12 volts but not enough amps puzzle. Ideas?

[Burnham Bob]

One of my interior lights failed. It was an ebay LED cheapie so no problem. Bought a new one. But before I fitted it checked the power wires with a meter. 12 volts fine but before fitting it tried the light unit. No light. Tried an LED strip off a friend and that just about glowed but no real light.

So I'm getting 12 volts but not enough amps (am I right?) All other lights running fine and battery fully charged. All there is between the switch panel that controls all the lights and the light power wires in question is simple plastic covered copper wire. Any suggestions? A short would surely blow the fuse. The wires are of course impossible to access behind the inerior mouldings.

 

[RichardS]

Sounds like a bad connection somewhere, either in the light or in the wiring leading to it. Could be in the +ve or -ve cable.

Richard

 

[savageseadog]

Most likely corrosion in a connector or switch or crimp etc

 

[elton]

You can only measure supply voltage under load, otherwise the meter reading is meaningless.

If you connect (say) a 5W 12V bulb, and the voltage at the bulb drops significantly below the battery voltage, it means there's something wrong with the wiring or the bulb holder.

 

[macd]

Sounds like a bad connection somewhere, either in the light or in the wiring leading to it. Could be in the +ve or -ve cable.

+1.
Your best bet is to jury rig a pair of wires from known good connections to the light. If the light then works, it's as Richard suggests. If you're lucky, the poor contact could be in the fitting itself.

For future use it's worth while knocking up a bulb tester -- simply a 10W or so bulb introduced into the circuit much as you would your meter. If it lights, you know the circuit can deliver amps; if it doesn't...

 

[Burnham Bob]

thanks but tested the bare wires with a meter and then a bulb. got 12 volts but bulb did not light up. so assuming the wires behind the internal mouldings have not moved, I'm still puzzled.

 

[VicS]

thanks but tested the bare wires with a meter and then a bulb. got 12 volts but bulb did not light up. so assuming the wires behind the internal mouldings have not moved, I'm still puzzled.

You have a bad connection somewhere as Richard and others have suggested.

I bet you used a digital meter to measure the volts.........They take so little current that they will give a normal reading even when there is a bad connection. It often better to use a bulb on a couple of wires for this sort of testing. Bulb lights brightly if all is well, dimly if there is a bad connection not at all if there is a very bad connection ( or no connection)

Try measuring the volts across the bulb .... you'll see a very low figure if we are right.

 

[AngusMcDoon]

thanks but tested the bare wires with a meter and then a bulb. got 12 volts but bulb did not light up. so assuming the wires behind the internal mouldings have not moved, I'm still puzzled.

Imagine having a water pipe with a tap at the end with an almost complete, but not quite, blockage upstream. With the tap closed you measure the pressure at the tap, and it's fine, because a tiny amount of water got past the blockage and provided the pressure downstream. Open the tap, and only a dribble comes out.

That's the situation you have. The blockage is the bad connection somewhere upstream of where you measured the voltage, similar to the water pressure.

 

[skipmac]

Imagine having a water pipe with a tap at the end with an almost complete, but not quite, blockage upstream. With the tap closed you measure the pressure at the tap, and it's fine, because a tiny amount of water got past the blockage and provided the pressure downstream. Open the tap, and only a dribble comes out.

That's the situation you have. The blockage is the bad connection somewhere upstream of where you measured the voltage, similar to the water pressure.

Good explanation of the problem. I always like the water analogies to explain electrical.

In electrical terms, you measure voltage on the bare wires and it looks good but measuring like this that wire is not putting out any power (amps) at all. When you connect the bulb it is then trying to get power through that wire but there is somewhere a bad, loose or corroded connection that won't let the amps go through. Take the bulb away and you can see the voltage again.

To demonstrate this, have a friend help or use some clips to hold the meter on the wire at the same time you connect the bulb. You will see the 12V reading drop to a much lower value, maybe close to zero. Remove the bulb and watch it pop right back.

