Vintage BMW Motorcycle Owners

Anyone ever take a stab at rebuilding these themselves?

I’ve torn a non-op horn down, cleaned it up internally and familiarized myself with the design and how it all goes together but I can’t say that I’ve effectively “fixed” one yet. Before I did anything I hooked it up to a 6v battery with a couple jumper wires and saw zero sparks when I touched the negative wire to the ground. I then tore it down almost completely, cleaned all the parts thoroughly and reassembled. Now I hear a very faint tone and see some weak sparking when I connect power. I haven’t installed the horn in the bike as I’m just bench testing. I tried turning the adjustment knob in/out but no real change was observed in tone. Is there anything written on this unit and how to restore it? Tips or suggestions?

I also have a few old speedos that are non-op and before I send them to some $500 a pop repair shop, I’d like to have a go myself. Surely they’re not a Rolex in there and I’ve got nothing to lose. Does anyone have any resources on rebuilding the speedo?

Thanks!

The horn only sounds in a narrow band along the adjusting bolt. Keep track of where the bolt was set originally, then try it for each half turn in and then out.

Is it a Bosch horn and what model number

The placement of the curved washer and order of the other spacer parts are very important
I may have your model and will get some pictures if it matches mine

The horns I’m working on are the French made Klaxon variety indicative of the /2 era.

Should these sound on the bench when connected to a 6v battery or am I missing something? Does the center rear stud need to be grounded as well?

Not a Klaxon, but rebuilding of a Bosch horn was written in the Fall 2010 issue of the Vintage BMW Bulletin.

schrader7032 wrote: ↑

Wed Mar 27, 2024 6:30 am

Not a Klaxon, but rebuilding of a Bosch horn was written in the Fall 2010 issue of the Vintage BMW Bulletin.

Thanks for that reference! I have 'fix old Bosch horn' on my list, this is great.

schrader7032 wrote: ↑

Wed Mar 27, 2024 6:30 am

Not a Klaxon, but rebuilding of a Bosch horn was written in the Fall 2010 issue of the Vintage BMW Bulletin.

Volume 37, #3

Slash2 wrote: ↑

Wed Mar 27, 2024 6:19 am

The horns I’m working on are the French made Klaxon variety indicative of the /2 era.

Should these sound on the bench when connected to a 6v battery or am I missing something? Does the center rear stud need to be grounded as well?

Yes, best way to test and adjust to eliminate any wiring/switch problems
The center stud is only for mounting the horn, Only the 2 wires, battery + and _ to power the horn, polarity does not matter.

Note:
The Bosch and Klaxon both have several different working voltages (6-36 volts) and they look similar
Google your model # if not noted on the horn body

You also may have dirty contacts on the spring loaded electromagnet switch and need to be cleaned, should be close to 0 ohms

Also try 12 volts will not damage the horn
Still am using my Bosch 6 volt horn on a 12 volt upgrade

Don't be discouraged if your initial attempts at fixing these devices don't yield perfect results right away. That's not my Neighbor

Go for it, just be careful with the “guts”, maybe just sand the contact points and be very careful since everything is 60+ years old. The Klaxon is similar to this, I try not to rip the paper gasket and have always “re-used” it no issues. This is a Bosch horn for early Earls fork that I restored for my 58’ R69, pre-Klaxon fitment. There are also 3 different types of Klaxon horn

Using a good 6 volt battery, attach + & - leads to the studs and then slowly turn the adjusting bolt in and out until you get sound then fine turn it. You can also do the adjusting with an ohm meter similar to adjusting the static timing on the motor, its a lot quieter.