Everyones favourite subject! Trailer lights

Need some help with this one

Last week I discovered the running lights werent working on my trailer. Meaning at night no sidelights or illumination of the rear lights. Brakes and turn signals worked fine. Didnt matter if the lights were set to "auto" or I had manually turned lights on.

I had a brand new set of LED lights at home so I decided this was the time to switch them out. Installed full new wiring harness with the lights. Everything. Drilled new holes for the grounds. Everything!

.....and the results are the same. Still no running lights. I checked the voltage coming out of the brown wire (ground) on the vehicle side and it is fine. I have one of those little trailer testers that plugs it and it lights up that bulb too.

Im stumped. Any ideas?

Try more than one truck?

Sent from my SM-G960W using Tapatalk

Quote:

---

Originally Posted by SongDog

Try more than one truck?

Sent from my SM-G960W using Tapatalk

---

Not an easy option. Im going to try a different trailer tomorrow.

If you have a multimeter you can check the ground on the vehicle side. I've had trouble with this before and the trailer lights would work after a few bounces down the road it would ground through the hitch but the vehicle ground through the connector was corroded where bolted to the frame. That 30 amp fuse is likely for a brake controller if you had it as an option. Or a 12 volt supply for 7 pin connector.

I had weird things happening with my trailer lights too, to a point of replacing the entire wiring harness needlessly ! Turned out to be a bad ground on the trailer frame, caused by corrosion. Now I have 2 sets of wiring on the trailer!
Put a good clean grind to the trailer frame, re-attach the ground wire and try again. Good luck ..

To eliminate the ground.... Use boaster cables to ground the truck directly to the trailer.... So boaster cable on a solid frame part of the truck to a solid frame part of the trailer.... That will make sure the ground is there....
Then it is easy..... If power is coming out of the clearance/parking connection (prong) which is the brown coloured wire, it must work....
Trailer wiring is easy if you think about current being water..... If it is coming out of the truck prong, it must go along that wire to the light and if the trailer is truly grounded it must work.....
Right is green
Yellow is left
Brown is clearance/parking
Black is ground...
I wish I was closer, I'd come out and get it done for you...

Quote:

---

Originally Posted by fratri

To eliminate the ground.... Use boaster cables to ground the truck directly to the trailer.... So boaster cable on a solid frame part of the truck to a solid frame part of the trailer.... That will make sure the ground is there....
Then it is easy..... If power is coming out of the clearance/parking connection (prong) which is the brown coloured wire, it must work....
Trailer wiring is easy if you think about current being water..... If it is coming out of the truck prong, it must go along that wire to the light and if the trailer is truly grounded it must work.....
Right is green
Yellow is left
Brown is clearance/parking
Black is ground...
I wish I was closer, I'd come out and get it done for you...

---

That's a slick trick, I'll store that one in my brain for later:thumbup:

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)

Update - not sure exactly what happened the first go around but I double checked the vehicle end of the connection and I'm only getting 6-7v on the running light connection. Soaked in vinegar and a polish with a brass brush in a drill didnt do anything so I guess I'm replacing that plug tomorrow.

Hopefully that solves the issue!

Quote:

---

Originally Posted by blasted_saber

Update - not sure exactly what happened the first go around but I double checked the vehicle end of the connection and I'm only getting 6-7v on the running light connection. Soaked in vinegar and a polish with a brass brush in a drill didnt do anything so I guess I'm replacing that plug tomorrow.

Hopefully that solves the issue!

---

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 ..

Quote:

---

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 ..

---

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

Thanks all for the advice.

Quote:

---

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)

---

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

Quote:

---

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.

---

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..

Quote:

---

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..

---

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

Quote:

---

Originally Posted by rick_iles

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

---

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.

Quote:

---

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.

---

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.

Quote:

---

Originally Posted by 410001661

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

---

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

Quote:

---

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)

---

same here
i checked the fuse panel on my truck and there was a blown fuse.

Update - It was a wiring harness in the vehicle.

Nothing is simple or cheap in new vehicles.

Put a good clean grind to the trailer frame, re-attach the ground wire and try again. If you have a multimeter you can check the ground on the vehicle side BRANCH SWIVEL-NICKEL PLATED - BR-SWIVELNP-XX. I've had trouble with this before and the trailer lights would work after a few bounces down the road it would ground through the hitch but the vehicle ground through the connector was corroded where bolted to the frame