Horn Button Meltdown?

[expavr]

The horn button on the 40 Tonner had a meltdown about 3 weeks ago. I finally got around to changing it out thanks to a Fordbarner who had a button similar to the original. The question is what could have caused this to happen? Usually I kill the ground by disconnecting it at the battery terminal when the truck won't be used for a while. The meltdown happened over about a 4 day period when it was parked and the ground was connected. No blaring horn to indicate that the contact surface was touching the tit. Photo 1 shows the original button next to its replacement. Photo 2 shows the contact surfaces on the back of each button. I had to drill a hole in the replacement button due to the fact that it wouldn't clear the tit on the steering column. The drilled hole doesn't affect the horn's operation. It will blow when the new button is pressed. Photo 3 shows the contact point in the column. The hotwire that extends from the steering column and connects to the battery lead from the wiring harness at the steering box is in good condition - no bare wire touching the box. I discovered that the wire in the harness is always hot even with the ignition switch off. Is it supposed to be that way? Other than that what am I missing? Why the meltdown?
Les Williams

[JSeery]

There shouldn't be any "hot" wire related to the horn in your steering column. It is a ground wire. And yes, the horn wire (from the starter solenoid to the horn relay) is always hot. That is if the trucks use a relay, if not it is hot wire from starter solenoid to horns and ground wire from the horns to the steering column and up to the horn button.

[rotorwrench]

The old Fords didn't have fusing for all of their circuits and the ignition switch was for the coil and gauges to turn on. Lights & horn were independent from the ignition switch. The hot wire for the horn should connect to a relay on most cars. The early cars didn't use a relay but as horns were doubled up, the need for one became evident. Horns draw a lot of current. Something had to cause the heat so there was likely a circuit going there somewhere.

Crazy things can happen sometimes. I was surprised one time by a melted carpet in a helicopter. When investigated, we found that the sun light was getting refracted through the plastic bubble just enough to concentrate some heat energy on the carpet. As the day progressed it would melt it in a line across the floor. That was the damnedest thing I had ever seen. The customer either had to install a cover or pull it back in his hangar to keep it from happening again. This was one of the old bubble types (Bell 47J). They had quite a green house in there.

[Kahuna]

39 Fords and up all used a relay. And,
as J mentioned, that wire thru the steering shaft
is a ground wire.

[drolston]

Yes the horn button provides the ground, whether there is a relay, or not. The wire will be hot in either case. The relay just means a little bit of current will flow through the horn button wire when the horn button is pressed, instead of a lot of current if that wire is carrying all of the horn power.

Horn power is hot all the time, like the lights, so you can use them when the ignition switch is off.

[JSeery]

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Originally Posted by drolston

Yes the horn button provides the ground, whether there is a relay, or not. The wire will be hot in either case. The relay just means a little bit of current will flow through the horn button wire when the horn button is pressed, instead of a lot of current if that wire is carrying all of the horn power.

Horn power is hot all the time, like the lights, so you can use them when the ignition switch is off.

The term "hot" normally is used to refer to the power side of a circuit vs the ground side of a circuit. Current flows through the entire circuit to the finial ground point. It gets a little confusing on the positive ground systems because the modern mindset is positive is hot and negative is ground. Because of this I use the term "hot" to refer to the power side of the circuit.

[expavr]

Thanks for jarring my memory. There is a relay for the horn-its tucked under the horn mounting bracket on the frame. Attached is a photo showing it and the electrical connectors on it with the voltage at each POC. Should the ground wire to the horn button be energized (6.6V) at all times as is the case now? If not does this mean the relay is bad?

[JSeery]

The Yellow-Green wire connects to a coil inside the relay. The other side of the coil connects to the Yellow-Blue wire. When the Yellow-Blue is grounded the coil is energized. You are measuring the voltage at the two sides of the coil, the difference is the voltage drop due to the resistance of the coil.

[rotorwrench]

I don't know about the pre-war vehicles but later Horn relays are marked with single letter characters at each terminal. S for switch, B for battery, and H for horn. This makes it less confusing to hook them up unless the wires have lost their color. Then it can be confusing no matter what you do.

A person may want to try and pull that horn wire up out of the steering shaft and see if it has worn insulation that could cause a short. Even an intermittent short could cause some heat. In any case, a good inspection of that circuit wouldn't hurt anything.

[Kahuna]

I think an intermittent short would only honk the horn intermittantly.

[rotorwrench]

If a person is around when the horn goes off then you can do something to stop it. If your away working or visiting or what not, you may not know the damn thing is sounding off. It would only blow as long as components or battery power would let it.