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MG MGB Technical - Horns: Metal or Plastio

i must be missing something here... have 2 12v plastic horns in my '71 B and decided to put original cast iron back in. Ordered from Moss, two wire horns and installed them and get a meow sound from just one of the horns. Placed plastic back in, work fine... hmmm, pulled 2 cast iron horns out of a '69 mgb parts car, cleaned the contacts and installed them,,, get a meow sound from one horn.,.. placed plastic back in, worked fine,,,

what am i missing? suggestions:? thanks! BART
bart

The first thing I'd check is the amount of electricity getting to the horns. THe metal ones take the full voltage to really wind. Test the metal hors with a spare battery straight. If they sound off, you are losing current (or amps, I stink at electrics!) somwhere in your car, or your battery/alternator isn't putting enough out.
Maury Drummond

Hmmmm...I thought it was just my set of horns. When I rewired my '67 MGB GT I also installed a new set of horns I bought from a Moss distributor, because my original horns were so soft. With the engine off, the new horns were barely audible, although with the engine (and alternator) running they honked moderately loudly. I replaced them with a set of very similar looking FIAMM standard horns (not air) which are enormously loud, by comparison, with or without the engine running. My battery is fairly new, so I suppose it could just be an awful lot of resistance moving those electrons from almost the rear of the B all the way to the front.

~Jerry
Jerry Causey

There are two adjusting nuts on the horn. A very large and a very small. Only the very small is to be adjusted. The small adjusting nut is reverse threaded--be very carefull--I made that mistake and ruined an original horn. Have someone lean on the horn while you very very slowly turn the nut. You'll know when it's right.

Paul
Paul Hanley

hmm, interesting. something I noticed as well, when driving with the headlamps on, the tach lowers slightly. never thought much of it as it didn't effect drivability. Could the two be related? I see that the horns go to the fuse black & to the indicator "pod". The headlamps go roughly same areas. Could the issue simply be a bad headlamp switch or sloppy ground?
bart
bart

Before adjusting the horns (particularly if two sets make the same noise or lack of it) check the voltages as suggested, like the turn signals they are sensitive to low voltage/high resistance connections. Connect a voltmeter between the purple wire and ground at the horns and see what happens when you push and release the button. Ideally it should show 12v all the time, if it drops more than a few tenths with the button pushed you have bad connections back through the purple fuse. Then do the same between the purple/black and ground. Ideally this should show 12v with the button released and 0v with the button pushed. Any more than a few tenths with the button pushed shows bad connections back through the horn button and column ground (wheel centre button) or black wire to ground (stalk button). The column ground is prone to going high-resistance, particularly with collapsible columns. After about 76 the horn push fed 12v from the purple circuit onto the purple/black to the horns, the horns having a local ground through their mechanical fixings, which are subject to corrosion.
Paul Hunt

Have you tried sounding the horns with them sitting on the ground? It may sound crazy but if you don't have the right brackets and you have them tightened down good, they won't work right. They need to wiggle. Of course if you test them out of the car you need to run the right wires to them.
Dana Wilson

"sound crazy" is probably preferable to "meow"

:o)
Paul Hunt

tried them just sitting on the cowl. meow....
bart

Is there some reason why a relay is not a good idea here ? . It's usual with horns to switch the earth ( this why they often sound after a crash ) . My air horns have tiny plastic cased relay , and it works fine . A chunky 12 V wire connects to it , and the coil is earthed by the horn push , the 12V links across to the contacts and the switched output goes on to the compressor . Its possible that horns will not sound if they are not bolted down , the body and diapragm may both move , and cancel the sound .
S Best

Good thought.

Tried them tightly bolted,
Tried them bolted light,
Tried them just right
tried them laying loose
and still failed to toot.

:)
bart

The new Dr. Seuss.
J.T. Bamford

now, now, it's not like I'm tooting my own horn....
bart

Using a relay *might* work depending on where the bad connections are. Mt V8 has high-resistance in the column but even more in the (motolita) horn switch which will probably break if I try to dismantle it, so I have used a relay to good effect. But ifd thre bad connections are in the purple supply to the horn, or at the horn, then a relay will do nothing.

Whilst MGBs switched the ground for many years this was changed to switching the 12v from about 76. These horns have a bolted ground instead of wired, so just think of the money BL could have saved if they had adopted that from the beginning. For that reason Bart's horns (71 and assuming factory wiring) are not dependant on the mechanical fixings.

Try the cast iron ones directly on the battery. If they still meow they are either knackered or 24v horns ...
Paul Hunt

This thread was discussed between 29/02/2004 and 02/03/2004

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