Troubleshooting Trailer Running Lights with Potential Grounding Problem

Question:

I just wired up a 4-pin setup to my utility trailer. All lights work with one problem. Each tail light has a bulb facing rearward and also a side light. Each tail light also has two black wires and a white ground wire. On the right side I connected one of the black wires to the green wire turn/tail and on the left side I connected one of the black wires to the yellow turn/tail. I connected the white wire of the harness to the white wire of each tail light. Every light works as it should except the rear facing bulb on each tail light running light. What is confusing is that the side light on each tail light comes on running light when the vehicle lights are on, but not the rear facing bulb. This certainly means I have power to the brown wire, but it isnt getting to the rear bulb. Inside the tail light it looks like the rear facing bulb and the side facing bulb are in connection with each other. Im baffled. One note is that since all the other bulbs and functions are working, the white wire from the harness is not grounded to the trailer. Could this be my problem? I tried using a jumper lead and grounded the white wire to the trailer during troubleshooting and it didnt seem to help. I hope this all makes sense. Thank you if you are able to help me!

asked by: Gene

Expert Reply:

It sounds like you have the lights wired properly, but if the trailer's main white ground wire is not grounded to the trailer frame it is quite possible that some light functions will work while others will not. Often it can happen that a weak ground will allow enough current for some lighting functions to get through, but not enough for other functions. Running lights, which stay on, draw more power than other lights (brakes and turn signals) that are on only briefly. This could be why the smaller side lights work but not the larger rear-facing lights which demand more power.

I suggest you find a clean bare metal spot on the trailer frame to solidly attach that main white ground wire from the trailer harness. This is likely to be the fix and it will take little time to try it. Once you have the ground wire attached and all lights are working, I suggest you apply dielectric grease like # 11755 to help protect those connections from moisture and other contaminants.

expert reply by: Adam R