 

[lw395]

It could be a bulb not intended for 12V battery operation.
LED bulbs vary from simple LEDs in series with a resistor through to complex electronics inside.
A) Does it work a lot better with the engine charging the battery?
B) Does it work if you use test leads direct from bulb to battery?
C) I assume you've tried turning it around?

To test the wiring, try something like a 5W car tail light bulb.

 

[cpedw]

I recommend measuring voltage across the switch (is there a switch?) and any other connections in the cable, with the light fitted and switched on. There should be next to 0V. Any connection or switch with several volts across it is faulty.

 

[savageseadog]

It really isn't a puzzle, the answer is in the first few replies. To find the fault use a 12V bulb (not an LED) one side to a probe, the other to a lead long enough to reach the battery. Clip onto battery minus and the plus with the long lead and probe the wires.

 

[Burnham Bob]

the puzzle is why did I suddenly get a bad connection (I agree that's what it must be if the amps aren't there when there is a power drain) when the wires haven't moved? I know all the causes but I'm at a loss to see how a previously good circuit suddenly failed. I assume ( 'cos I can't get at the wires to see) that a connection must simply have corroded. hope none of the others decide to do that. have replaced several lights because they were corroded and past their sell by date but the wiring has always been ok.

However, these have solved the problem. One where the failed light was and one above the switch panel which has never had a light but was in shadow.

https://www.amazon.co.uk/gp/product/B00IXOSA7A/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

 

[Topcat47]

Poor connectivity can have quite a variety of causes. Auto wire corrodes and eventually crumbles. If you bare the wires, you may find the colour anything from dull brown to black. Marine quality wire is silver (tinned). Similarly broken wires, usually close to a crimp terminal or screw connection like a Choc Bloc will also allow 12v but no power. Fuse holders corrode as do fuses, esp auto blade type.

Insulation cracks with age and eventually the wires within break around the crack.

Finally, not every new lamp is good to go. Check the fitting by connecting it directly to a 12v source.

 

[jbweston]

I had a similar problem some years ago with my old boat. Good voltage at the bow nav lights and good bulbs but they wouldn't light. The problem turned out to be crumbled wires. Untinned copper that had crumbled down until it was pretty much black dust - so thin it couldn't deliver but still gave good voltage when there was no load.

I found it hard to spot because I hadn't come across the problem before and the crumbling wasn't obvious because it was hidden by the wire's plastic insulation rather than staring at me where the wire met the terminal in the fitting.

 

[GrahamHR]

The clue is 12V measured. 12.7- 12.8 V would be normal. Either a discharged battery, or a circuit with a serious continuity issue caused by corroded/ poor connectors etc.

 

[skipmac]

Like others have pointed out, over time things on a boat tend to fail. Could be corrosion, loose connections from vibration, wire going bad.

But now for the good news. 99% of the time the problem will be at one of the ends of the wire or somewhere there is a connection, crimp fitting or something. Start at the ends of the wire and you're almost certain to find the problem.

Of course if you haven't lived a clean and moral life the wire could have been chafed through where it passes through a totally inaccessible hole in a bulkhead that will require to take the deck off the boat and pull the engine to get to the spot.

 

[savageseadog]

Like others have pointed out, over time things on a boat tend to fail. Could be corrosion, loose connections from vibration, wire going b

Of course if you haven't lived a clean and moral life the wire could have been chafed through where it passes through a totally inaccessible hole in a bulkhead that will require to take the deck off the boat and pull the engine to get to the spot.

You can draw through new wring with the old..............usually. Best way is to solder stripped butt ends together to avoid jams.

 

[skipmac]

You can draw through new wring with the old..............usually. Best way is to solder stripped butt ends together to avoid jams.

If you're living right and the gods smile upon you it could happen. When I tried to pull a new spreader light wire in my mast I discovered that everything was all tied together: anchor light, steaming light and even the masthead VHF antenna coax. So I pulled all new wiring including coax.

Same thing happened running new wires several places around the boat. A whole bunch of wires all tied together and screwed to bulkheads and always behind a cabinet, paneling or some other boat bit that was not removable.