[rotorwrench]

The horns were series wound motors much like a starter motor only much smaller since the torque requirements aren't near as much. These type of motors draw a lot of current. When the horn is only powered off the battery, it can draw more current that when the generator is carrying the load.

Proper maintenance of the horn and all related electrical connections will definitely help with reliable function but sometimes the motors develop armature, brush, bearing, or field problems that require repairs.

I have no doubt that they functioned well when they came off the assembly line but that was 90+ years ago.