Troubleshooting Non-Functioning Running Lights on Trailer  

Question:

Expert Reply:

When all lights on a trailer work except for the running lights this often has to do with a grounding issue. If a ground wire on the vehicle or trailer is loose or attached at a rusty or painted surface it may be able to pass enough current for some lights but not others. Turn and brake lights do not remain on continuously like running lights do. This is why that lighting function, with it's higher current demand, tends to be where a weak ground connection shows up.

This issue has an easy fix.

Check the white ground wire on both the vehicle wiring harness and the trailer connector. Make sure both are tightly affixed to a clean bare metal spot. You can try sanding/cleaning the existing grounding points or create a new grounding point if you prefer. Be sure all paint, primer, rust, grease, dirt and salt are cleaned away before you make the new ground connection.

I suggest you protect that connection by applying dielectric grease like # 11755. You can apply this to the ring terminal before securing it and also after, where the grease will act as a barrier to incoming contaminants.

You can also directly test the running light circuit on the trailer. Simply unplug the trailer and apply 12V input power to the running light circuit (this is the pin next to the ground). Connecting a 12V battery directly to the trailer lighting circuits will tell you what's happening on the trailer without the vehicle wiring coming into the picture also. If the running lights work in this way then you know the issue is in the vehicle.

expert reply by: Adam R

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