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MG MGB Technical - Confusing Electrical Problem - Horns

My car is a 1973 MGB. I have been restoring it for the past four years. I have completely gone through the electrical system of the entire car, testing, fixing and cleaning every connection, plus adding additional ground connections through out the system.
The problem I am now encountering is with my horns. When the car is parked, turned off, "sometimes" only one of the horns will blow/work (the front right, sitting in drivers seat). The front left does not make any sound. However, if you start the car, they both will blow/work perfectly. When I stop the car, they both might work or just the front right one - there is no pattern.
I have re-checked/re-cleaned/re-verified the connections at the steering wheel and they are fine - tight, solid, and no loose connections. I have re-checked and re-cleaned the connects at the horns. In addition, both horns have their own individual direct ground connections.
If the horn is going bad, why would it work when the car is running? Also, the battery is fairly new and strong and everything else works fine without starting the car (wipers, radio, lights, etc.), so I do not think it is a power loss problem. The alternator is also less then two months old.
Suggestions??? Comments????
Thank you.
Robert Browning

I can't even begin to guess why it works when the car is running vs. not but there is an 1/4 inch hex head screw on each horn that adjust the points inside the horn. Extreme Caution--the hex heads are reverse threaded so it's clockwise to loosen. (they are very easy to break off if you go too far counterclockwise--trust me on this one!) You might want to employ an assistant to lean on the horn while you make slight adjustments to the screws. Worked for me on my '64. Don't know if you have the same horns. Good luck.

Paul
Paul Hanley

Robert, You may have a low voltage at the horn or there may be some resistance in the ground circuit. Look for defective/dirty bullet connectors in the 12 volt purple and the purple/black horn button ground circuits. Clifton
Clifton Gordon

As for Paul's suggestion, I hate to start adjusting the horn when it does work. However, thanks letting me know about the left hand threads.
As for Clifton's suggestion, the connections have been cleaned and checked. As for the ground being a problem, when fixing the electrical system, I installed a new separate ground wire direct to each horn, to be sure there was a solid connection. With a volt meter, it is registering 12 volts at the horn - and indicates a solid ground when checking the ohms.
However, I am still wondering as Clifton suggested, could there somehow be a low voltage problem at the horn?
Other suggestions??
Thanks
Robert Browning

Robert, The horn ground I'm speaking of is supplied to the horn through the horn button, slipring and associated wiring to the horn through the purple/black wire to turn the horn on. You could have some resistance in any part of the circuit. Of course that may not be the problem. When I got my 74 one horn would growl and sounded sick but it would always act the same, turning the screw mentioned mentioned by Paul solved the problem. I'm assuming that with your problem being intermittent the adjustment may not help, but it's worth trying, just remember how much and which direction you turn it. It just takes a small fraction of a turn to make a difference. Clifton
Clifton Gordon

Robert,

Before you do any adjustment, use a little PB Blaster or your favorite loose juice on the adjustment screw. It could be rusted together. Break the screw while it's out of adjustment and the horn is all but fubared. (Yes, I know you can drill out the rivets, but that's another discussion)It seems that due to the age of my horns or some other unknown reason, my horns go for months working just fine and then, out of the blue, they'll stop and I'll have to readjust them. Many, many, good, original horns have been tossed out as no good when all they needed were their points adjusted! No big deal--when you're confident that the screw is moving freely, and your horn has stop working, just have a friend push the horn while you turn the screw a little in either direction. Make sure you know where you started from. If it doesn't work one way, go back to start and try it the other way. My left horn has a sweet spot of about a half a flat where it likes to work. The right horn has a wider range of maybe 2/3 flats.

As a side note, my horns are not working now. The slip ring contact on the turn stalk has broken off and I need to solder it back together. The last toots they gave were at the tech inspection at the Concours event at MG2004. Whew!!

