I have a problem with the trailer light connector on my '05 F250 diesel. The test indicator shows right blinker and left blinker but no tail lights. No lights on the trailers (have connected boat and utility trailer to truck) work. All the truck fuses under the dash are fine. I dropped the spare tire and removed and wire brushed the ground wire connected to the frame rail. Still nothing. What should I try next? Is there a wiring harness or converter that I can check?

There is a tail lights fuse for the truck, but not for the trailer.
First, check that your left tail light on the truck is working, if not fix the problem (new bulb, bad contact, fuse).
Then the obvious suggestions regarding the trailer are: Check the tail light portion of the bulb to be sure it is not blown. If good (best to put in new one to be sure), then check the wiring from the front of the trailer all of the way to the non working bulb, including every connection and at any place it is secured to the trailer frame. If all is good with the trailer circuit and bulb, then check the truck's trailer connector, including any adapters. If by chance the truck is wired with a flat 4 wire harness the problem may be solved (but usually the blinkers are the problem). Modern trucks require a 5 wire harness which self adapts to the standard 4 wire trailer circuit. Check for dc power (12 volts) from the truck's female connector. One wire will be ground, others will be taillights (turn on headlights), brake lights (have someone press brake pedal), and blinkers (turn on switch and blinkers).

If the blinkers on the car are working when the trailer is not connected but they fail when you connect the trailer plug, it is because the LOAD of the bulbs on the trailer exceed the capacity of the BLINKER CIRCUIT of the vehicle. I would recommend replacing the tail lights on the trailer with LED tail lights. Did that on my travel trailer, and it solved the problem 100%

I would recommend taking out the tail light. Get a test light, have an assantant step on brake. U will see the bulb in tailamp light up. Probe the wire w/your test lamp. That is brake wire. Repeat that process for blinker.. Drop down those two wires. Hook that to the same side on trailor. Repeat for left and you should be all set.

It does sound as though the trailer connection is the source of the problem, check to see if the wiring has been resting on the exhaust pipe and melted through, shorting the wires. Also check if it is squashed or damaged anywhere. Failing this you may have to pull the trailer connector apart and see if the wires have come out inside and are touching, this can happen if the trailer connector is pulled hard.

is the trailer attached to the ball on the van?? could be a ground problem,,run a jumper from hitch to trailer see if that works,,

Possible corrosion break in the Vehicle Trailer Plug.
Regards,DT

I beleive the diagram is on the inside of the plastic fuse cover, mine is.

• 4 way flat connector
• brwn-tail lights
• yellow-left turn
• green right turns
• white ground

1. 5 way connector
2. brwn tail light
3. yellow left
4. green right
5. white grnd
6. blue back up for surge brakes

i know of some more kinds but the 4 and 5 flat are the most used on trailers...word!

brown to the tail lights. yellow to left turn signal. green to right turn signal. this is a basic 3 wire hook-up i use all the time w/4wire connectors for all trailers. white wire is not used. ground is established when you hook to the ball.
to find the correct wire to splice to, use a testor (has a clip to the bare wire, and a sharp point to the truck frame). using a needle, push it through a wire to rear light harness. turn on the turn signal. check if the connection is the blinker, testing until you have the blinker wire. the tail lights can be attached to the license plate wire. brakes and blinkers use same wire, so brake lights will work whenever the turn signal is attached.
to be sure everything is working right, have a second person go inside the truck and work the signals, brakes and lights one at a time while you are at the back of the trailer telling what to do.

exactly what does it do? please explain what happens when you press the brake turn on the light than brake signals ect.....