Trailer Running Lights Not Working when Connected to 2017 Ford Transit T250

Question:

all lights on trailer work except running lights? i assumed it was a blown fuse, went through owner manual and have not been able to find a bad fuse related to trailer lighting.

asked by: Caleb

Expert Reply:

You can determine if the vehicle side is the issue by testing its trailer connector with a circuit tester such as # PTW2993 or a meter if you have one. If the running light pin shows power when the vehicle headlights are on, then the issue is on the trailer side. If it does not show power, then the issue is on the vehicle side.

The easiest thing to check is the trailer connector itself (on the vehicle and on the trailer). If there is enough corrosion then there won't be a good connection and the lights will not work on the trailer. This happened to a friend of mine and once we cleaned out the corrosion everything worked well.

You can also inspect the wiring as much as you are able and fix any damaged or pinch wires.

In the passenger compartment fuse panel check location F12. That is the fuse for the trailer tow parking lamp relay. The relay itself in location R3 could also be bad. The easiest way to see if the relay is the problem is to use a known good relay in place of the potential bad one. There is also an auxiliary fuse panel and the fuse in location F6 also protects the R3 relay for upfitting applications so that is likely not what you would be looking for. So basically if you do not have power on the vehicle side and the fuses are all good then it is likely the trailer parking lamp relay.

If there is power on the vehicle side then the issue is on the trailer. If cleaning the trailer connectors does not help then there could be a ground issue. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

expert reply by: Michael H