Cheers--Paul
Paul Hanley

This weekend I will pull the steering column covers off behind the steering wheel and recheck the connections again. At the ring contact behind the steering wheel, is there just the one purple wire, which when you press on the horn button completes the circuit? Is there any ground wire at this point or is it just picking up the ground through the steering column? What still confuses me, is why the one horn works perfectly all the time. I will try as Paul suggested, adjusting the horn. But I hate to adjust it since it does work most of the time? I just stepped out to my garage and pressed on the horn button and it worked fine. Go figure.
Robert Browning

Robert. From what you have said, I tend to agree it is an adjustment issue. When the engine is running, the system voltage is between 13.5 and 14.5 volts. When the engine is not running, system voltage is about 12.5 volts. So, looking at this another way, we can define your problem as: "the horn operates fine at 13.5+ volts but does not operate properly at 12.5 volts". This would be an adjustment issue. Les
Les Bengtson

Les, when you say an adjustment issue, are you referring to the adjustment on the actual horn? Also, not to be stubborn, but why does the horn work some of the time - like right now and yesterday, but not earlier today or last Friday?
Robert Browning

Maybe the temperture in the engine compartment affects the resistance in the circuit, making it more or less sensitive to voltage? Expansion and contraction could maybe also change tightness of grounds?
Joe Ullman

Robert. Yes, I mean on the horn. From the information you have provided, the only guidance I can offer is that the situation seems to be voltage dependent. As I remember (did not click on the "view the entire thread" this time) your problem is that one horn works well with the engine running, but does not work well with the engine shut off. The only real difference, in this case, between engine on and engine off operation is system voltage. Thus, an adjustment problem is the most likely source of such a problem. I suspect that your adjustment, there is a signficant range in which the horn will actually sound and a "sweet spot" where it sounds the loudest, is marginal. This allows it to work with the higher system voltage when the engine is running and makes the operation marginal when the system voltage is battery voltage. But, I am not an expert on horns and their construction and operation. I have, however, had similar problems over the years which were corrected by adjusting the horns. Thus, since this is the easiest thing to begin with, not requiring tracing wires and removing steering wheels and cowls, it is where I would start if my problem was similar to yours. Les
Les Bengtson

Well put Les esp. your last sentance. Robert, I dug out the horn that I broke and it appears that the 1/4 inch nut on top of Lucas brand horns appears to be made of aluminium and thus very fragile. (The break point showed no signs of rust after a year and is still quite shinney.) I would not use an open end or rachet wrench on the nut but rather a nut driver to insure that you don't over torque to the point of breakage. If you have the aftermarket, French made horns, or anything besides original Lucas horns, none of the above applies and therefore you would need a replacement horn--it would have gone "bad" since the points won't be adjustable. The points get pitted after a while and therefore sensitive to different voltages. On the genuine, they're repairable----adjustable.

Paul
64B
Paul Hanley

OK, Thank you everyone. I will try to adjust the horn and let you know what happens. This sounds like the most logical next step.
Cheers
Robert Browning

I know you have done some voltage measurements but I would do more before adjusting them as recommended early on in this thread. Measuring ohms is no good as bad connections can vary according to how much current is passing through them, you must always measure voltage when the circuit is actually under power and not working as expected. With a 73 you should have a permanent 12v supply at each horn (from the purple fused supply) and a ground from the horn push on the purple/black. With the engine off measure the voltage between each 'purple' horn spade (not the wiring connector) and a good ground, with the horn push released and then operated. In an ideal world you would see the same voltage each time, but in practice the battery voltage will drop slightly under horn current and you will get a little bit of voltage drop in the wiring and the many connections, including the fusebox, but it should only be a couple of tenths. Similarly test between the horn purple/black spades and a good ground. This time you should see battery voltage with the horn-push released and 0v with it operated, but again it will be a little above 0v in practice. Collapsible columns seem to be prone to high-resistance grounds, mine would only operate a single moped horn which I refused to tolerate, so I boosted the ground using a relay which operates the correct twin horns very well. Eventually I decided to find exactly where the column resistance was coming from with a view to eradicating it at source, but found that I had more than twice as much in the motolita wheel horn push. As this is fastened together using spring clips on plastic pegs discretion was the better part of valour and I opted to leave things as they were. Having said that if only *one* horn, and always the same one, doesn't work with the engine off it could well be a faulty horn which *may* respond to adjustment. But also bear in mind that the left-hand horn has more connections in circuit than the right-hand and so has more likelyhood of bad connections.
Paul Hunt

Thank you Paul. This is more along my thinking but I was not sure how to proceed or what I was suppose to be looking for.

I will let you know what happens.

Thank you.
Robert Browning

This thread was discussed between 07/07/2004 and 08/07/2004

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