Trouble shooting mast light circuit

[Harry2k]

The steaming light on my SJ 28 is out. It's original equipment from 1978. I'd like to trouble shoot the system before climbing the mast to determine if the problem is a short in the wiring circuit vs blown bulb or other issue with the fixture itself.
Electrical issues not my forte but I can handle basic wiring, rewiring and I have a basic volt meter. Is it as simple as running a test from the panel switch to check the integrity of the circuit?
The light mounts to the mast just above the spreaders. SJ 28 has a deck stepped mast and I can't access the connection at the base of the mast without pulling it. Aside from the VHF antenna connection, this is the only circuit and both wire runs are inside a dedicated channel and isolated from damage from the running rigging.
Thanks,
Harry

[GordMay]

Turn the light 'on' (however you do that, on your boat).

If it blows a fuse, or trips a circuit breaker (whichever you have); it's a short circuit.

If it doesn't, it's not (a short).

[Vartok]

Harry,


You can use your multi-meter to test if the lamp filament is still intact. Disconnect the wires from the battery and measure the resistance across the wires. For a 25w lamp you should see around 6 Ohms. If the filament is broken it will read open/infinity/x MOhms.


Regards,

[HeinSdL]

Hi Harry, It's either an open circuit or there is a short somewhere. A pity you can't access the connection at the base of the mast without pulling the mast, that's a bit unusual I think.


So then I would go back to the switch panel and find the two leads which lead away to the mast and then onto the light fitting itself. Disconnect the leads there, both positive and negative. Then check that the panel is supplying 12V when the light is switched on by measuring with the volt meter the same connections from which you just disconnected the leads. So you'll know the panel is working OK.


Measure resistance between the two leads, if it's a filament bulb (and not an LED which I would imagine is the case, engine is running anyway so no need for low power solutions here, notwithstanding age of boat) I think you should see a resistance in the range of 20 Ohm. If nothing is measured then it's an open circuit, most likely the bulb but could also be a loose connection somewhere.


If you measure a very low resistance then there may be short somewhere between the two conductors (in which case the fuse in the panel should have blown). Finally you should measure both leads resistance to mass (eg the mast, requires a lengthy probe lead), this should be infinite (if there is a short between positive and mass then the fuse in the panel should also have blown).


Were I to place a bet, it would be on the bulb being blown. If someone can help you the quickest first step might be to just go up and replace the bulb.


EDIT: yes agree with the above, resistance should be closer to 6 Ohm.

[Harry2k]

Thank you Gord, Vartok and Hein,
I wasn't certain how to use mu multi-meter to test the bulb but get that now as well as solid information on testing the circuit itself.
Cheers and Happy 2021 to all!

[Wotname]

Good electrical advice upthread so let me add - if it is original equipment form 1978 then it is likely (very likely IMO) the fixture is full of corrosion and needs replacing.

The corrosion is causing the electrical problem but if you are really really lucky, the fixture is well sealed and the bulb is blown.

[Harry2k]

I assume a cooroded fixture would give the same reading as a blown bulb but would not trigger the circiut breaker, correct? If thats the case seems like its time for an upgrafe on the fixture to a combo syeaming/deck light.

[Wotname]

Originally Posted by Harry2k

I assume a cooroded fixture would give the same reading as a blown bulb but would not trigger the circiut breaker, correct? If thats the case seems like its time for an upgrafe on the fixture to a combo syeaming/deck light.

Yep, that is a reasonable assumption.

[bjp447]

Just a thought.
Start with the simple stuff like a blown bulbs.
I always have to remind myself to do that otherwise
I am sorting thru major components when it is a maintenance
thing. Don't make it more than it is.

[Spot]

Originally Posted by bjp447

Just a thought.
Start with the simple stuff like a blown bulbs.
I always have to remind myself to do that otherwise
I am sorting thru major components when it is a maintenance
thing. Don't make it more than it is.

I think the trouble on this one is the fact that the wiring is buried and the mast is stepped putting the bulb and fixture out of easy reach. Where is that skinny deckhand when they're needed? Send them up the mast with tools, a spare bulb, and a meter.

[Harry2k]

My skinny deck ape is my son. Unfortunately, he is in New Orleans while the boat and I are in Seattle. My wife, in the meantime, frowns on me hauling my 72 year old butt up the stick alone
Fuse hasn't blown so seems bulb failure or fixture corrosion is the culprit. Maybe a good excuse to upgrade to a dual LED steaming/decklight fixture and see if I cant find a volunteer deck ape.

[Quadrille in JB]

While you’re at it I would replace the fixture with a combination LED steaming light and spreader light.
The LED spreader light illuminates the deck like a street light. The only problem is that it can attract a lot of bugs.
Bill in JB

[GordMay]

Originally Posted by Quadrille in JB

While you’re at it I would replace the fixture with a combination LED steaming light and spreader light.
The LED spreader light illuminates the deck like a street light. The only problem is that it can attract a lot of bugs.
Bill in JB

LED lighting is generally less attractive to insects.

An LED lamp with a correlated color temperature (CCT) below 3600K (above 550 nanometers) will emit warm, or ultra warm, white light, that’s not easily seen by bugs. Bulbs with CCTs of 3600K, and above, will emit natural or cool white light, that is more likely to attract bugs. LED lights produce little to no UV light, and a minuscule amount of heat, which makes them less attractive to bugs; so long as they emit longer wavelengths of light.

Yes, you read correctly, a lower correlated color temperature is a warmer light, and vice versa.

[Jammer]

If I were in your situation I would fit a temporary masthead light and investigate the problem the next time the mast is unstepped. The requirement for boats your size is somewhere along the fore-and-aft center line and at least 1 meter above the side lights. A 1 meter pole on the pulpit would do it, use e.g. an antenna bracket and antenna extension pole.

Then when the mast is down you can figure out whether it's the connection at the base of the mast or something in the fixture.


When I fixed mine last the bulb contacts and the plug at the base were BOTH bad.


Whether you want to replace with an LED fixture depends on your plans for the vessel and how much you are out at night. Generally there isn't much benefit to LED replacement for the masthead light because it is only in use when the engine is running and electrical power is abundant. An LED upgrade to the anchor light, on the other hand, will make a significant difference in the power budget on a small boat.


Question becomes where do you stop. If you're doing wiring in the mast you might as well replace any doubtful electrical components on the whole thing including lighting fixtures, connectors, VHF antenna and feedline, and wind instrument. Can get spendy though

[Disailor]

I find that resting my left elbow (I'm right handed) on the cabin top helps a great deal when sighting targets up on the mast.
Oh wait, you meant troubleshooting!
Sorry.