1.5V

[Jonah]

Hi all,

The dome light in my aft cabin hasn't worked since I bought the boat. Finally pulled it down last night and exposed the wires leading to it. My multimeter is reading 1.5V. I was expecting something closer to 12 or 0, and so I wonder what could lead to 1.5.

Is 1.5V the kind of voltage you might get with a loose connection or corroded connection? Or is it the kind of voltage that might leak through a switch somewhere, and maybe I'm wrong about what switch controls the light? (Other than the one on the light, of course! Unfortunately the wiring seems to be original, and it runs into the carpeted ceiling, making it difficult to trace. For now I'm assuming that it goes to the same switch that powers all the rest of the cabin electronics.)

For clarity: I am touching the multimeter leads to the wires above/before they enter the light.

 

[seasick]

You have a bad connection or switch somewhere. It could be the hot lead or the ground. If you can connect one lead of your meter to a known good ground and then connect the other to the two leads of the light. If all were well, you would see 0 volts on one wire and 12+ on the other.
One other check is to measure as you did at first, verify you have 1.5 volts and then turn the light OFF. See if the voltage you read now increases to about battery voltage. If it does you have too much resistance in something, like as I said, a connection, switch lamp fixture it self. etc.
Ohm also find out if you can, which fuse feeds that circuit. Pull it to check for corrosion on the fuse or its socket.
For tracing problems like this, I prefer to use a 12 volt test lamp like the ones that look like an awl. The reason is that they draw more current and high resistance issues are easier ti isolate. The test lamp either glows brightly or generally not at all.

 

[Jonah]

Nice, thanks very much! Will try this out soon.

 

[Fishtales]

Should be 12v. I'd first check the power block and fuse, you could have had a short that interrupted the power. Separate the dome switch from the circuit. You should have 12V coming to the dome. If not you need to chase the wire back toward the power block. You can test the dome switch with the meter and if you have a access to a battery that way.