The MGA With An Attitude

At 02:37 PM 1/22/03 -0500, Richard Koseluk wrote:
>>"I would be grateful if you could provide information on the assembly of the horn wiring. Specifically, the sequence to assemble the brown/black, brown/green clips, spacers and upper part of points."

Okay. Check this first: Direct circuits - fused without switch on. You will notice that power is supplied to the horn from the fuse which is always hot (not switched), so the horn will work even when the key is switched off. The circuit is completed by making a ground connection with the horn button in the dash. As such, the entire circuit inside the horn itself has to be isolated from the case so it is not grounded on the chassis of the car.

Inside the horn an electrical solenoid pulls on a metal diaphragm when energized. The circuit is completed through a set of contact points which are normally closed. As the magnet pulls, the diaphragm flexes, allowing a small motion at the center. This motion breaks the electrical contact at the points. The solenoid then stops pulling, and the diaphragm springs back to its original position, at which time the connection at the points is restored, and the cycle starts over. Rapid vibration of the diaphragm displaces air, creating the acoustical output wave that we can hear.

See the workshop manual section N.11 for original factory maintenance notes. If the horn does not sound at all, first check the external wiring for power supply and ground continuity through the horn button. If this does not restore operation, you will need to remove the side cover (original horn) for access to the contact points and adjustment screw. With power applied, try adjusting the set screw (with a lock nut) on the side of the horn. This adjustment is used to limit the travel of the contact points. When the horn is working, you can adjust this screw to get maximum volume from the horn (not to change the pitch). If that doesn't work, try cleaning the contact points.

You should never have any reason to remove the diaphragm from the housing. Once I had a horn that was so rusty that the diaphragm motion was restricted. I drilled out rivets around the perimeter for disassembly, cleaned out a lot of rust, and reassembled it with machine screws. It worked, but not very well. It may have also had a crack in the diaphragm. If you can't get a horn working by cleaning and adjusting the points, it's probably a lost cause.

Now it sounds like you are asking about how the contact set is mounted and wired inside. Here both sides of the contact set have to be electrically isolated (insulated) from the body and case. Everything is wired in series, starting with the constant on power input wire, first side of points, second side of points, then a wire leading to the coil, and the other wire from the coil leading to the output connection, which in turn goes to the horn button for grounding. The key to proper installation of the contact points is that they have to be electrically isolated so they only contact each other, the connecting wires, and the adjusting screw.

See Aftermarket Horns for information on non-standard types.

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