you push the centre knob of the steering wheel. Check the voltage at the two contacts on the horn. Both should be about 12volts (battery). (If not then I would suspect the horn). When the horn button is pushed, one of the contacts should drop to almost zero (if not then button and its wiring is suspect).
Dave D

Sorry; I forgot to say "Check the voltage at the two contacts on the horn RELATIVE TO THE CAR'S METAL CHASSIS". If the car is Negative Earth then both horn contacts will read about +12 relative to the car chassis and if the car is Positive Earth both contacts will read about -12 volts. Again when someone pushes the horn button one of the contact voltages should drop to almost zero and the horn should sound.
Good luck; Dave D

Have replaced the clicking horn with a Japanese type which works very well. So circuits should be OK.
Pulled the old horn apart and found rust like deluxe67 said!
Cleaned the points and removed most of the rust. Will reassemble with some M4 screws and try again.

I have one that started working after I bashed it on the bench and loosened up the 1/2 kg of dirt that had caked in there... once that was out it worked like a champion

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Fixed the horn. Cleaned the points and the rust.
Took a while to get the points adjusted so it honked, then the big adjuster in the centre that controls volume.
Needs a spray to finish it.

well done!

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