9 volts at bow light

[Berkeley 30]

I discovered, at 9:30 at night, in a dense fog, while trying to view the San Francisco fireworks, which for one more year were shot up into the fog bank, that my bow light was not working. I'll spare you the story of the epic sail home, navigating by two dim blurs of light that I took (fortunately correctly) to be downtown Oakland and downtown Emeryville. Several weeks of procrastination later, I discovered that the light fixture was corroded into oblivion so I replaced it, but then discovered that there was only 9 volts running through the light. (The bulb definitely works.) Sure enough, the wire was corroded too, so I cut back to clean copper and still got 9 volts. A previous owner has spliced a new cable from inside the chain locker, up through the pulpit, to the light. The obvious next step would be to cut the original cable, before the splice, and test the voltage there, but before I create the resulting headache for myself (I will have to cut into the fiberglass where the cable is glassed into the side of the chain locker) I wonder if anyone can imagine why there might be 9 volts at the bow light, even though there is 12 volts at the stern light and at the circuit breaker. Any thoughts would be appreciated.

 

[jonelli]

3 Volts in dropped voltage is about equal to if you had 30 gauge wire all the way to the light. You probably don't have 30 gauge wire, you probably have something bigger, so that means you a huge amount of resistance somewhere. Corrosion overall, corroded splice connectors, etc....

 

[Johnb]

There is a high resistance somewhere in either the positive or negative wires that lead to the light.

Well done that you actually measured the voltage. If you want to continue being scientific you could take further voltage measurements to determine which wire has the problem and where it is. If the wiring is fairly old I might be tempted to just replace the whole run.

At this point I would not cut into fiber glass if avoidable. Run new wires back to the panel, cut off the old and abandon in place those that are imbedded. Try and find a routing that keeps them away from damage. To protect the wires in the chain locker run them in well secured plastic conduit.

 

[Don S/V ILLusion]

Ohm's Law tells us it's a bad connection assuming you have 12+V at the source. - rather than create a new splice, it would be preferable (and probably easier) to pull a new cable through.

 

[Berkeley 30]

Thanks to both of you. I understand the wisdom of rerunning the whole cable. I truly hope I can avoid that. I'll test back along the cable, and if the problem isn't too far back, I may splicing a new section in a dry place. (A butt splice sealed with electrical tape in the chain locker probably wasn't the best idea in the first place.)

 

[mikeracca]

I just found out that Luna has only 1.5v at the bow lights. So I am going to need to find where the splice was located and see if it just needs some corrosion repair or if it’s going to need a while new wire run. My stern light works perfectly.

 

[RoyS]

You may find that the ground wire is shared by every light fixture in the cabin. Somewhere, perhaps everywhere, there is a poor connection causing your voltage drop. Open all the fixtures and check the connections or run a new duplex cable from the breaker to the nav light (that is what I did).

 

[jssailem]

The boat was launched in 1985. Likely the original wiring. Does the old corroded wire owe you anything? If good navigational lights are of importance, perhaps new wire and new LED CG approved NAV lights would be good bling for your boat.