How would you suggest that I check that.... I took an ohm meter and tested from the trailer wire to the frame of the truck ???? Does that make sense ?? So then if you plug the trailer in to the plug and the plug is grounding to the truck... That should do it right >?????????????????

 

One of the pins is a ground. Check it for continuity to good ground on truck. If the trailer is not on the ball and you plug in light connector with open ground on plug lights will not work. Also, the coupler on the ball does not always make a good ground.

 

The ground pin has continuity to the truck.. the trailer was hooked to the truck.... If I run an extra ground wire from the truck to the trailer will that serve as a pretty good indicator of the grounding circuit ?????

I've also been told that there may be another fuse for the tail lights on the truck that may be causing the problem. One fuse could provide enough feed back through the other to produce the voltage, but when under load it can't produce enough ??? #38 is the fuse for the trailer park light circuit. What about the truck tail lights ??????????? which fuse runs those ??????

 

If you ran a separate ground wire between truck and trailer and still had the same problem that would prove it's not a ground problem. With neither trailer lights working it has to be the truck.

See this PDF for fusing and relays. It's for a 2001 but believe 2003 is the same.

 

S................................................. ..........................................

 

you just mentioned running lights,how about brake lights,turn signals, brakes,do they work?
#38 circuit should be trailer tail lights

 

Brake lights and turn signals works fine ! Just the running lights don't work and there's power to the terminal ! It doesn't make sense !

 

It looks like the 2003 and 2001 fuse panels are different. The numbers for locations are quite different and the lay out is not the same...

Does anyone know if in the 2003 there is another fuse panel. I see the main one under the dash and then a couple little relay boxes behind the airbox area ! My owners manual pdf file doesn't show where the tail light circuit for the truck is ?? This is starting to get under my skin ! There has to be another relay or circuit breaker for these lights that isn't providing the power under load, but will show power when not under load...

 

This is just a thought. Are you sure you have the brown wire on the right pin of the plug?.

 

It is the brown wire that I'm testing yes ! As far as being on the right pin... it is a brand new wiring harness directly from ford. plus it was working before I towed that POS trailer from u-haul ! Everything else works fine as well... you would think if was hooked up to the wrong pin there would have to be something else not working correctly. Plus its the running lights on both trailers... one has a 4 pin and one has a 7 pin... They can't be both wrong !

Its got me good !

 

don't know if you can find anything to help in repair manual but here it is
https://www.motorcraftservice.com/pu...20/S3OMAIN.HTM

 

I FOUND the problem....

It was at the connection between the rear plug and the truck wiring harness.. The brand new harness that I put in a while back had the retaining clip broken.. I guess it was hit by a rock or something. I pulled it a part and shoved it back together and bingo... running lights are working.. I don't know why I was getting power there and they wouldn't work.. but when I pulled that connection apart and reconnected it... they work... Must have been enough of a connection to read 12.4 volts but not enough amperage to light all the lights up when connected.. That was a weird one...! I connected it up again and taped the living sh#@ out of it so it would stay together... If it happens again... I'll no where to look right away !

Thanks to everyone that helped out with wiring diagrams and ideas, I'll be sending all of you reps that helped out !!

Russ

 

Robin.... It won't let me rep ya... so I'll thank you personally for your help.. I appreciate it !!! I'll rep ya when it lets me ! Stupid thing !

 

Good deal on the fix Russ. Sometimes it is the simple things that stump us. With the connector being loose, you had voltage until a load was applied, then it would drop out.

 

Exactly what I figure to Jim... Glad its over !