Help with Horn

David in San Antonio

David in San Antonio

Well-known member
Old horn worked fine until roadster was in a parade and I was frequently awooga-ing the horn until it suddenly quit. No blown fuse, just stopped working.( I tried to rebuild it and got in over my head, but that’s a different story.)
I was given a brand new six volt horn that had been stored about 30 years in a cardboard box. Connected to six volt battery it worked perfectly. These things are unbelievably loud inside a two-car garage.
I put a continuity tester (6 volt lightbulb) on the two horn leads from the wiring harness and it lit up when I pushed the horn button.
Pulled the 30 amp fuse, installed the new horn, and the moment I replaced the fuse it blew. I unbolted the horn and tried again thinking maybe I’d inserted the bullet connectors wrong. It blew the fuse instantly, I didn’t even push the horn button.
Horn is ok on bench with spare battery.
Horn circuit in car works lights up a 6 volt test bulb when button is pushed.
But Horn hooked up to car immediately blows fuse.
I have a digital volt-ohm meter and zero practical experience dealing with Model A electricity.
Suggestions, please. Explain it like I know nothing, because I don’t.
Thank you.
 
Use your multimeter on the ohm setting (resistance) and measure the resistance between both terminals in the horn and ground. Measure one terminal at a time. The resistance should be infinite. If you get something less than there is a short between one or both terminals and ground in the horn. That may explain why it was sitting in the box for 30 years.

Regarding the horn that quit working, did you try to adjust it?
 
Last edited:
I will try the meter as you suggested. Thank you!
The horn that stopped working - it did not spin under power even with the adjuster all the way loose. It had not been serviced since around 1970. I used the Les Andrews book to disassemble it, soaked everything in contact cleaner, ordered new electrical parts (brushes and the rest) but got confused trying to reassemble it and stopped. The Wretched Roadster is a rolling project. I’m not looking to restore it, no paint in its future, just want to make it reliable and fun.
 
The motor is a basic shunt wound DC motor so the motor won't change rotational direction if the leads are swapped. In other words, there is no way to connect it up wrong.

The modern fuse in a model A is just to keep wiring from burning up if there is ever a dead short of power to ground. The horn circuit has power available to it from the generator connection. The ground circuit portion connects to the horn wire connection at the light switch. The horn rod assembly controls the light switch and has the grounding switch for the horn with the actual ground being at the steering shaft. Shorts can be anywhere in the circuit. The OHMs meter is your friend here. Checking the ground circuit is just a matter of access to the ground circuit either at the horn location or the light switch for probe application. The other probe can be anywhere on the engine or frame. There should be no continuity with the horn switch open. Push the button and there should be continuity with as little resistance as possible.

The power wire has to be isolated from it's source and end terminal at the horn. It should have continuity through out the wiring segments with no continuty from wiring to ground. The horn motor by itself should have continuity between terminals with normal field resistance levels but no continuity to ground or any other part of the motor assembly. The armature has to be checked separately to see if there is a short from it's coil to ground or an open circuit. This has to be checked from the appropriate commutator segments per any normal armature test procedure. A growler is generally used to check for shorts but with the little armatures that may be difficult. It depend on the growler design.
 
if it now blowing a of fuses. I have been running to this lately people either use to long of screw in the motor cover or they tighten the screw down too far and the grounds on the brush holder.
 
jw hash said:
if it now blowing fuses. I have been running to this lately people either use to long of screw in the motor cover or they tighten the screw down too far and the grounds on the brush holder.
Click to expand...

What John said! Easy to check, try it without the cover.
 
To verify what John said, measure the resistance of the wire going to the horn from the generator connection. Without removing the cover or changing the attachment screw, disconnect the horn wire from the generator. Use your multimeter to measure the resistance to ground at that wire. If not infinity then you have a path to ground. If the resistance is very low, less than one ohm, then you have a short to ground. Then remove the cover as Blockhead said and measure the resistance again. If you still don't have infinity then the short is internal to the horn.
 
You must log in or register to reply here.
Back
Top