Troubleshooting Left Trailer Running Light That Does Not Work

Question:

We purchased a trailer for a cross country move. The right tail light running light mode is out. Turn and brake lights work. Thanks

asked by: Mike E

Expert Reply:

Thank you for the photo of your trailer's 7-way plug, which appears to be in good condition (no signs of discoloration or corrosion that could be causing a signal transmission problem). On a 7-way plug like yours there is one contact that handles running lights for BOTH sides of the trailer and this contact is noted by the arrow which I added to your photo.

Since the running lights are working properly on the left side of the trailer, the things you can check first in your troubleshooting effort are the ground on the left light (that does not work) and the main ground for the trailer wiring harness. Some lights use a separate ground wire, often white, to make the ground connection to the trailer frame while others will use the light's metal mounting stud to establish ground. Paint, primer, grease and dirt on a trailer frame can interfere with the ground, which requires bare metal to bare metal contact.

The ground problem could also be on the vehicle side. Find the point on the vehicle where the wiring harness white ground wire attaches to the frame. Make sure this point is clean bare metal and that the ring terminal is tightly secured. You can use sandpaper to make a new clean grounding point if that's easier.

It can happen that a weak or marginal ground will pass enough current to allow some lighting functions to operate while others will not.

expert reply by: Adam R