BSA Owners' Club Forum

The BSA Workshop => Twins => Topic started by: RESTORER on 17 July, 2021, 20:39:26

Title: Lucas horn problem. 1954 Flash.
Post by: RESTORER on 17 July, 2021, 20:39:26
Hi all,  Firstly, please understand that I know very little about electrics but lets face it on this bike they are pretty basic. My machine has a 6 volt Lucas horn, similar to the one shown in SS 807 except that there is just one screw in the back, the notched adjustment screw. The horn was working fine with a good loud vibration. Now, on the machine, when I press the horn button I just get a soft beep and no vibration. I have fitted a new chrome front ring, domed nuts, and tone disc, which has been painted to match the rest of the bike. I am using 2 new style 6 volt 4.5 amp/hrs batteries, wired to give a capacity similar to the old black box type, but the horn has worked fine with these. I have not moved anything behind the tone disc (the screw and locking collar etc). When I remove the horn and bracket assembly, mount it in a vice and wire it direct to just one of the 6 volt batteries, as soon as I touch the terminals I get a very loud sound and vibration (maybe too loud). With everything back on the bike, the horn goes back to the soft beep and no vibration. Can anyone suggest what the problem maybe and how I could sort it? Thanks for reading. 
Title: Re: Lucas horn problem. 1954 Flash.
Post by: Mike Farmer on 17 July, 2021, 21:13:04
 :) :) :)

 Maybe bad earth

 Mike 8) 8) 8)
Title: Re: Lucas horn problem. 1954 Flash.
Post by: idie on 17 July, 2021, 21:53:06
You may have voltage drop. Use a meter and measure the voltage on the live wire. If it is battery voltage put the meter between the live 6 volt wire and the earth wire then press the horn button you should still have battery voltage. If not you could try cleaning the contacts in the horn push. Failing that do as I have done and fit a relay.
Title: Re: Lucas horn problem. 1954 Flash.
Post by: Mike40M on 17 July, 2021, 22:43:03
As it works as intended off the bike, the horn is good. So the problem is somewhere on the bike. The most common cause is on the earth side of the horn. If you have a horn button that when pushed makes contact between the button and the handlebar (which is usual), you probably don't have a good electrical contact between the handlebar and the earth side of the battery. So test it by connecting a wire between the earth side terminal of the horn and the earth side of the battery. I'd guess a -54 bike has positive earth.If this test gives a loud signal, fix a better earth path between the handlebar and the engine or the frame. Sometimes the earth path from the handlebar goes via the steering stem bearings. sometimes via the throttle or choke wire cable. None of them ideal.
Title: Re: Lucas horn problem. 1954 Flash.
Post by: Mike40M on 18 July, 2021, 04:34:10
On a lot of bikes, a fairly common earth path is a wire between engine (or frame) and the headlamp shell. Then to the handlebar. Paint can isolate the shell.
Title: Re: Lucas horn problem. 1954 Flash.
Post by: RESTORER on 27 July, 2021, 19:46:04
Hi and thanks for your replies. Having tried to do all you suggested, I think you have identified the problem. The earthing, which with a few experiments on re-routing I believe I can fix. Strangely though, the cut-out button on the same h/bars, has always worked fine and I have no idea why this is.  Anyway, I cannot get full sound and vibration unless I go directly to the battery earth terminal but there is a further problem.  Disconnecting the horn button cable and using a multi-meter between that cable and the earth terminal on the battery, the sound and vibration is very loud but the meter fires up to 10 amps before bouncing about at 7-8 amps. "Must not exceed 3-4 amps" is stated in SS 807.  I have tried using the adjusting screw but nothing changes until about 7 or eight notches clockwise when the vibration starts to drop off and the sound goes lower but the current does not drop.  This may be the point described in the last paragraph of page one of SS 807 when the contacts do not separate.  Can anyone suggest what causes this current use or how I might lower it. Thanks again.
Title: Re: Lucas horn problem. 1954 Flash.
Post by: RESTORER on 21 August, 2021, 21:19:47
Hi, Horn working as it should now. I tested all the possible positions I could and all were OK, so I thought that if the cut-out button operates OK then handlebar earthing must be adequate.  I stripped the horn button, which is a new one, and found that the connections were wrong and needed bending to be sure of a good connection. The high consumption (10 amps) was due to the horn being adjusted at some time way too far in the clockwise direction. I turned it back until it was obviously too far for the horn to sound as it should reading 1-2 amps and then adjusted it one notch at a time. I now have a horn reading 3-4 amps, loud and vibrating. Thanks all.