Horn sounds bad on 12 volts - Fixed!

[drolston]

Stuck at home looking for some little project in the garage. The sound of the horn on my 12 volt '41 coupe sounds bad. Though I would share the fix.

With only very intermittent use, the 6v trumpet style horns would probably work indefinitely on 12 volts, but they sound scared and angry, instead of the correct authoritative tone. I tried the same voltage reducer that worked fine on my windshield wiper motor, instruments, and fan motor, but the chopper circuit in those devices does not play well with the spikey current demands of the horn vibrator.

A resistor in series with the power to the horn would reduce the voltage at the horn and not be bothered by the spikey current demand. I put a 10 Ohm rheostat in the circuit and turned it down until the horn worked. The wiper was on the last winding of the rheostat. Best I could tell with my cheap Ohm meter was that the resistance was under .5 Ohm. So I bought a batch of high power (5 watt) 1.0 Ohm resistors and started wiring them in parallel, until I got the resistance low enough for the horns to work. It took 8 1.0 Ohm resistors in parallel, for a resultant resistance of 0.125 Ohm. With 5x8=40 watt capacity it can probably handle a stuck horn long enough for me to disconnect the battery.

The pictures show the assembly. It is kind of a "Farmer Fix", but I tucked the resistor assembly behind the horn, so it does not look too bad. I am still looking for a smallish 0.125 Ohm resistor with the power capacity to handle the high horn current. In the mean while, I think I will install a battery switch. Been wanting to do that anyway.

[rotorwrench]

The horns were adjusted by current draw measurement during manufacturer. Some of the old 6-volt horns like 8-amps but I don't know what specs the Ford horns were adjusted to. Using a quality wire wound resistor like the Ohmite ceramic coated ones should work. As long as a person doesn't lay on the horn very long then it won't have time to get too hot. Using a good controller type adjustable rheostat is a simple way to find a good value. You don't have to do the math and it's predictable about the results. The math does a person no good If you don't know what the current draw for the horn is supposed to be.

[drolston]

I got close to finding the correct Ohms resistance using the rheostat, then did trial and error by adding resistors in parallel to get the resistance down to the point where the horn worked. I am using 8 5 watt wire wound 1.0 Ohm ceramic coated resistors, but have since found 10 watt .5 Ohm resistors. Four of those in parallel should work and be a little more compact.

[koates]

A 6 volt Ford horn working on 12 volts requires a wire wound .5 ohm 50 watt resistor in the power feed to each horn. You can buy these on line pretty cheap. The resistor can be fitted under the dome cover of each horn and then cant be seen. The 50 watt rating is required to handle the horns current draw so don't opt for a lower rating. Regards, Kevin.

[JayChicago]

Thank you all for this info. My new-to-me ‘37 came with 12 volts and modern horns, but I love that “authoritative” sound of original trumpets. Now I Know how to make that happen. It’s going on my to-do list. Thanks.

[Burt Lake Barn]

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Was that a .5 ohm or a 5 ohm?

[rotorwrench]

Ohmite Metal-Mite 850FR50E would be a basic dropping resistor to drop 12-volt system power source to 6-volt. There are other brands and percentage ratings too but these types are commonly used for dropping voltage in automotive use. Fifth Avenue Internet Garage sells units like this for continuous use for heater fans and such. It might be overkill for an intermittent horn circuit but it would work.

[drolston]

Originally Posted by koates

A 6 volt Ford horn working on 12 volts requires a wire wound .5 ohm 50 watt resistor in the power feed to each horn. You can buy these on line pretty cheap. The resistor can be fitted under the dome cover of each horn and then cant be seen. The 50 watt rating is required to handle the horns current draw so don't opt for a lower rating. Regards, Kevin.

The Ohmite Metal-Mite 850FR50E resistor is 0.5 Ohm rated for 50 watts, continuous, only if it is mounted flat to a sizeable aluminum chassis. The horns draw over 50 watts, but only for short duration, so the resistor does not have time to overheat. Those high wattage resistors are about 2" long and about 0.5" in diameter. Been a long time since I tore a horn apart, but I believe it would be a challenge to fit that size resistor under the dome. And my experiment says that each separate horn would work with 0.25 Ohm but not with 0.5 Ohm. Two of those resistors in parallel would give the right resistance but be an even bigger problem to fit under the horn dome.

[koates]

Originally Posted by Burt Lake Barn

Was that a .5 ohm or a 5 ohm?

0.5 ohm or half an ohm if you like, 50 watt. This gives plenty of power to the horn for a long blow if you need to get angry at some other driver. Regards, Kevin.