Having launched into a full strip down restoration on a car I've owned and driven since my 1980s "refurbishment" I have come to the horn situation. As the song goes two out of three a'int bad. All are rusty, two have innards that are somewhat acceptable, one has mud and rust inside the housing!

I've decided to try and find new ones and think I have a solution however there are very few people interested in the Volvo 1800 horns posting anywhere on the internet. I've been trying to decipher Bosch's numbering for the three horns and also determine the frequency of each in order to attempt to replace two maybe all three with newer compatible models on my '64.

To start one of the smaller "City" horns is totally rusted internally, the other is ok inside but the exterior is also a mess, the "Highway" horn is the best of the three.
The two "City" HO/FDG horns on my car have clearly different frequencies, but numbering on the front grill pieces are identical other then "290" at the bottom centre of one and "362" on the other, does anyone know if these are the frequencies in Hz?
The "Highway" HO/FSA 12/27 horn with the attached centre ID bears no frequency information at all.

My plan is to adapt the FDG front grills to the current Bosch 0 320 223 021 and 0 320 223 022 horn models(page 5 of the Bosch catalogue attached) which are 300Hz and 375Hz respectively. As with the original horns these new models are 125mm diameter, and other then now being riveted together I hope will be a simple task to convert.

Similarly I have found on eBay a pair of unused 156mm diameter Bosch 0 320 213 010 and 0 320 213 011 390Hz and 520Hz which have the same front grill as the old horn but if possible I plan to replace with the original FSA grill.

I have found and interesting manufacture of reproduction Bosch patented horns, I'm guess with Bosch's permission, and made in Tbilisi with a Facebook presence in the US.

I've searched and searched and cannot find any definitive source of information on any of the Bosch products, only bits and pieces here and there on BMW, Mercedes and Alfa sites and of course what Volvo owners have posted. If anyone knows of such a listing or catalogue please let me know how to obtain a copy.

On the wider world of Bosch horns it is strange that this seems an area where no one has chosen to write the "Encyclopedia According to Bosch" and maybe in my retirement I might do just that, if only I could crack the Volvo question.