Every trailer I hook up to my F250 does the same thing.  I've eliminated the possibility that this is a malfunction of wiring on the trailer side.  The problem is on my truck.

Basically it's like this.

Upon plugging in the trailer (whether to the 7 RV or 4 Flat extension) the left tail light comes on.  Just the left one.  Not the aux lights, not the right one.  The headlights in the truck are turned off.  For some reason, the brake light on the left side works, but the blinker will not.

Taking a tester and going from ground to the yellow wire with the left blinker on gives me a flashing indication.  This tells me that voltage is being sent to the light, but it won't blink.  

The right side uses the same brown wire for tail lights as the left one, but they do not come on unless I turn the headlights on.

Same for the aux lights.

Banging my head against the truck bumper resulted in no change in it's behavior.

try looking at the fuses?

always cut the green wire ..........or is it red...........just cut one already
is it the bulbs sometimes wrong bulbs, or blown bulb, will cuase other bulbs on that side to act funny

Quoted: try looking at the fuses?

Using the tester, everything gets power when it should.  There are no broken circuits.

Quoted: always cut the green wire ..........or is it red...........just cut one already is it the bulbs sometimes wrong bulbs, or blown bulb, will cuase other bulbs on that side to act funny

4 trailers I've plugged this into all do the same thing, and I've actually replaced the whole wiring harness and light kit on the boat trailer once as well.

Another thing, on the brake controller in the cab, for electric trailer brakes, when I flip on the right blinker, it indicates it by flashing.  However when I flick the left indicator, nothing.

maybe check for loose connections/ bad splices near the controller

bad ground on the truck side....................if you checked 4 trailers and replace one totally I would rewire the truck side checking every wire for breaks, etc..............but it's a ford so who knows

ok it sounds like power is back feeding the yellow wire.if with the trailer unplugged the test light blinks then that cicuit is working,however if when you plug in the trailer and the left light comes on then power from somwere else is getting to the yellow wire.if you are getting 12 volts constant to turn on the yellow light then the blinker function will not matter as the wire is getting power from another wire.i bet it is your trailer and possibly it is wired wrong or two wires are shorted together.
7 pin not any specific order
1-yellow is left turn                       first four are the 4 way flat also.
2-green is right turn
3-brown is tail lights
4-white or black is ground
5-blue is trailer brakes from in truck controller if installed.
6-orange i think and is accessory power(to power inside trailer accessories if needed)
7-light blue for reverse lights.
unplug the trailer and test all circuits on truck side,they will test good i bet unless your truck is old and the wiring is fubared.also if you can test another trailer since the 4way on the trailer you have is directly connected to the 7 way on the trailer just like it is on your truck.you can get wiring testers at pepboys for 4 ways and i think 7 ways.
try putting 12 volts to the trailer without it being hooked up and test all the pins while having a test light on the yellw pin and when you find the offending wire the test light will light up.make sure to ground the proper pin on the trailer also.i have been a tech for 17 years and doing hitches and wring for over 6 and have seen very few problems on the truck side.if your truck is under warranty and the trailer tests fine than go there.if you have no luck pm me and i will give you my number to call and i can try to walk you through testing but it will be very hard over the phone(work load permitting).

it is the brake controller,the controller should not blink when the turn signal is on when a trailer is hooked up.i bet it is wired incorrectly or the two wires are shorted on the truck side.trace all the wires under the truck ,you will have to un tape and take loom off.sounds like the brake controller is giving the yellow wire power when the trailer hookup completes the circuit

ignore most of my first post,i didnt know you tried several trailers,i would test the pins on the truck side to see if they all function as they should,is the control an aftermarket or is the truck new enough to have a factory one.

There's no power on the yellow wire unless I turn the blinker on.  Then the tester blinks at the regular interval.  I can't see any power backfeeding onto the yellow wire.


I also have an adapter from 7 RV to 5.  This is one with small LED indicators for LTail, RTail, LBlink and RBlink.  When I plug this into my socket, the LTail comes on, even with no trailer attached.  When I turn the left blinker on, the LBlink light flashes, just like my tester, however the blinker on the trailer will not when I hook it up.

After market controller that was installed by the previous owner.

have you ever used it before i am drawing a diagram and thinking check back in a few min

try unplugging the brake control from the back,it should be a 4 pin
it is wired to power,ground,cold side of the brake light switch and to the brakes of the trailer

still thinking

Quoted: After market controller that was installed by the previous owner.

I would guess it's the controller and the way it's wired.


I have rewired 4 army trailers and about 4 or 5 boat trailers and every single trailer light problem I've ever had has been one of these things in this order:

1) bad grounding or no grounding
2) shorting of wires in the wiring loom. usually on the trailer side but once on a fubared wire setup on the jeep side
3) Bad connection to the actual light in the back,  just pure corrosion (1x).


I'd pitch that light controller first,  it seems to be the common denominator.  You know the trailers all do the same thing.

i am stumped,i was starting to think that maybe the pressure of a plug was causing a short on the trucks seven pin then i remembered it was happening with the 4 way also,if it was wiring in the truck the tail light would be constantly on.it has to be something goofy with the wiring in the trucks trailer harness,something is powering that tail light when a trailer is plugged in.with the truck off see if there is power to any of the wires at the 4 and 7 pin connectors,only the auxillary power wire goes directly to the battery and i think that is the center pin if it is used on that truck