I don't mean to hoot my own horn - because I can't!

[Marshall V. Daut]

Ha, ha, ha! I am helping a friend sort out a 1925 Coupe he recently bought and we've fixed all the electrical bugaboos left in it by a previous owner - except for the horn. It's the last thing to be fixed. So that we're all on the same page, it's a typical Model T electric horn, not some other off-brand. I have by-passed the horn button and run a direct wire from the battery to the horn's terminal, but no-go. Nothing. No even a chirp or an attempt at making a sound. I have removed the back cover and inspected the wire connections and the overall condition, which appears to be good. No broken wires or burned coil strands. The soldered connections look good. It's not rusted or oily inside. I backed off the tone adjusting nut and turned the adjuster screw both ways, likewise without audible success. 400 sandpaper was used to clean the points. The points gap looks about right at 0.010" or so. I have read the Ford repair manual's description of servicing the horn and followed its instructions, as well as what Ted Aschman wrote in his "Tinkering Tips" booklets. Still no sound.
What am I missing here? It can't be THAT complicated to make this thing honk the way a Model T horn ought to! Failing any magical suggestions, does someone on this website repair Model T horns?
Thanks in advance.
Marshall

[MichaelPawelek]

Bad ground?

[speedytinc]

Bad ground?

Or missing insulator on the terminal? Have you tried putting power to the wires inside with the cover off? Checked continuity, also with the points closed. Of coarse you have. Cleaned the oxidation off the points?

[Marshall V. Daut]

"Bad ground?"
I used two wires directly from the battery to the horn, with the cover in place and without it, on the engine, on the workbench. No improvement.

"Cleaned the oxidation off the points?"
Yes, as stated, I used 400 grit wet/dry sandpaper between the points. No sound.

"Checked continuity, also with the points closed"
That's the next step on Monday. Not being an electronics wizard, what should I be looking for on my meter? The dial set where? Touching the probes where? And what will that tell me? Not my strong suit.

Marshall

[Wayne Sheldon]

Any chance it could be a magneto horn? They only work on Alternating Current.

[Marshall V. Daut]

"Any chance it could be a magneto horn? They only work on Alternating Current."
Mmmm. Well, the horn looks like the other electric horns I have seen (and have owned) and the innards match the photo in the Service Manual with the coil, points block and all. Shouldn't the magneto horn look different inside? I'll take a photo of the horn tomorrow or Monday, but I'm pretty sure it's an electric horn. 'Not sure why a magneto horn would be on '25 Coupe with its full complement of electrical equipment (generator, starter, battery). But who knows after all these years? 'Could have been swapped by a previous owner.
Marshall

[Steve Jelf]

A magneto horn has an adjusting screw in the center of the diaphragm.

[vech]

I have worked on BMW motorcycles for over 40years, and like all old electric horns, they have a set of points in them also, so let me give you some advice. You said you ran sandpaper through the points to clean them. That is the thing to do. But, after you cleaned the points, did you take a white business card, and drag it through the closed points? You see, the sandpaper will leave dirt/grit on the points surfaces, and while they may appear to close to the naked eye, the grit is actually holding them apart ever so slightly.
If you take it back apart, and clean the surfaces with a slice of white business card, and put it back together, you may very well find that it works again!

[Dave Sullivan]

While at rest, there shouldn't be a point gap..Dave in Bellingham

[RajoRacer]

It's quite hard to mistake a magneto horn from a battery horn - mag horn has a very small back end compared to the larger backside of the battery
horn !

[Marshall V. Daut]

"A magneto horn has an adjusting screw in the center of the diaphragm."
As stated, the adjusting screw/bolt is inside the cover that I attempted to adjust for sound. I wouldn't think this horn has an adjustment inside the bell and inside the cover. I'll check when I get the horn later today.

"...clean the surfaces with a slice of white business card..."
I'll do that, too.

"While at rest, there shouldn't be a point gap."
There isn't. The points separate easily by pushing the lower part of the points down to create the gap.

"...mag horn has a very small back end compared to the larger backside of the battery horn."
I have robbed an image from the Internet that is the same horn as I have. See photo below.

Marshall
[image][/image]

[Lil Teezy]

