MG Car Repair: horn switch repair, fuse panel, purple wire


Question
78 MGB. Sometimes the horns work fine. Other times only 1 & lom note only, or no sound at all. Horns themselves are OK. Can the push switch, which also incorporates the turn signals, be dismantled & repaired??

Answer
Hi Tony,
The horns (both) are powered by a purple wire (hot all the time) and grounded (both) by the horn push via a purple with a black tracer wire. the horn push is grounded by a black wire.
One horn can fail to sound if you have a poor connection anywhere in the complete circuit because one horn can draw more current than the other. Since at times neither horn sounds it is most likely a poor connection somewhere in the circuit.
Since it only fails at times it will be difficult to locate but it can be done by monitoring the circuit at different points. I can tell you how to do this or you can do like many mechanics do in many shops and that is start throwing money at it by replacing parts.
If you want to find out what the problem is before buying anything, do this:
Solder a long wire to a small 12v bulb on it's end and one on it's case and tape it up so as not to have it short on anything. Connect one lead to the purple wire at the horn and the other lead to a good ground. Put the light bulb on the dash or anywhere you can watch it each time you push the horn. Note that the light dims slightly each time you blow the horn. Note what the light does when the horn fails. If it stays bright when the horns fail, then the problem is not the power supply. If the light goes very dim or out when the horns fail then it is one of two things, you have a bad connection between the horns and the fuse in the fuse panel or you have failing horns.
If the light stayed bright move the wire from the purple wire over to the purple with black wire up close to the horn push. The light will again burn until you push the horn button and then it should go completely out. If it don't go out when the horn fails then the horn switch itself has failed or the black wire from the button to ground has failed.
As for the switch repair, I have found the horn push wires broken and was able to repair them sometimes.
I hope this helps, let me know.
Howard