Vehicle trailer hitch wiring problem

[Scottintexas]

94 Ford f350
I have a 7 round with a 4 and 5 flat spliced in.
Last month every thing worked great using both my boat trailer and utility trailer.

This weekend I did my good deed and pulled the boy scout trailer to camp. It had an issue of the left turn/stop light not working on other peoples vehicles. I decided to try to trouble shoot it.

I swear the first thing I did with the multi meter was confirm I had power on all the terminals at the truck.
I then took the light off on the trailer looking for any obvious issues.
Not seeing any I was going to run a bypass wire straight from the truck side to the light to try to confirm the problem was somewhere in the trailer wiring.
I looped the end of the wire around the left turn terminal on the truck side of the 7 pin and ran it back to the trailer but got nothing.
Confused I got the multimeter back out and now I have no power going to the left turn terminal on the truck side on any of the plugs but the truck it self doesn't have any issue, all the lights work.

I could understand if I blew a fuse but wouldn't it have blown and my truck lights wouldn't work or the all other terminals on the 7 pin?

I don't think anything special has been done to the truck but would it be common for each terminal on the hitch to have it's own fuse?

Just looking for some ideas before I have to have a bunch of dirt fall in my face trying to undo all the connections under the bumper.

 

[itsdgm]

@Scottintexas I feel your pain. Reading your post, I can see several potential causes of your problem.

1. It's entirely possible that there's a grounding/wiring issue in the trailer.
2. It's possible that at the start, your truck's wiring is all in order.
3. It's possible that while running a jumper wire directly from your 7 round plug to the trailer, you bumped another wire in the plug causing a fuse to blow in the truck. Also, did you have a ground hooked up to the trailer frame? If not, the circuit wont be made enabling the trailer light to work.
4. Yes it is possible to have all of your vehicles service lights working, but have issues with the trailer outputs. I had this very issue on an older suburban. When I'd signal left, trailer light would signal right turn, brake lights only worked on one side of trailer. All the time vehicles lighting was normal. This was caused by a faulty turn signal relay under the dash. Somehow the trailer wiring and vehicle wiring are different and they come together at the turn signal relay.

Anyway, It can be hard to diagnose some of these electrical issues in person, and even harder over the keyboard. But for me in the past, usually trailer wiring issues are related to a bad ground somewhere. On boat trailers, it's usually been at the swing away tongue (if so equipped).

Good luck, hope you figure it out soon.

 

[Speedling]

Check the trailer on someone elses truck first. That will narrow down where the problem is.

 

[Scottintexas]

thanks guys,
the trailer does have problems on other vehicles, that what lead us to think "it's a trailer problem", I checked my truck side just as a precaution in trouble shooting to eliminate it, that's why I was so confused when the second time I checked it had no power,

I had a few minutes to go and look at the truck this afternoon, after reading the manual (to much action, heat exhaustion at camp to think about that!) I found the truck does have a fuses dedicated to the trailer harness and they are specific for each terminal. Sure enough the left turn/stop signal trailering fuse was blown.

I don't have a multi meter with me today to confirm it's good after replacing it but I would believe so,

 

[jawsf16]

My old trailer went through a spat of blowing truck trailer fuses. I too was interested to see that they were separate for the main truck fuses. I eventually found that the ground was bad, standard trailer light issue problem it seems. Hopefully that will square away your issue.

 

[-klb-]

It is surprisingly common for trucks with tow packages to have separated protection for the tow wiring. This is because rat chewed wiring is way too common on trailers stored in work yards, and it makes sense to protect the tow vehicle.

When I set up wiring on a small truck that had separated turn/brake lights, I actually designed the converter to isolate and protect the trailer completely separately from the truck wiring. Always fun running new wire all the way from bumper to bumper for the new power feed.