I need to tow this weekend and my truck will not light up the trailer lights. I hook my dad's truck up to the same trailer and it lights them right up so I don't believe there is anything wrong with the trailer. I have 12 volts coming from the 3 inputs on my 4 flat connector. I have also run a long jumper wire from my neg. battery terminal and wired it to the ground wire on my 4 flat connector, just to make sure I have a solid ground. Still no lights. My question is, are there any fuses/relays that are not allowing my truck to send the power through to the trailer? I jumped all connections and still no lights, so it's not that my plug is making bad contact. My friend has the same truck, 97 f150, with the same problem on this particular trailer. I would love to be able to tow the boat this weekend, and any help is appreciated. Thanks guys.

 

Not sure what "I have 12 volts coming from the 3 inputs on my 4 flat connector" means, that makes it sound as if you always have + DC Volts at the pins that should be either parking lamp ( when the headlamps are on ) or the stop / turn for each side ( brake & turn signal dependent ).

From the owners manual

Engine compartment fuse panel fuse # 16, 40A This feeds the Stop/Turn fuse & relays and the relay for the battery charge circuit ( on the 7 pin )

Engine Minifuse panel, Fuse #2, 20A Trailer Tow Stop / Turn lamps ( both relays for the L & R sides ).

Engine compartment fuse panel fuse #1, 20A trailer tow parking lamps & backup circuit ( 7 pin adapter ).

The fuses start on page 137 in the 1997 owners manual, and don't forget to check both fuse panels in the engine compartment.

 

Follow your trailer connector's wiring harness forward under the bumper to the truck's wiring harness. Is the connector spliced in, or does it look like it was made with the harness? Do you have the smaller relay box attached to the main one like SSCULLY's pic shows?

 

Thanks so much for the detailed reply. When I first began diagnosing this, the small 5A was blown. I replaced it and it lit up the test light for brakes/ turn signals/ running lights when I tested the connector pins. I went home, plugged it in, and nothing. I thought what happens if I connect the boat trailer to my hitch? So I hooked everything up and went fishing. Pulled the boat out of the water afterward and to my dismay the lights quit working again. This made me think it must be grounding through my hitch. So I cleaned up my ground wire and where it connects, thinking that would for sure fix the problem. It didn't. So I ran a long wire from the neg. battery terminal and connected it to my ground wire on the plug. Still nothing. Long story short, I am very frustrated and I checked all those fuses and they were still good. Any other ideas??

 

The engine minifuse panel, 5A is the PCM keep alive circuit, nothing to do with the trailer tow adapter, so if replacing engine minifuse panel fuse #1 make the lights work, you have a problem with the connector or how you are testing.

If you installed a 5A in position #2 in the engine minifuse panel, that would explain the fuse blowing, you short changed the fuse by 75% ( should have been a 20A in position #2 ).
This would make it work with a test light, but the 1st time a real trailer load was placed on it, a 5A would blow for the Stop/Turn lamps.
Each is 26 W, total is 52W, or 4.33 A load with the brakes on, without any resistance added to the circuit from a dirty trailer tow or bulb connector, or weak splices on the trailer.
This is with a standard dual element bulb ( like the truck has ).