Why Wont Running Lights on Trailer Work When Towing with My 2008 Ford F-350  

Question:

Expert Reply:

Whenever a single trailer light function suddenly stops working with a tow vehicle like your 2008 Ford F-350, the first thing that comes to mind is a blown fuse or bad relay under the hood. I looked up the owner's manual for your truck and it looks like there's a 30 amp fuse in the #42 position and a relay in the #55 position inside the fuse box under the hood in charge of the trailer running lights circuit. Check to make sure these are still in good condition and replace if not.

If that ends up not being the issue, the next thing to look at will be the 7-Way itself at the rear of your F-350. Inspect the pins to make sure debris or corrosion haven't built up inside the connector, cleaning out as needed. Do the same with the 7-Way on the trailer side as well.

If the problem persists, use a circuit tester like # PTW2993 to test the running lights pin on the truck side 7-Way and see if it's getting power when you've got the lights turned on in the truck. If you do get power, then the issue is actually somewhere on the trailer side.

Either way, once you're at this stage of the process, it's really just a matter of inspecting as much of the wiring on this circuit as you can - both going back from the 7-Way on your truck and along the trailer back to the rear lights - looking for any exposed, frayed, or otherwise damaged wiring that needs to be replaced.

Just let me know if after all of this the running lights on the trailer still won't work and we can dig in even further.

expert reply by: Chris R

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