Project Vespa Smallframe - Horn Repair

Rehabilitating the horn is actually a pretty easy task. The rivets can be ground off of the back side so that they press out with a flat punch, allowing the unit to be disassembled. I did this with a Dremel cutoff wheel. The inside was really corroded, which prevented any good contact from being made. The two terminal screws can be removed and the mechanism cleaned. The adjustment screw is also removed from the rear side. I suggest running the screw the whole way in first counting the number of turns so that the adjustment can be returned nominally to the original place.

Current flows through contact directly connected to the coil on one side. The other terminal is on the opposite side of a set of contacts. The contacts are normally closed completing the circuit. When the current flows through the coils they act as electromagnets which open the contacts stopping the current flow through. The contacts close again completing the circuit. This cycle repeats as long as voltage is present at the terminals. The adjustment screw adjusts the tone of the horn.

I used a wire wheel to clean the components. The final step was to use some contact cleaner such as CRC 2-26 which is safe for the insulator components. Assembly is the reverse order as disassembly.  

The rivets were made of soft aluminum. I just happened to have a soft aluminum wire (typically used on a residential satellite television dish). This is very malleable and makes good material for rivets. The wire is gripped in a set of pliers and a ball-peen hammer can be used to flatten the end to form the head. The wire is nipped off with a set of cutters to an appropriate length. This makeshift rivet is then inserted into the original hole and pounded flat to secure the horn.

Beep Beep