Nav Lights, a baffling electrical problem:

BobnLesley

BobnLesley

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Our stern light stopped glowing, not the bow lights, they're both working fine.

Changing the bulb made no difference, so I tested new and original bulbs, both were fine.

Disconnected the whole light unit, tested it and found continuity everywhere there should be.

Checked voltage on the wires coming in and got 12.1 volts (more cable-loss than I'd like, but not enough to cause a problem)

Connected a couple of new wires to the unit, touched these to the battery terminals and it works fine, it's an incandescent bulb, so would even light-up with only 6 volts through it, therefore 12.1v should be more than enough and polarity not an issue either.

So, the unit/bulb is fine, the feed wires say they're sending 12.1 volts but connect the two and nothing happens: So, any ideas, before I start trying to withdraw/re-wire all the way back to the panel switch?
 
12.1 might be ok with no load, but if there is poor contact somewhere in the system, that figure will drop dramatically once you want to draw an amp or two off it.
 
Agree with Pye End. I've fallen foul of that trap before. Multimeter reads 12V+ but test it with a bulb and you get nothing. You will find a loose/corroded connection or (almost) broken wire somewhere in the circuit.
 
BobnLesley said:
Our stern light stopped glowing, not the bow lights, they're both working fine.

Changing the bulb made no difference, so I tested new and original bulbs, both were fine.

Disconnected the whole light unit, tested it and found continuity everywhere there should be.

Checked voltage on the wires coming in and got 12.1 volts (more cable-loss than I'd like, but not enough to cause a problem)

Connected a couple of new wires to the unit, touched these to the battery terminals and it works fine, it's an incandescent bulb, so would even light-up with only 6 volts through it, therefore 12.1v should be more than enough and polarity not an issue either.

So, the unit/bulb is fine, the feed wires say they're sending 12.1 volts but connect the two and nothing happens: So, any ideas, before I start trying to withdraw/re-wire all the way back to the panel switch?
Click to expand...

Testing with a high impedance meter ( digital? ) guaranteed to make a fool of you if there is a poor connection. Meter draws next to no current so reads normal volts. .... Been there, done it .. got the T shirt

A reading of 12.1 instead of the full battery volts should ring the alarm bells ...... next to no current flow should mean next to no drop in volts.

Use a test lamp instead of a meter or put a load ( the test lamp) across the meter terminals.

As the others have said

Somewhere there is a connection so bad that it barely exists at all ..... could be in the negative or the positive.......
 
With the lamp put together and the bulb in place try giving the negative side of the bulb another earth to a known good earth and see what happens. I had a very similar problem and it turned out to be the negative wire had corroded (google "black wire corrosion").
HTH.
 
Having just replaced all the bulbs on a moody because all the wires had so much corrosion in them (amazing how far it spreads along the wires, I agree with everyone else).

If you cannot re-run a new cable, stick an LED in there for now (25w filament draws 2 amps approx), get an LED that draws 500mA max and see if it works, if not go with a lower rated LED - all temporary fixes of course). If all else fails stick a bike LED light on there to get you home.

You don't need tinned wire if you seal the ends properly when rewiring - use Contralube, the magical gel that costs more per gramme than silver ! But it is VERY good and one tube lasts a lifetime. Works out cheaper than tinned wire.
 
alahol2 said:
Agree with Pye End. I've fallen foul of that trap before. Multimeter reads 12V+ but test it with a bulb and you get nothing. You will find a loose/corroded connection or (almost) broken wire somewhere in the circuit.
Click to expand...
Yup! +1

In my case it was a steaming light that wouldn't work. Rigged up a bright LED headtorch for the night element of the passage, then went up the mast 5 times and replaced the bulb with LED before discovering a badly-corroded deck joint. Mix of embarrassment and relief, but also a valuable lesson!
 
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I had a similar problem last Sunday when, right after berthing, the port light went off.

Took out the festoon bulb - still good.
Tested the clips - zero voltage.
Tested the crimped female sliders on the wires - 12.5 volts.

In the end I took the clips apart, sanded the L-shaped metal pieces and the holes where the bulb is held and then reassembled the lot. Works fine now.
 
I also had this problem last time I was on my boat. Found the sprung bulb contacts on the aqua signat stern light had corroded, the voltage at the wiring joint was OK but no light. Replaced the bulb holder and all was OK.
 
Had similar problem on aquasignal steaming light which in the end proved to be an almost invisible 'coating' (merely a darkened surface) on the contact points of the incandescent bulb that prevented contact when in the holder but which I had first tested satisfactorily with the prongs of a meter before reviewing all the cabling.
 
All those who suggested a weakness/corrosion in the wiring were correct. Whilst I've not actually finished the re-installation (it got around to beer o'clock, so I had to stop) the discovery of four joints in each of the feed/return cable (one crimped-spades, two with chocolate blocks and one two-wire twist) three of which were in damp/condensation-prone locations makes me confident the problem's solved, Why do people extend/joint cables in silly places? Four in less than 20' (three were in the last 6 to 8 feet!) and it was probably more hassle to squeeze into the back of the cockpit locker and the lazarette to make those crappy connections, than it would've been to just thread/draw a new cable
 
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