I was wiring up an LED light strip inside my trailer. When I tested it the light strip as well as two existing light fixtures were really dim. I checked the voltage and it was only 7.4v. The lights on the other side of the trailer were working fine.

I checked the fuse box and none of the fuse failure diodes were red. I then checked a few other things out, disconnected the new light strip and scratched my head for a few minutes. Back to the fuse box and a 15 amp fuse was now showing as failed. But the LED light strip and two other fixtures were still on and dim!

I replaced the fuse and the dim lights went back to normal brightness.

What's going on?

 

Would guess the original fuse was going bad then failed. I am assuming once the fuse failed the lights would no longer light.

Steve

 

No, that's the strange thing. The lights were still on - though dim - when I pulled the failed fuse.

 

Feeding back then off another circuit. Pulling fuses one by one should enable you to determine where it was getting power. Had to be somewhere. Strange!

 

Crossed circuits? Must be in parallel, not series, otherwise I would end up with more than 12 volts, right?

 

My thought was there are two circuits running side by side where somewhere along the line they make a weak connection. Since all your stuff works and it is not causing you a problem, likely not worth worrying about.

 

Look close in your fuse panel to make sure you don't have one strand of a wire at the connections touching another circuit.

Denny

 

If you pull the one fuse, the lights on that circuit remain dim as opposed to 'Off'?

If so, am with Denny, try to find any obvious places where the current is jumping circuits. Stray current gives me the w i l l i e s (even at the 12v level) as it can lead to a bunch of not so fun things. Based on the description, if there is nothing obvious (like a strand of wire or poor solder joint on the board housing the fuses), I'd suspect the board itself and consider swapping it out.

Another possibility is that somewhere along the line a light fixture was somehow connected to two circuits. But if the issue suddenly surfaced, I'd continue to focus on the fuse panel area including any wire looms and any crimp connectors near the panel.

Moisture can play the same type of trickery, so based on the other thread about the marker light fixtures, might want to try and rule out water as a source.


Unless I am missing something, No, in DC, the line voltage of a properly configured circuit will remain at the line voltage (unless another device steps-up the voltage) through the system even when introducing an oddity like combining / crossed circuits. Aside, Yes, when putting batteries in series, the battery potential will increase by the sum of the batteries as long as the line voltage supports the total (combined) potential. In the RV's case, I'd only expect a max of 12v at any one point (allowing for the effect of distance / resistance) even if there are two positive lines going into one fixture / circuit in either a serial or parallel manner. Naturally, am open to correction

The 7v reading is an indicator of a short / leak and may explain why that one fuse slow-failed.

 

I agree with you regarding the voltage. It stays constant. The thought about a failing board is an interesting one. The fact that the voltage when the fuse had failed was only 7.6 VDC suggests a pretty small circuit.

Here's a question. If HRTKD pulls the fuse now. Does the condition reappear?

Steve

 

I'll have to try pulling just that one fuse. This problem only cropped up due to me adding an LED light strip, daisy chaining it off of an existing light fixture. I didn't run a new line from the fuse panel for the new LED light strip. I probably crossed the wires I was working with at some point while the light fixture was turned on. Poor technique on my part.

The trailer is at the storage lot so it could be a while before I have access. So don't hold your breath waiting for an answer.

 

Got it. Thanks, just a curiosity.

Steve

 

Good point. Overlooked the possibility of wiring at / for the new fixture.

Just asking, was it daisy chained in parallel with the existing fixture or added in series? If the LED was set-up properly as a parallel circuit off of the existing fixture, it probably does not need a dedicated run to the panel. At the risk of insulting, by parallel I mean to use both the positive *and* negative wires of the original fixture for a parallel circuit so that the electricity follows the same path all the way through (versus a separate ground point for the LED fixture). Worst case is that if it is too much current, the fuse will likely fast-blow with all the lights 'On' for that circuit.

It is possible that the crimp / joint of the new / old fixtures is not tight enough to allow a good current flow. Or the LED fixture was somehow added in a serial manner. Or that the new LED fixture is bad.

My vote is to first remove the LED fixture, put the wiring back the way it was, and see if the original fixtures (without the LED) return to normal. Second is to check the LED fixture independently (via spare battery, battery charger, car, etc) to see if it is working normally. If both of those isolation techniques are good, then double check how the LED was wired to the original fixture and try again. Oh, and make sure to stop by the store to pick up a few more fuses

 

I used the positive and negative wires available at the existing light fixture. This is the fourth LED strip light I've added. None of the others had any issues. I'm not sure that the new light strip actually caused the problem. It's likely that while wiring it up I touched + and - together causing the fuse to blow half way. But that still doesn't explain how the lights were still on when the fuse wasn't even installed. I'll have to experiment a bit and see which other fuse is involved.

 

Sounds like a polarity issue somewhere. Incandescent bulbs are not very picky, but when you ad an led to that system it can do some weird things if not correct.

I had a similar situation this past weekend when rewiring my boat trailer. Was necessary to add a ground and I ended up having one led light reversed. It caused all sorts of havoc until I realized what I had done.

 

When I wired up the LED light strip I had to verify wire "alignment" as it only works one way. So I know I have it wired the right way.