Everyones favourite subject! Trailer lights

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Originally Posted by fratri

Bypass the plug, I mean if you are thinking of changing it, what will be the harm, this will only confirm........... Right behind the plug on the truck, find the wire that is bringing power to that prong, cut and strip it then run a bare wire right (bypassing the truck plug)to the trailer plug end.... This will confirm your suspicions ..

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Did just that, and its reading low voltage on the wire. Going to have to replace the wiring harness.

Thanks all for the advice.

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Originally Posted by rick_iles

I had a similar issue with my trailer. It turned out to be a blown fuse in the truck.....some of my trailer lights worked, others didn’t ( clearance lights etc.) don’t ask me why, but the dealer checked my truck fuses and one was blown. (2014 Ford F-150)

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Yes most if not all trucks now have multiple separate fuses for trailers so if something shorts out you won't lose truck lights.

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Originally Posted by Loony

Yes most if not all trucks now have multiple separate fuses for trailers so if something shorts out you won't lose truck lights.

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There must be multiple fuses for trailer lights, because some of the lights worked where others didn’t....

Remember back in the day all you had to do was find the wiring harness in the tail light assy, strip back the insulation and tap in that wiring harness from Canadain Tire.... took maybe 30 minutes and cost about $5. Fuses ??? HaHa..

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Originally Posted by MikePal

Remember back in the day all you had to do was find the wiring harness in the tail light assy, strip back the insulation and tap in that wiring harness from Canadain Tire.... took maybe 30 minutes and cost about $5. Fuses ??? HaHa..

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Yup, but “back in the day” was when wiring harnesses might have had a substantial amount of copper to tie into - these days it’s the minimum amount they can get away with, and if your vehicle’s taillights are LED It may not be much

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Originally Posted by rick_iles

There must be multiple fuses for trailer lights, because some of the lights worked where others didn’t....

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Yes, most of the newer trucks have a section the the big fuse block under the hood, I have left, right, and running lights separate from the truck system.

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Originally Posted by Fisherman

Yes, most of the newer trucks have a section the the big fuse block under the hood, I have left, right, and running lights separate from the truck system.

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That explains it... thanks for the info. I’ll have to keep that in mind...

Best tip I ever got for trailer wiring was ground it to the truck. Avoid all this frame grounding nonsense. Run ground straight back to your wiring harness and thus back to the truck itself.

I had the exact same issue in my old boat trailer. My left side running lights were not working - but I had signal and brake lights!!??

Being an electrician this drove me nuts so one afternoon I ripped it apart to find the culprit. Here was my order of operations
(1) check your truck to make sure your plug is working
(2) check the trailer connector
(3) check the condition of the cable from the boat plug to the light termination at the back of the trailer (make sure the wire is not grounded or broken) - check the terminations at the lights as well.
(4) check the ground connections at the lights - if you have a folding tongue on the trailer check the jumper cable at the hinge
(5) check your lights

The issue in my trailer was a bad LED driver in the left light for the running light circuit. LED lights are very robust but not bullet proof - yes they do fail. When wiring your trailer please do NOT use butt crimp connectors - they will last for 1-2 years but will ultimately fail on you - even if you heat shrink them. I prefer to strip the wires long enough to twist together then twist back on one of the leads then solder them and then heat shrink them - just remember to put the heat shrink tube on BEFORE you solder the wires.

Also some abrasion resistant tubing or material when the wires enter or exit the trailer frame goes a long way.

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Originally Posted by 410001661

- just remember to put the heat shrink tube on BEFORE you solder the wires.

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DOH, been there, done that. Cut the wire, slide the heat shrink on right away.