Found the mini fuses for our trucks..for further search reasons their called "low profile mini fuses".

 

I've got the exact same problem with my 2010 F150 XLT 4x4.
Brake and turn signals work, but running lights don't. I'm 100% sure it's a wiring problem with my truck. I brought my truck to the dealership and they replaced the mentioned blown fuse, only for the problem to come back again.

When I first noticed the "no running lights" I hooked the trailer to my fathers truck and everything worked fine, I also hooked it to my mothers SUV and everthing worked fine. After the fuse was replaced, the running lights worked briefly then quit working again. And again I hooked the trailer to both my parents vehicles and everything works fine.

I'm 100% sure there is a wiring problem, but the dealership keeps telling me I have a problem with the trailer. I checked all the wiring and it's fine, and It works fine on my fathers truck. Just made a 6 hours run with the trailer hooked to his truck and had no issues...

Anyone have any idea's? There is defintely a wiring problem with my truck as it just keeps blowing fuses, and there 8$ a pop at the dealer!

 

In 30+ years of dealership service, the truck wiring is rarely the culprit. 99.99999% of the time there is something wrong with the trailer wiring. It's usually a wire pinched or bared or a lamp socket or the plug on the trailer. If the truck fuse is blowing, it's doing the job it's supposed to do.... protect the circuit.

If you truly believe it's the truck's fault, replace the fuse but don't plug anything into the trailer socket for a couple of weeks and then check the fuse again. If it blows (and I don't suspect it will) then you can blame the truck wiring and begin looking there. If the fuse remains good... then you have to find what part of the running lamp circuit on your TRAILER is at fault.

The fuses for the trailer plug only protect the wiring to the plug (on the truck) and the wiring/lamps/sockets/etc. on the trailer when the trailer is connected. They do not protect the lamps and wiring that go to the lighting of your truck. Those circuits are fused separate.

If your lighting/wiring problem is intermittent, it may not blow the fuse right away so simply plugging the trailer into another vehicle may not be reason enough to say the trailer is without problem. It could be in the trailer's plug and simply by unplugging from one vehicle to another it might 'cure' the problem for a short time. It could be a wire that shorts out only when hitting a bump or when the trailer twists a certain way. Some trailer's wiring is enclosed behind interior walls or through the frame so it might not be cut and dry as to where the problem exists. A good bit of detective work might be involved.

In any case I hope you have good luck with finding the culprit.

 

Checked fuse #31 under the hood and found it was blown after taking it to the dealer and the service advisor setup an appt. Both trailer plugs have power for running lights. I've been told to turn the truck off before plugging the trailer wiring into the truck. Also need to make sure the lights are grounded on the trailer properly.

 

Bad ground. This thread was a long time ago, but if I remember right I had a bad ground and would blow that fuse. When I replaced the fuse it would blow again when I plugged the trailer in. Fixed the ground no more blowing fuses, and lights work properly.

 

Hey all. I am having this same problem except my fuse is not blown. Scratching my head!!!

 

I got a 2010 F150 2yrs ago. Put the trailer on to move a horse...Signal lights good,brake lights good,no running lights.
After checking connection pins with tester there was nothing. If you start at the guide slot, count 3 pins clockwise. That should
be the brown wire, running lights.After checking further,they have the blue wire for brakes on that pin,the brown wire is on
the blue wire brake pin. I run a hot wire with fuse to number 3 pin &everthing is working great.