Model A horn resistor
 

Anyone know the dropping resistor value used on a 6v Model A horn in a 12v system?
Thanks, JC

Re: Model A horn resistor
 

about 2 ohms. One of the white resistors found on the firewall on 50/60's chevy ignition systems will work fine.

Re: Model A horn resistor
 

Wattage is just or more important than resistance. The small ones that mount in the horn housing can burn out easily. The larger ones that mount on the frame work fine. You can also rewind the horn for 12 volts. That is what I did.

Power is I squared R. So at 10 amps (probably high) the power dissipated is 200 Watts. 5 amps would be 50 Watts. The common 1/4 Watt resistors clearly will not work and would make a fuse with a smoke signal to let you know it burned out.

Re: Model A horn resistor
 

If you know how many amps it draws on 6 volts you can figure it out.

Re: Model A horn resistor
 

Katy, That is mostly correct, but the horn has induction which is part of the reason why it has the distinctive ah-og-ga sound. So the current starts out higher then goes down as the reverse EMF builds at higher rpm. So the dropping resistor works but the sound is slightly different.

To do the math, first measure the current on a correctly adjusted horn with 6 volts. Say you get 5 amps. R=V/I so the equivalent resistance would be 6/5 or 1.2 ohms. Power is W=I X V so the wattage would be 5 X 6 or 30 Watts. To be on the safe side use a resistor with a higher power rating.

It would seem that the 2 ohm resistor is assuming the horn draws 3 amps with 6 volts. It probably takes some experimentation to get the right sound.

Re: Model A horn resistor
 

As stated above, the Ahooguh horn is an inductive animal, and a dropping resister is not the best answer when converting a Model A to 12-volts. Most dropping resisters sold by the suppliers cannot handle the power requirement and they will burn out with repeated use.

The best solution is to rewire the field coils, it is not difficult, anyone can do it. See attached article.

Tom Endy

http://www.santaanitaas.org/wp-conte...orn-to-12v.pdf

Re: Model A horn resistor
 

My 6-volt horn burned out. I bought a new 12-volt horn from A&L before the pandemic. You can still get them from the major suppliers.

Re: Model A horn resistor
 

X2 what Tom Endy said.

Re: Model A horn resistor
 

I re wired mine. It was not hard to do.