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We have Canadian owner to thank for Patrick Hiron for this guide to repairing Windtone Horns:
Wind tone horns, developed by Lucas in the 1930s, were originally fitted to high end motor cars. Two separate horns, a high and a low note, as Lucas said, "To give a Pleasing Harmonious Chord which remains constant under all conditions”
As time went by these horns went down market and were fitted to just about every British car in from the 1950s onwards. There are many variations in Windtone horn design and construction, as time went by their materials and construction became increasingly cheaper. However all Windtones work on the same principle.
The 1938 Lucas catalogue says that:
"The note is produced by the excitation of a column of air, just as in a concert instrument - -  the pitch and character of the sound  of the instrument is decided by the correct acoustic shape and scientifically determined dimensions of the trumpet. For these reasons the note cannot vary”
The "excited column of air” is produced by a vibrating diaphragm actuated by a powerful electromagnet.
Problem areas:
Windtone horns live a neglected life, out of sight and out of mind, until they fail.
There are three potential problem areas, the horn mounting points, wiring and earth connections, and the horn internals
Horn mounting points:
My Lucas catalogue has a full page of Windtone mounting brackets. These vary from elegant laminated spring steel mounts to solid agricultural looking lumps of steel.
They are all designed to absorb the considerable vibration produced by the horns. Poorly mounted horns can produce off sounds and vibration in the structure of the car, so check that the horns are solidly mounted.
Wiring and earth connections:
 Interceptor horns are wired through relays. The low power side of the relay is earthed through the horn button, this pulls in the relay making a high power connection feeding power to the horn electro magnet and then to earth, completing the horn circuit. 
First check the fuses are intact and that the fuse holder terminals are tight and clean before moving on to check the power wiring. The horns use a great deal of power, so it's no use pushing the horn button and using a multi meter to confirm there is power at the inlet connection to the horn and  to earth. You need a hefty load to replace the horn when checking out the capacity of the wiring.
I disconnected the horn and used an old head light wired across the horn power feed, first directly to earth and then as a connection between the power feed and the horn's earth wire. This shows whether both the power feed to the horn and the wire connecting the earth terminal on the horn to ground are OK.
The earth connection to the car is often poor. Cleaning this connection often solves horn problems. If the power circuit and ground connection are sound the horn itself is faulty.
If the fuses are OK and there is no power to the horns then you need to work back through the horn circuits to find the problem. The most likely problem areas are the connections on the relays which often need to be cleaned, or the relays themselves.
Horn internals:
If the horns have to be worked on, remove them from the car and work on them in comfort in the garage. Work on one horn at a time, keeping the other horn as a reference. Mount the horn in the vice; bring in the battery and some heavy wire to connect power to the horns.
The "Column of excited air '' is produced by a vibrating metal diaphragm which is moved upwards by a hefty electro magnet energised by a make and break switch in the horn's internal wiring. The electromagnet  attracts the diaphragm. The diaphragm in turn moves a rod which opens the make and break switch contacts, cutting off power to the electro magnet. The diaphragm then returns to its rest position, the rod drops, and the switch contacts close, powering the solenoid and flexing the diaphragm. The diaphragm, electro magnet and the make and break switch must all is in good condition, so remove the horn's top cover and look inside.
Windtone horns generate a great deal of heat in operation. I found out the hard way that it's not a good idea to use penetrating oil or flammable solvents to clean up the horn internals. Spray cans of brake cleaner, and computer cleaner "air” are a much safer bet.
The diaphragm is solidly fixed to the bottom trumpet section by either rivets or 2 BA nuts and bolts, depending on Joe Lucas's cost accountant's whims. Clean off the top face of the diaphragm using an air line or a spray can of air. DO NOT separate these two components "to see what's inside” You may have to replace loose rivets or nuts and bolts, or even strip the horn if the diaphragm is cracked or rusted out .If you separate the diaphragm be careful not to damage the thin rubber gasket .
It's easy to check the electromagnet’s heavy wiring using a multimeter across the inlet and outlet terminals
If the diaphragm and electromagnet coil are OK the problem lies with the make and break switch. Many horn problems result from unhappy attempts to tune the horn by "adjusting” the switch.
Make and break switch:
This is typically Lucas, Rube Goldberg inspired confection. The contacts are often misaligned, loose, dirty and burned. They should be tightly screwed down, and aligned and clean. It possible to align the contacts by fiddling with the tiny screws holding them in place and clean them in situ with 600 wet and dry moistened with brake cleaner. If this doesn’t work you will have to strip the contacts out and either polish them on oil stone or replace them. I have found NOS Windtone contacts on E Bay.
Be warned! The switch is a glorious mix of tiny BA screws and bits of [Mica, Tufnol,] insulation and sproingy metal. This why you should repair one horn at a time, you have an unmolested model for reference if things go pear shaped. If you have to strip out the contacts take digital pictures at each stage and lay out the parts in sequence. 
 Stripping out the switch will let you clean up the actuating rod buried in the horn internals. It's worthwhile pulling out the rod and polishing it with chrome cleaner, and then flushing the recess it lives in at the diaphragm. The objective is to have the rod move cleanly. Do not be tempted to help it along with any sort of lubricant.
By now you should have confirmed that:
1.  The diaphragm is in good condition and is solidly attached to the trumpet base   of the horn
2.  The electromagnet's windings are in good condition
3. The actuating rod is clean and moves freely
4.The contacts are polished and centred.
5.The switch is properly installed and bolted firmly into place.
The last job is to set the make and break contacts. You will need: a multimeter, a pair of very thin 3BA open ended spanners [I had to grind down the faces of two old spanners to make suitable tools] and a great deal of patience.
The horn depends on the make and break switch operating cleanly, snapping decisively between open and closed. If it is maladjusted there will be either a dead silence or a "mad MIG welder display” of arcing.
The contacts must be adjusted so that the switch is just closed with no power at the horn. Use the multimeter to adjust the points so that they just break, then turn the adjuster half a turn so that the contacts are fully closed. Use the lock nut to lock the adjuster screw in place.  
Test the horn on the bench using heavy wiring before putting it back on the car