Trailer Running Lights Malfunction When Towed with Ford F250
Updated 03/29/2014 | Published 03/28/2014
Question:
the running lights on the trailer do not come on when first plugged in. I usually stop at a store and when i come out the lights are working. Please what is causing this? This only happens on my F250 truck. This does not happen on the F150.
asked by: Jeff P
Helpful Expert Reply:
Since the problem is only occurring with one of the vehicles, the problem likely lies in the wiring of the trailer connector of your F250. When you plug the trailer in and notice that the running lamps are not working, use a circuit tester like part # PTW2993 on the running light pin of the trailer connector on the truck. I've edited a photo for you showing the running light circuit on a 7-way and a 4-pole trailer connector. If you are using an adapter, first test the connector, plug the adapter in and test the adapter to rule it out as being the problem.
If you do not get a signal on that pin, since the problem is intermittent, check the trailer connector on the vehicle for damage or excessive corrosion on the sockets or pins. If you can see any corrosion on the connector, the corrosion might have spread to the interior of the connector and might be preventing the signal for the running light circuit from getting through the connection reliably. Replace if necessary. Check the connections behind the trailer connector, make sure everything is secure. Check the ground wire on the trailer connector, making sure it's securely grounded to the vehicle, preferably to the frame, and make sure the connection isn't corroded.
If all else fails, there is possibility the problem IS in the trailer wiring. Some vehicles are more sensitive to ground problems or short circuits than others. It is certainly plausible that the trailer may work fine with one vehicle, but be problematic with the other. Make sure the connector on the trailer is in good repair, with no corrosion or damage. Check the point on the trailer where the ground connection is made and ensure it is tight and not corroded. Check the running light circuit for loose connections, pinched wires or any cracked or worn wire insulation.
If none of the above checks out, get back with me and we'll look into some other possibilities. I've linked you to our trailer wiring help article.
