Horn Repair PDF Print E-mail
Written by BMWVMCA NEWS   
Sunday, 26 June 2011


by Slamaway Hamfist


     Often, these days after many years of sitting, many old Bosch horns don’t work.


     Rarely are they burnt up inside. Usually, the problem comes from corrosion on the points, which are made of Tungsten, really hard stuff!


     See the photo of a horn with the cover removed. There is enough white powder around the points to tell us what’s going on. The white stuff acts as an insulator, preventing the horn from working.



    These horns are electro magnets as you can see the iron core in the center and the copper wire wrapped snugly around it. Current passes through the contact points (supposed to when you push the horn button) creating a magnetic field, drawing an iron plate to it. The iron plate is bolted to a thin stamped steel disc that acts both like a speaker in your stereo system and a spring to suspend the iron plate.


     When the magnetic force is strong enough to pull the iron plate down against the iron core of the magnet, the plate depresses a tab attached to the electrical contact points, breaking the circuit, which collapses the magnetic field, turning off the magnet. The disc’s springing part returns the iron plate to its original position at the same time releasing pressure on the contact points, which close, completing the electrical circuit and repeating the magnetic/mechanical cycle. And ‘BEEP’ it goes.


     When these contact points become corroded, the horn won’t work.


     The next time you find one of these old horns at a swap meet, you can tell the vendor that it probably doesn’t work, and therefore you should get it really cheap.


      Hey! Hold up there Mr. Hamfist, those vendors happen to be our members! They are reading this as you tell it, and they’re on to you and your sleazy ways! They know these horns are easy to fix, and they are getting harder to find, therefore worth more! 

     Yeesh! I hate it when you do that. You’re a regular ‘Dudley Duright’! Let’s get on with it.

     Okay, so you might have to pay a little more for your horn. It’s still easy to fix. Remove the six (some have five) screws that hold the cover and sound disc to the body of the horn. Do not remove the iron plate, which will be held in place with a single long threaded stud.


     The points can be gotten to by depressing the composition phenolic tab and slipping a small bit of abrasive paper between the points and sliding back and forth to clean them. Do this several times to each contact point. Blow the dust away with compressed air.



     Test the contact points for continuity. Reassemble the horn in the reverse order from how you took it apart.


     When  you test it, it will probably need to be adjusted. This is done by rotating the adjuster screw at the rear of the horn. Be careful! Only slight movements on this screw will make a big difference.


     The adjuster screw can be seen from the inside in the first photo on the previous page. Note there are two screws sticking up. The one that does the adjusting is the one with the spring below it.


Last Updated ( Friday, 25 January 2013 )
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