The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: Tone on 28.04. 2009 19:25

Title: Horn testing
Post by: Tone on 28.04. 2009 19:25
Hi All, To test the horn works do I connect the two wires on the horn and put a feed to them?
Title: Re: Horn testing
Post by: groily on 28.04. 2009 20:19
Unless you've got a relay in there, one wire is live from switch or battery, the other goes to the push button and the horn won't work till that's pressed to make the circuit. Ie the live is always connected and the button makes the earth. Easiest test - just connect one wire to each terminal of a battery. 12v is good - if there's life that'll usually jar it into making some plaintive yowl. If it doesn't, try turning the adjustment screw on the back of the thing (sometimes inside, depending on what you've got) a tiny bit in either direction - it might work. If it still doesn't, take it apart and clean the trembler contacts, reassemble and mess with the adjuster screw again (they're pretty sensitive). The coils are tough as old boots and it takes a lot to bust them - which is why a lot of so-called 6v horns are giving blaringly good service while being zapped by 12v batteries. The windings are very chunky.

If there's a relay on the circuit (which is nice as the live isn't permanently connected), one horn wire will be connected to the frame (the earth), and the push button sends battery power to the relay, which closes and sends the ooomph onwards to the horn.  But you can test the horn just the same, direct from a battery if you want.
Title: Re: Horn testing
Post by: trevinoz on 29.04. 2009 01:27
Groily and Tone, if you pull the horn apart you have to go to all the trouble of setting the armature movement.
The late Altettes were the easiest as the pole could be moved but the earlier ones can take a long time as they have to be shimmed with gaskets.
I have found that 12V applied to a 6V horn makes a lot of noise but doesn't sound right.
The windings are not that "chunky" The wire in the late ones was .8mm and the early series was finer.
The points having a film of corrosion tends to stop the horn working.
A quick test without dismantling the horn is to measure the resistance across the terminals with a digital multi meter.
A 6V horn should be 0.6 - 0.7 ohms. 12V around 1.1 or so.
Title: Re: Horn testing
Post by: groily on 29.04. 2009 16:53
Well I guess chunky is a relative term Trev. I agree, they aren't THAT chunky! But they certainly handle a 12v battery's current without melting down. You're right about the sound though - not the same  . . . but for testing whether there's life in the thing . . . ? (And for the man with the peaked hat from the government who wants an audible warning device which is probably quieter than most of our exhaust notes . . .)
Title: Re: Horn testing
Post by: trevinoz on 30.04. 2009 03:43
Groily, if you set the clearances correctly and adjust properly, the Altette is quite loud  Trev.