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New Trailer Lights Not Working

Old 09-30-2020, 06:15 PM
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Default New Trailer Lights Not Working

I just bought a pair of nice $70 Submersible LED trailer lights from Northern Tool. My previous set wouldn't work either but I had used the original wires and plug from an even older set and thought that might have been why. This time I hooked everything up and used all the new wires and plug and double checked all of my work which looks correct. The thing is that only the right tail light will come on but the signal or brake will not work on that though. My ground is very tight and secure. I was worried that something might be wrong with my harness on my vehicle so I went to Northern Tool again and bought some trailer tow magnetic lights and everything works great with those. Now, this just doesn't make sense. Something is odd because this is two sets of lights that haven't worked now when hooking them up. The fact that the one light does come on makes me think that the signal and brake should at least work with that even if something was faulty with the left tail light. Any ideas of what this could be is greatly appreciated. Thanks
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Old 09-30-2020, 06:30 PM
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First thing is to check if the new lights work themselves. I believe you can hook them up to an external battery/power source.

Did you check ground on BOTH the vehicle and the trailer?
Old 09-30-2020, 06:33 PM
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The symptoms you're experiencing point to faulty ground, though.
Old 09-30-2020, 06:34 PM
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Originally Posted by Shewillbemine View Post
First thing is to check if the new lights work themselves. I believe you can hook them up to an external battery/power source.

Did you check ground on BOTH the vehicle and the trailer?
The ground is definitely solid. The removable trailer tow lights are working so obviously the harness on the vehicle has to be fine.
Old 09-30-2020, 06:48 PM
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Originally Posted by ERock82 View Post
The ground is definitely solid. The removable trailer tow lights are working so obviously the harness on the vehicle has to be fine.
One way to double check would be to unplug the ground from the light and run a wire directly to the ground of the towing vehicle connector, bypassing the trailer's. That way you could be certain that the trailer ground is A-OK.
Old 09-30-2020, 06:49 PM
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The ground must come from the tow vehcle, through the plug or dedicated, a test light is a simple way to diagnose your problem....start at the tow vehicle side plug, using the ground in the plug, check running, turn signal etc....
Old 09-30-2020, 07:07 PM
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Make sure the ground is good at the hitch, and the ground is good at all the lights. If it is not the ground. Run new wiring to the lights.
Old 09-30-2020, 07:11 PM
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Unlike incandescent bulbs led bulbs require correct polarity, use a VOM to confirm you have correct polarity.
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Old 09-30-2020, 07:24 PM
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How are the wires in the trailer harness? Most will develop black wire disease after 2-3 years. Once that happens reliability is hit or miss. I've learned my lessons. When I go to repair trailer lights and encounter black wires I bite the bullet and buy a new harness. At that point the frustration is just not worth it.
Old 09-30-2020, 07:35 PM
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Originally Posted by slickster View Post
The ground must come from the tow vehcle, through the plug or dedicated, a test light is a simple way to diagnose your problem....start at the tow vehicle side plug, using the ground in the plug, check running, turn signal etc....
What does a test light consist of?
Old 09-30-2020, 07:37 PM
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Originally Posted by Cable Mariner View Post
How are the wires in the trailer harness? Most will develop black wire disease after 2-3 years. Once that happens reliability is hit or miss. I've learned my lessons. When I go to repair trailer lights and encounter black wires I bite the bullet and buy a new harness. At that point the frustration is just not worth it.
These are brand new wires. Everything is brand new.
Old 09-30-2020, 07:43 PM
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Originally Posted by jayyy View Post
Unlike incandescent bulbs led bulbs require correct polarity, use a VOM to confirm you have correct polarity.
Do you mean a voltometer?
Old 09-30-2020, 07:46 PM
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Originally Posted by jayyy View Post
Unlike incandescent bulbs led bulbs require correct polarity, use a VOM to confirm you have correct polarity.
That is very interesting....Never heard of that before. How would I determine the correct polarity?
Old 09-30-2020, 07:51 PM
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Originally Posted by uchu View Post
One way to double check would be to unplug the ground from the light and run a wire directly to the ground of the towing vehicle connector, bypassing the trailer's. That way you could be certain that the trailer ground is A-OK.
Ok, so I will run a wire from the short ground wire that comes out of the tail light to the hole in the plug from the vehicle?
Old 09-30-2020, 08:03 PM
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Originally Posted by ERock82 View Post
Ok, so I will run a wire from the short ground wire that comes out of the tail light to the hole in the plug from the vehicle?
Correct. That sums it up.
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Old 09-30-2020, 08:13 PM
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Fuses?
Old 09-30-2020, 08:30 PM
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Try plugging your trailer into another known good tow vehicle. A poor connection at the trailer plug can show full voltage (with a meter) at the lights but fail when a load is on it.
Old 09-30-2020, 09:37 PM
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I'm interested in knowing more about the polarity thing with LED lights. The last 2 sets that didn't work were LED and the tow lights are incandescent which makes me think that could be the issue.
Old 09-30-2020, 10:05 PM
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Check fuses in tow vehicle. Depending on your truck there will be fuses for the trailer lights. In our GMC there is one fuse for right turn/ stop and another for left turn/ stop lights
Old 09-30-2020, 10:19 PM
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Originally Posted by ERock82 View Post
What does a test light consist of?
It looks like an icepick with a wire coming out of it, clip the wire to a ground and touch the tip to a connector or pierce a wire to detect power....a 12v lightbulb in the testlight will light up....sold at any autoparts store....can also clip wire to power source and probe frame,etc for ground, works either way....basic diagnostic tool that's easy to use....all you need to figure out your lights....

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