no power

I am restoring a 17 year old boat and am doing some rewiring. Everything was going fine until I got to the horn. I measure 12 volts with the switch on between the 2 wires and the horn disconnected but nothing with the horn hooked up. The horn works when I wire it direct to the panel. I thought maybe the ground was bad so I left the hot lead on and grounded it to the panel, again nothing. I've never run into this before, any help would be appreciated

Thanks,
Shawn

 

Corrosion in connections?

It could be that that there somewhere a bad connection, likely due to corrosion.
What sometimes happens is that small currents do not "see" or measure this type of bad connection; ie the current of a multi meter (when measuring voltage), or a LED do not notice this large resistance, as voltage drop is relatively small.
But if one connects something with a large current ie horn, then the large resistance stops the horn from working.
I have seen it often happen with radios: it works OK in standby and receiving mode (with small current draw), but soon as one tries to transmit (with 5 to 10 times the amps drawn), the unit stops working altogether, and lights on the display dim or go out. Stop transmitting, and the unit works OK again (in receiving mode).

You can test this scenario visually by connecting a ordinary 20 or 50 Watt globe across the (horn) terminals and test the circuit. If the light is considerably dimmer compared when connected on the battery itself: large resistance. In addition you could connect a small pilot globe or a LED (with the appropriate series resistor if not built in). If this is working OK, a large resistance in the circuit is nearly certain.

Another way and better of identifying the fault is to disconnect the supply wires at the panels. You can leave them attached, if the battery is disconnected (and the fuse removed). Then remove the horn, connect the horn wires to each other, leave the horn (push)bottom switch in situ as that may be the culprit.
Then use a digital multimeter and connect the probes to the supply wires at the panel/switchboard side. Now press the horn button and measure the resistance.

If a large resistance is noted repeat the measurements with moving the probes to the next connector (towards to horn terminals). This way you will find the offending connector. However it may be that each connection is marginal and that the 4 or 6 marginal connections in the circuit combined silence your horn.

And what is a large resistance? One Ohm could be too much already as a horn may draw 12 amps, which results in a voltage drop of one Volt (V=IxR).
I realise that a one Ohm resistance would not stop this horn working, but a resistance of 6 Ohm or more could.

 

Thank you for the excellent reply. The distance from the horn to the panel is very short. The only connections are the 2 on the push button switch. I left the negative wires as is and then ran the hot wire straight to the fuse panel and the horn beeped. I am pretty sure that the switch is bad, but I will play around with it a little more just to fully inderstand all you are saying before replacing anything. Thanks again for your help.

Shawn