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MG MGB Technical - Laxy horn

Hi,

I have a slight problem with my high tone horn on my '67 mk1 MGBGT. It doesn't to work when the car is being run at low engine speeds, but does function at speeds exceeding 2.000rpm. The problem is predictable.

On disconnecting each horn in turn, each horn will work however. The car does charge: the changing brightness of the headlights on gear changes indicates this. I suspect that the horn is lazy; that said why does it work when the low tone horn is disconnected?

Is there a simple test that can be carried out to assess the horn - I have a multimeter. Will this problem fail the car's imminent MOT test?

Many thanks in anticipation
Brian
Brian McIlvenna

As long as at least one horn works reliably it isn't an MOT failure.

Sounds like a bad connection in the horn circuit somewhere - at higher voltages (higher engine speeds) or lower current (one horn disconnected) there is enough voltage, but not for both horns at lower voltages which requires double the current of one horn. It could be just in the wiring to the bad horn, or could be to both and it is just that one horn requires more voltage than the other.

Use your multi-meter to measure the voltage from each horn terminal (i.e. 4 terminals) to a known good ground while the horns are operating (stuff the trumpets with rag to avoid annoying neighbours and frightening the cat). Ideally you should have 12v (engine off, 14v with engine running at fast-idle) on the purple and 0v on the purple/black. Voltages less than 12 or greater than 0 indicate bad connections, but bear in mind the horns take quite a bit of current so some volt-drop can be expected, what you are looking for are significant departures.

Of course, it could be a dodgy horn.

PaulH.
Paul Hunt

Brian,

If this is an original OE horn there will be a "tuning" screw and locknut on the top of the horn body. Your symtoms also suggest that you may need to adjust by either turning in or backing out this screw while having an assistant testing the horn.

Cliff
Cliff Maddox


Thanks Paul and Cliff for the help. I will check this out later. Any other suggestions will also be welcome.

Brian
Brian McIlvenna

Check for variance in electrical resistance between the two horns. A higher resistance (ohms reading) may dictate horn replacement. Otherwise, it may simply be a matter of electrical terminal corrosion. Try switching the mounting locations of your two horns, switching the electrical leads as well. This will determine if the problem is in the horn or the wiring.
Jeff.
Jeff Schlemmer

Brian

I had a similar problem a few years ago after I'd upgraded a Honda Civic to twin tones. After a few months I only got a sort of f*rting noise from the low tone horn with the engine idling although it worked ok at higher revs (it was quite amusing actually!). Being a tamperer I took the unit to bits and found that it was full of condensation and needed a bit more power than the high-tone to work correctly.

With that in mind, I'd say that your problem's likley to be a damaged unit although a bad connection must come a very close second.

Bob
Bob Everick

I had a similar problem on my B. I think what happens is that the funnel of the horn scoops up water when driving and once it is damp then it requires a higher voltage to work (as Bob suggests). I solved my problem by pointing the horn funnels inwards slightly and once they had dried out (with some help from a hair dryer) I used the tuning screw, tightened it right up and then released it all the way a couple of times. This appeared to restored what I suspect is a bit of cardboard inside to the correct shape and the horn works. Its also worth checking the earths as these are prone to corrosion being in an exposed position. Hope that helps.
Climber

You could also put them on a relay cct,
Tatty

I have a similarproblem with my 72 roadster. I put new horns on last year (two terminal type) and they worked fine. Earlier this yearthe car was mediablasted, and painted. Now the horns dont work. I put on two NEW horns and they do the same, a slight, sporadic, squeakly sound if the button is held down. Each works fine when connected directly to power and ground, but they wont when in place on the car.
william fox

William,
Resistance in the circuit. Probably corrosion at the ground point or in the switch contacts would be my guess.

Joe Ullman

This thread was discussed between 24/10/2001 and 26/10/2001

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