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Cross-posted from P30 forum:

I have a 1976 P30 with a non-functioning mast head light. After five trips up the mast, I have determined that

a) The mast head light works fine. - I tested it with my VHF wires inside the cabin
b) With the mast head light removed, I get 14+ volts from the black and white wires.
c) Holding the negative lead from my multimeter to the shroud, I also get 14+V
d) Whenever I put a load at the top of the mast, (i.e. light bulb) the voltage drops to zero.

I suspect the problem is in the mast step somewhere, but can't figure out where the wires enter the mast from. I suspect they go from the port side, up through the headliner, and then under the compression post. I also suspect that the wiring problem is at the base of the mast. Does anyone have any troubleshooting suggestions? This weekend I am planning on using a wiring toner to trace the line and rule out any issue inside the boat.

Frustrated in Deale, MD -

Scott
 

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If you have wire strands broken in the casing, or something of very thin gauge, your situation is possible. A voltmeter has very high resistance, so when you take the voltage at the masthead, it makes sense to get the same voltage you would get at the battery terminals, because the circuit is not loaded. DC circuits suffer from voltage drop; when they are loaded (the masthead light is on) , the voltage decreases, because a battery is not a perfect voltage source, and the wires have resistance. As the circuit heats up, the problem gets worse.

I would think that if the masthead light worked before, and stopped working, that you have some area that does not conduct well under load, which explains why the multimeter is throwing you a curveball! There is not a lot you can do, in a practical fashion, to find these in a continuous run of wire. I would suggest looking in places where the wires bend, entering the mast, and perhaps elsewhere, and carefully inspecting these sections, then replacing as necessary.

Good luck finding your ghost. This is part of why marine grade wire is a good thing!
 
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