12v showing on circuit tester but no power?

[Ursula123]

I am certainly no electrician but am trying to get my stern light working on a yacht I recently bought. I switched on, no power at contacts, renewed baring fresh wire, re-test 12V, well 12.3 to be precise, excellent, reassembled fitting bulb, looked ok, didn't work. Tried new bulb, didn't work, tried exact same bulb out of bow nav light that def works, didn't work. Plenty of pressure on contacts, springs good, cleaned contacts in fitting and on bulb, recheck with volt meter at contacts - 12.3, switched off, nothing, re-tried bulb, nothing, you get the picture.
Tried touching both wires together, no spark, nothing.
So how can you have 12.3 volts but not enough power to light a bulb or create a spark. This has pushed my knowledge to its unimpressive limits so any input would be appreciated.
May not get back straight away, dreaded wedding to go to (not mine) but wanted to post it before leaving for the day.
Cheers

 

[PaulRainbow]

You have a bad connection somewhere. Try connecting the bulb to the positive at the fitting and a temp wire to negative, then vice versa, that will show whether the bad connection is in the negative or positive part of the circuit.

 

[brman]

Just to be clear, with bulb removed you get 12.3V at the bulb contacts, then with the bulb fitted our measure 0V at the contacts? ie measure 0V, not just that the bulb doesn't light? (this double checks it is your wiring not a faulty new bulb)

Assuming yes, then I agree with PaulRainbow. If you have a long bit of spare wire you can do has he said.
Alternatively, whip off the switch panel and test behind that.......
-Isolate your battery (just in case of shorts)
-Turn off switch to stern lights
-Short the wires at the light end
-Find the panel end of the stern light wire (probably output of a fuse)
-measure *resistance* from the wire positive to wire negative. If it is high then you have a problem with the cable (or a connector in the cable). If not.....
-measure wire negative to battery OV. If it is high then you have a problem with the wiring at the switch panel to battery 0V.
-Assuming the above is ok, work backwards through the fuse, switch to battery isolator, measuring resistance from positive to ground. At some point you will find a high resistance showing where the fault is. Don't forget to close the switch when measuring from its input but also don't forget to keep the battery isolated or (best case) you might blow a fuse......

As Paul says, it looks like a bad connection or switch or something so testing resistance rather than voltage is a much more reliable way of finding it.

That is how I would approach the problem anyway.......

EDIT: btw. You might well have a complete break in the circuit (eg bad switch or fuse hold or fuse, or corroded wire or connection) and still be reading 12V because of leakage (eg damp etc) from other circuits (or across the switch/fuse/break etc). That is the trouble with modern multimeters, they take very little current from the circuit when measuring volts so can give a good reading even when the supply connection is basically non existent. Hence my preference for resistance readings, or the good old fashioned bulb on a couple of wires which gives a decent load. You can get low resistance voltmeters (or the old fashioned analog ones) but they are not common and still not foolproof.

 

[lpdsn]

I had a similar issue with a brand new Aquasignal last year. Turned out that there was a minor manufacturing fault so there was no pressure on the contacts when the bulb was pushed in. I took it apart and compared it to a working one and was able to fix it by bending a wee bit of metal that should've been crimped around a plastic block.

However, voltage drop when bulb is connected as suggested above is a pretty likely cause.