2012 f150 lariat crew cab, re-wiring my small trailer, got all wires run and started to hook up lights, wire harness goes all the way to the taillights. so just to make sure lights light up i plugged just into truck's harness, no power to lights... checked at truck's plug..12vdc, I ran to back and .4dcV. so I rechecked all wires even the ground all the way in back of trailer, everything has continuity. after 2 hours I decided to put trailer on ball of the truck and WOW it all works now....truck has an onboard app for trailers, would this shut off power when it isn't hooked to ball? that's what it seems to do, last year I rewired my boat trailer and same thing happened until I hook up to the ball on truck....the main ground wire is a 14ga. from front to back with no breaks or splices. anybody have that happen???

 

Interesting. I always attach the hitch first, then the chains, plug last, always lights up immediately when the truck's lights are on.

I don't think there is anything monitoring the trailer tongue being connected. What can happen is a trailers can pickup a ground through the hitch if the paint is worn enough. Have you performed a resistance test on your ground pin?

 

Your grounding circuit is insufficient so your trailer is grounding through the ball hitch to the truck.

 

thats what I thought too, but its a new wire harness H/D the wires are a lot bigger gage than any harness from ebay to walmart to trailer stores, I have the ground directly to rear lights first then tap off for marker lights. could it be that just being an aluminum trailer has anything to do with it. I used a 12volt battery from my tractor and plug into ground and used power to each wire for , left, right, stop and running lights and all is fine. thats the weird part, what and how do you test the grounding circuit?

 

Is the trailer grounded anywhere or just the lights? I'm thinking it's your trucks grounding circuit that is broken or insufficient if you were able to use a seperate battery to make it all work. Check for continuity between the ground on the trucks trailer plug to the frame somewhere as FlamingTaco mentioned and make sure it's good.

 

the ground goes straight to the rear, I used a s/s bolt and made it into a grounding stud from there it goes to the two taillights then to 2 marker lights, I made 3 more grounds from stud to rear frame and each side frame, should I make a ground from frame to tongue? the trailer is a tilt snowmobile trailer and that's the only thing I think may help...

 

how do I test grounding circuit? pics would help for that too.

 

Is it all of the light functions that are not working or just 1?

 

Attach one end of a copper wire 16 14 gauge long enough to get to the back of the truck, and connect ine end to the negative battery post and bring the other end to the back of the truck. Set your meter to ohms and touch the two probes together to get an idea of what your meter reads. (Zero ohms in a perfect world) Then put one probe on the wire and one on the ground stud. The ohm reading should be near zero or match close to the meter reading you got when you touched the probes together.

 

quick update.... all is good, ground on truck coming out towing package harness, bolt almost completely out of the hole so intermittent at best ground. dropped tire and cleaned everything up and re installed wire to frame. good ground now. Thanks for the help, I thought it could only be trailer... but I was wrong!