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MG MGB Technical - Electrical issues with horn and headlights


I've recently dragged my B GT out of the garage and tried the horn. Initially I got a click, indicating that there was power going to the horn and at one stage, after turning the adjustment nut, got a beep... then nothing! I am assuming that the horns are defective, anyone think anything different?

Furthermore, because I am exporting the car to Europe, I have switched to LHD headlights and discovered that the right hand headlamp is considerably dimmer than the left. They are the sealed beam units with new Lucas bulbs. I thought that I may have initially touched the bulb which I gather is the worst thing you can do to those bulbs, but on swapping back to the original headlamps, the problem persists. Is this most likely to be a connection problem? I wonder whether this is due to a corroded wire causing higher resistance and therefore not enough volts are getting to the bulb, thoughts?

If anyone can shed any light on the matter ( pun intended!) then I would be excessively grateful.


ESJ Young

Ed. I'd remove the horn and try it across a 12v battery. AFAIR power goes(probably +'ve ?) to the horn and it is earthed through the horn push. A meter or test bulb will test for both these with the horn removed.

Headlights-I'm confused you mention sealed beam (large, round sealed glass with reflector built in and terminals on the back) but then touching a bulb- so presumably halogen type? Assuming it is halogen, then touching the bulb shortens life but unlikely to make it dimmer. More likely whilst changing it you have either "discovered" a corroded feed or corroded earth. The connectors behind the grill are notorious for corroding. Try a length of wire from a good 12v source and see if its brighter- if so "bad" power side. Then a wire from the bulb to a good earth, if brighter then its an earthing problem. Remember 60w bulb = 5 amps (with an initial surge of more) so use decent cable to test. When you have located the fault and use the original wiring switch etc, don't expect the bulb to be QUITE so bright, as you have old wire, old switch etc, but at least they should be the same as each other! If it is a sealed beam unit of the "old" type, exactly the same tests apply but you have to hang on to a larger bit of kit! Halogen bulbs get bloody hot bloody quick......
Michael Beswick


I'll try the horn tomorrow, I took the precaution of ordering a new one, at 8.50 it seems daft not to!

As for the lights, it turns out that the connections were corroded very badly. A quick strip and reconnect and the lights are as bright as one could hope for. Now to get them re-adjusted so that they point the right way.

Thank you

ESJ Young

What year? It makes a difference.

The *last* thing you should have done was adjust the horn, the problem was more likely to be a bad connection, which can be in one or more of many different places. You need to use a voltmeter in various places along the circuit looking for incorrect voltages. But if you have a full 12v on the horn *spades*, then it probably *is* the horn itself. Do you only have one horn? If you have two and both do the same thing it is probably connectiond further back. Like the headlight bullets the horn spades are in a very unfriendly environment, electrically, and you may well find that simply unplugging and replugging the spades will have got it working again, except for having twiddled the adjuster.
PaulH Solihull

This thread was discussed between 24/08/2010 and 26/08/2010

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