A7B2FX4

Under the highly improbable but easy to check if your taillights are 1156/1157 types look for a broken filament dropped onto the other filament.

emmarose007

My truck tail lights?

sincitizen

I think he means your trailer lights.

sincitizen

I think he's talking about your trailer lights.

emmarose007

I'm not using a trailer to test... I'm using a tester.

There is nothing wrong with my trailer. All three of them test fine but work weird with the truck. The truck tests as outlined above. I am not having problems with trailer lights, I'm having problems with the truck trailer light plugin.

brulaz

Did you ever determine whether the truck has an aftermarket light controller or OEM?

After markets are usually wired in to the tail light wires and come together in a little "black box" that feeds the 4 pin connector.

If aftermarket, I'd rip it out and get a Ford harness ...

But if it's Fords ... no idea ... well, clip an after market into the taillight wires and see if that works?

emmarose007

Can't say 100% that its factory but I'm pretty sure it is. I do have aftermarket trailer brakes though.
I'm going to re-do all the grounds on my truck (even though they seem ok), spray the crap out of everything with dielectric and cross my fingers. If that doesn't do it then I'm going to go drive off a bridge (not really, but it's tempting at this point).

Dirttracker18

^^^

Well first of all the bolded part is correct, you should not have any trailer running lights when your truck lights are off, so stop worrying about that.
The trailer running lights are only active when the lights on the truck are on.

I'm surprised no one caught that yet.

As for the first part, those two circuits should be separate from each other. There should not be any consideration between them, tail lights and brakes/signals that is.

However the brake and signals are on the same circuit.

I would suspect someone has either made changes to the wires somewhere or put in an aftermarket trialer harness incorrectly.

I assume you have checked that this same issue is not happening with your truck tail lights and signals/brakes. Is this correct?

If so then we can narrow this down to the wiring that leaves the trucks main harness above your hitch.

emmarose007

I know that there are no running lights when the headlights are on. That's not a problem... I just put that in there to say what was and wasn't lit up, not because it was a problem.

Everything is wired correctly as far as the connector goes, The brake lights and turn signals are on the correct pins, as are the running lights, they just won't run at the same time.

emmarose007

And yes... There are NO other electronic or light problems with my truck other than the trailer light plug in. (That I have noticed). All fuses seem good.

Here's how I narrowed down what was happening:
Hooked an LED up to the "running" light pin of the trailer light plug in. Turned on truck headlights. LED lit up (as it should). Attached my meter to one of the brake/blinker pins an hit the brakes. Running light LED was still lit up but meter only read 1 or 2 volts on the brake/blinker pin...
Disconnected LED from running light pin... Pin measures 14 volts (running light pin). Brakes pressed, brake/turn pin measures 12 volts.

So the brake/turn pin is getting full voltage with the headlights on... Just not when something is connected to the running light pin.

That's why I'm thinking bad grounds... Even though they look good and I get continuity between ground pin/frame/negative battery terminal.