I second the notion of cleaning the points with a piece of paper soaked in alcohol or brake cleaner, etc. after filing them. The points also need a fairly even gap when the tab on the plunger pushes them open. Mine was pushing the lower points at an angle and they were burning up from the spark as fast as they were flattened. Apparently all the adjustment in the plungers travel was used up. The points mount in a little piece of spring stock which can be bent gently back to level and does need a bit of spring tension. If the points are super thin they may need to be replaced. The whole system is a little touchy and needs really good connections not to arc brightly at the points, burning them instantly after every test. Make sure the ground is really thorough to the horn body, and your power input terminal is insulated completely from the body. The old paper or phenolic washers in there break down and soak up oil and may need to be replaced. Also, “tapping” the wires to attempt continuity test will not work as well as being sure you have a solid connection at the terminal and use a good button or two wires you can push together easily and completely. Sketchy inconsistent connections seem to be hard on the points too. Chase the current through the wiring and see that the points are letting the coil energize and that opening the points disrupts the circuit. If there’s current in the Belden wire, a small air gap when the points are open, and the plunger is free to move inside the coil you should get at least a “croak” or a “bonk” or maybe just an arc at the points, but some sign of life! If you tap the plunger briskly the diaphragm should give up a little noise too, so removing it from the body and cleaning any grime out of the landing seems to help. I don’t know if it’s really possible and have not researched the actual composition but the points being some soft metal seem to have oxidized in mine and burned up super easily. I replaced them with some cookies made from large diameter solid copper wire, and added an electrolytic capacitor across the coil as well. If you get it to wake up, but there is a bright arc at the points you will be back at square one each time. I adjusted mine prior to replacing the points and it would be good for three or four honks, then fry itself before it even left the bench. It seems that after so many years in service the thin wire on the coils gets almost broken in a few places and in combination with iffy connections and insulation breaking down the current has harder time making its trip and heat is generated, toasting the points with every toot. Way easier to work on it when not mounted, but the best sound will be had adjusting the plungers’ throw on the car, not the bench. I guess they just sound better resonating the rest of the car. Some of this might seem trivial or repetitive, and I sure don’t want to say you don’t know what you’re doing! Just figured I’d go through everything I can think of for the benefit of future researchers. Good luck and shoot me an email if you want to chop it up further or need some pictures for reference. -Chris, in Boulder

[Marshall V. Daut]

Good input, Chris. Thank you for taking the time write up such excellent advice..

Here are four photos of the ELECTRIC horn in question. Do you see anything obviously wrong? Is the "drum" with the yellow arrow supposed to spin freely? It is sluggish and difficult to turn with my fingers. Could this be the problem? Or does it simply vibrate? I don't see where to oil the shaft if this is supposed to turn when juice is introduced.
I'd sure like to get this horn working for my friend and also gain a new Model T skill I can use to help other Model T owners locally, who also have non-operating horns. In this town, you NEEEEEEEEEEEEED a loud horn!
Marshall

[speedytinc]

Those "drums" are steel disks that are pulled down upon activation of the electromagnet. Note on the opposite side the disks open the points, shutting off the magnet. The proper opening of the points via the location of the disks & length of travel changes the frequency of the vibration effecting the horn tone. The disks location is to be moved/tuned by loosening the lock nut & rotating via the slot in the stud.
It appears in the picture (or its not shown well) that the points are not making contact @ rest as they should. Power pulls the points open with the electromagnet.

[JTT3]

Looking at the slotted brass stud and lock nut, they do not look like they are in the proper position. The brass stud should be exposed a bit more. If they are not in the right orientation you’ll get no sound. You can connect the horn to your 6 volt source and adjust just as Chris & John stated by loosening the nut first & adjust the slotted stud. These are not gross adjustments but rather fine adjustments.

[Jerry VanOoteghem]

The "drums" are not supposed to rotate and should NOT be oiled. They are just hunks of metal that the electromagnet pulls against to make the horn operate.

Show a close-up photo of the point contacts when the horn is "at rest". Also, the solder joint, shown in your first picture, looks kinda iffy. Maybe not. Hard to tell.

[Jerry VanOoteghem]

Maybe knowing how the horn is supposed to work will help you find the trouble.

The basic components are...
1. The flat spring
2. The discs, (you call them drums)
3. The coil/electromagnet
4. The contact points
5. The plunger, which you can't fully see because it's hidden within the coil, but its top end is what the nut attaches to.
6. The diaphragm, which is also mostly hidden from view.

- At rest, the contacts are closed.
- When energized, current goes through the points and energizes the electromagnet coil.
- The magnet pulls the discs towards it and in doing so, flexes the flat spring and draws the plunger closer to the diaphragm.
- Just an instant before the plunger is able to strike the diaphragm, the discs touch on the end of one the contact points, causing the points to open.
- The opened points turn off the electromagnet, but the momentum of the discs allows their movement, and the movement of the plunger, to continue on, until the plunger strikes the diaphragm: BANG
- The tension in the flat spring now pulls the disc/plunger backwards until the contact points are again allowed to close, reenergizing the electromagnet and causing the sequence to repeat.
- The whole sequence repeats very rapidly and the "voice" of the horn is nothing more than a series of rapidly repeating strikes by the plunger against the diaphragm.

The adjuster screw & nut determines when the points will open and close. Adjusted one way too far and the magnet will pull down the disc/plunger and just hold it there, not allowing it to return. Too much the other way, and the magnet will not energize, or energize only so briefly that the discs/plunger are not drawn down with enough force to eventually strike the diaphragm. You need to adjust for somewhere in the middle, i.e., the "sweet spot". To help get close to that sweet spot, there is an initial gap setting between the contact arm and the edge of the disc, measured when the horn is at rest. That dimension was mentioned elsewhere in this thread. I'll let you find it

[Marshall V. Daut]

Update -
I tried all suggestions and guess what! The horn now honks like a ruptured goose! I'm not sure which of the suggestions did the trick because I tried them all. I think, though, that as suggested, I backed the adjusting stud out a couple more turns. Hooking up the horn to a battery again, the horn came to life. Now all I need to do is adjust the tone quality. The first step was getting it to burp. Mission accomplished! Thanks to all for their help. My friend will be overjoyed. I know that I am!
Marshall

[Lil Teezy]

Excellent news! That sound is so goofy it makes them fun to get working, and I definitely need mine around here!