12-volt Lamp has Power, doesn't Illuminate

[jimp]

On my Wilbur 34 I have a 12-volt Perko light in the shower that has power (13.7V by multimeter) but will not light the bulb. The identical 12-volt Perko light in the head (3-feet way) works fine. I took the bulb out of the head light and put it in the shower light (that shows 13.7 volts at the two contacts in the fixture) the bulb that worked in the head doesn't work in the shower.

I swapped out the plastic fixture in the shower light with a new plastic fixture for the bulb, and no luck.

How can that be? Ideas?

Thanks.

JimP

[jimh]

There is a high resistance in the circuit feeding the lamp which does not work. When you measure the voltage with a meter, only a very small current flows, perhaps just a milliampere. When you put the lightbulb in the lamp it wants to draw an Ampere or more current. The high resistance in the branch circuit for the lamp creates a big voltage drop, and the lightbulb only sees a small portion of the 12-Volts.

Find the bad connection in the circuit feeding that lamp.

[jimp]

jimh--thanks.

I have a four-inch wire coming from the overhead. Short of taking down the overhead (destruction) it seems the only thing I can do is redo the connections that I can see. Also, might the wires be connected to the head light over the sink? I'll look at the wiring for that too.

Maybe I'll have to take showers in the dark? Well, at least that's better than the shower on my old Revenge!

jimp

[jimh]

In proper electrical power distribution there should not be ANY connection that you can't see. Every connection should be made in a junction box or other electrical enclosure.

Years ago we had a reading lamp in the salon of our sailboat that did not work. There was a short circuit on the wiring feeding it. After several hours of checking everything visible, I found the cause. The yacht builder had very craftily concealed the wiring inside the wood bulkhead. The wires ran down from the lamp inside the wall. A previous owner decided to hang some sort of nautical decoration below the lamp. He put a screw into the wall to hold the decoration. He placed the screw precisely so it would puncture the insulation of the concealed cable dead-center and short the two conductors.

[jimp]

jimh--thanks again. As I become more familiar with the boat I'm able to track down more "things". You've given me more ideas about what to look for and investigate.

Saturday will be fun. A bit different having to go to the harbor 11 miles away instead of working on the Revenge 50 feet away in the front yard.

jimp

[jimh]

I think that running power from one lamp fixture to the next in a daisy-chain fashion would be a common method. The bad connection you are looking for could be in an upstream lamp connection where the power to the non-working lamp originates.

[jimp]

jimh--Agree.

Thoughts on snipping the two wires coming into the fixture and connecting directly to the two contacts on the bulb? That would eliminate the light fixture.--jimp

[jimh]

Before cutting out the lightbulb socket in the lamp fixture, make sure there will be enough wire left to connect a new one.

You might try to find the fuse or circuit breaker for that branch circuit. You could do this by monitoring the voltage at the lamp with a meter. Find the breaker or fuse that cuts-off the voltage. Then temporarily disconnect the positive circuit from the breaker, so you have isolated that branch circuit. Then measure the resistance from the loose wire at the breaker to the contact at the lamp socket. One of the two contacts will be the positive circuit. Then measure the resistance of the other contact, the negative circuit, to the negative bus. Both the positive and negative feeders to the lamp should have very low resistance, something like 0.1-Ohm. I think you will find that one of them has a much higher resistance. That will be the part of the circuit you need to fix.

[jimp]

jimh -

I've got about 3" to play with coming from the overhead.

The circuit breaker for the lights in the head/shower fall under "Cabin Lights". Not too many lights on that circuit (4 lights(?).

jimp

[jimh]

Since the branch circuit for the lamp has a circuit breaker, and you have identified the breaker, then proceed as follows:

--switch the breaker to OFF; you don't have to disconnect any wiring;

--connect one lead of an Ohmmeter to the load side of the breaker; this should be a RED wire that feeds the lamp circuit you are investigating;

--connect the other lead of the Ohmmeter to the lamp socket contacts, one at a time. One of them should have continuity to the circuit breaker. Measure this resistance.

The resistance should be very low, on the order of 0.1 to 0.2-Ohm. If the resistance is much higher, say more than 6-Ohms, there is something awry with the wiring of that circuit.

You can repeat the resistance measurement of the other lamp socket contact to the negative bus. It should have similarly low resistance.

[jimp]

Thanks jimh, will give it a try. Will let you know.

jimp

[jimp]

jimh -

Quick update. Got nowhere? Went backwards?

Company coming (Tom W. Clark) so I put things back together. But before I did, I took one last look. Just two wires come out of the overhead to the shower light and two wires to the head light over the sink. Likely linked together, but I can't tell.

When I originally started trouble shooting I had 13.7V at the shower light (touching the multi-meter probes to the two contacts. Just before buttoning things up I took one last test with the multi-meter - 5.7V. What? No time to figure that out so I secured the light and scratched my head.

Taking Tom fishing tomorrow. Got our first fish on the boat last Sunday, a 12-14 pound silver (Coho) salmon.

JimP

[jimh]

Maybe your lamp circuit is leaking current into the sea, and that accounts for the ability of the boat to raise fish. (Just kidding.)

That salmon looks tasty. Nothing like freshly caught fish for dinner.

[jimp]

Took Tom out yesterday. Must be those electrical currents you're talking about.

[jimp]

jimh--I chickened out. I just cut the wires and put in a new light fixture, saved from my Dad's old 1977 Post 42. It works like a champ! Thanks for your ideas.

jimp