Trailer electrical troubleshooting

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Old 07-21-19, 08:50 AM
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Trailer electrical troubleshooting

Due to the way and places I bring my trailer (open bed) (off road areas) I'm constantly breaking my tail lights. Since I want to be in compliance with the law while on the road, tracing breaks in the line is getting to be a pain. The big problem is having to attach the car or towing vehicle to the trailer in an effort to test and troubleshoot problems. Is there a way, using an outside 12 v power supply that I can use to test tail lights, brake lights, hazard lights and turn signal lights without attaching the towing vehicle? All the trailer lights are LEDs and integral with each type of light. In other words each LED cluster will be, turn signal, tail light, hazard light and stop light all in one.
 
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Old 07-21-19, 09:11 AM
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probably could use some jumper wires hook up your ground and then apply power to each terminal. they do make testers with a power supply but they are expensive probably not worth the cost you could probably make your own also.
 
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Old 07-21-19, 09:28 AM
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Yea, I'm sure shops have a special apparatus just for this purpose and I agree probably expensive. Also I'm assuming to test the light all I really need is a 12 v source just to see if the light lite up. The function as to whether it's turn, hazard, or driving light is determined by the electronics in the tow vehicle.

Another quick question...In this case my middle light bar in the back got knocked off. All wires are in tact but the ground wire is no longer attached to the frame. They just used studs with a force fit type washer to hold the bar to the frame. I'm going use a screw and nut with lock washer. But since it's close to the ground, I 'd like to use stainless steel screws against plain steel frame. I'm sure that's OK for a grounding material?
 
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Old 07-21-19, 09:51 AM
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A trailer with a 4 pin connector really only has 3 functions: Running lights (brown), which the taillights are in lower output mode and the side marker lights if present, and left (yellow) and right (green) full bright on. The tow vehicle only does the controlling of the tail lights whether to be constant on when breaking, or flashing when signaling, but the light is the same brightness. Of course, the running lights are also from the tow vehicle but they are just a single on-off switch.

That said, yes, you can do all your testing with a 12-volt battery or 12-volt DC power source like a battery charger. If you connect the negative to the white pin, the positive will then be connected to any of the other pins (green, brown, yellow) to test. You can even get a vehicle side plug with wire leads for easier connecting to the trailer wires. I would also recommend installing an inline fuse in case your wiring has a short to ground to protect the wires and battery.

Yes, stainless hardware is just fine for grounding. In fact, it will likely be better than the existing.
 
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Last edited by Tolyn Ironhand; 07-21-19 at 02:03 PM.
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Old 07-21-19, 10:52 AM
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Thanks Tolyn. Also about the color configuration.
That said I did a quick Google on wiring harness diagram. What they tell me is:


vehicle side plug with wire leads for easier connecting to the trailer wires. I would also recommend installing an inline fuse in case your wiring has a short to ground to protect the wires and battery.
Isn't a side plug just the mating side to this? And the fuse should be placed at the white or negative terminal?
 
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Old 07-21-19, 11:08 AM
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And the fuse should be placed at the white or negative terminal?
No, the fuse should be on the supply or positive wire. That wire color code pictured is pretty much the universal standard.
 
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Old 07-21-19, 11:12 AM
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Trailers can be finicky because the grounds often aren't the best and you're working through the filaments as well as any sloppiness between the bulbs and sockets, but I have had some success with an ohmmeter or automotive type continuity checker, the kind with a battery and bulb in the handle, a spike, and wire with an alligator clip.
 
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Old 07-21-19, 11:58 AM
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No, the fuse should be on the supply or positive wire. That wire color code pictured is pretty much the universal standard.
Yes of course and it's all coming back to me. What's confusing is the fact that on an AC circuit I consider the black or negative as the hot and supply side and white the neutral or return side. But on a DC circuit it's from negative to positive. Am I wrong?

AKA, yes, I use to have one of those back when I worked on cars. Honestly I have forgotten a lot of what I use to know and with newer codes and electronics I'm a reluctant to try things.
 
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Old 07-21-19, 02:06 PM
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Sorry for the confusion. I posted the wrong color codes. I have edited my earlier post and Norm's post is correct.

Really the fuse could be installed on either side, positive or negative, but it is normally put on the positive wire. As long as there are only two wires coming off the power source current the flow through either one.
 
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Old 07-21-19, 04:05 PM
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Suggestion as to what size fuse to use?
 
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Old 07-21-19, 04:53 PM
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I have been running trailers like yours for 40 years, and I just want to add that most problems I have had over the years had to do with bad grounds, (unless I smashed one or more exposed tailler light assemblies) On the one I have now, which I built up from an old boat trailer, I made sure that I put the ground connection wire to the trailer in a very accessable place, so I can inspect it first every time.
 
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Old 07-21-19, 05:27 PM
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WML, I've had both types of problems. Smashed light assemblies and bad grounds. This latest problem where the middle light assembly was torn off and the ground was taken out. The assembly is still good. Just reattached and made sure there is a good ground. I'm not overly happy with the Carry-On brand trailer in terms of their wiring. Seems as though they could've done a better job. The trailer itself is good with tubular rails instead of angle iron.
 
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Old 07-21-19, 05:38 PM
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Well Norm, I can't speak for the Chinese light kit I bought from
Autozone to wire up this homemade trailer as far as quality, but unless I do a burn thru of the wires when repairing the frame etc it has held up pretty well. I should get some heavier wire and just re do it all, but hey it works, so I leave it alone lol
 
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Old 07-21-19, 06:27 PM
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Suggestion as to what size fuse to use?
I would bet a 5 or 10 amp fuse will cover you depending on the size of the wires.

I have never been really impressed with any trailer wiring. Something always goes bad for one reason or another. One reason is they rely on the trailer frame for the ground path and the connections get corroded and damaged all the time. I am always fixing them along with tires and batteries. OF course, I also have five trailers so that might be one cause.
 
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Old 07-21-19, 07:31 PM
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When I had my mobile electronic store I would get many trailers to wire and repair. I didn't use the trailer at all for ground. I ran a dedicated ground wire and never had any problems. All connections were soldered and heat shrunk with glue type tubing.
 
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