No Trailer Running Lights on Ford F-350 Super Duty

Updated 01/09/2023 | Published 01/07/2023

Question:

I have a 2015 Ford super duty F350 Dually. I just purchased a new gooseneck trailer when we connected the trailer seven pin plug. All the lights worked except for the running lights. We tested the trailer with their jump box test kit at the dealership everything worked perfectly, so I thought I probably had a blown fuse. I came back checked every fuse every relay they all work fine I did some testing with the test light and noticed that when the truck is running the tail Light Pen shows 12 V, but the lights on the trailer still don’t work when the truck is not running but the headlights are on the running light pin shows no voltage I also tried to clip the ground to the test, light to the ground, pen and check each pen and it showed no voltage. Could this all be a grounding issue?

asked by: Phil R

Expert Reply:

The trailer wiring circuits on a 2015 Ford F-350 may only be active while the vehicle is running to conserve the battery. If the vehicle is running and the circuits are not active then you know you have a problem. The first thing to check would be the fuses and relays in the engine bay fuse box. If all are good you will need a circuit tester to check for power at the trailer pins with the circuits active. If there isn't power here you'll have to take the truck into the dealer for service but if you do have power you can proceed by plugging the trailer in. If the lights do not illuminate, use the circuit tester and check the brown wire for power directly after the plug on the trailer. If you don't get power here you know the issue with the plug. If you do get power then continue checking the brown wire further back to see where the break in power occurs.

If this did not address you concern or provide a solution for you please just let me know and I'd be glad to assist further.

expert reply by: Conner L