Troubleshooting Lighting Issue on Boat Trailer Towed by 2004 Ford Ranger

Question:

I bought a boat trailer that has a new wiring harness four pin. I have a pre-wired 2004 Ford Ranger. After messing around with it, I bought a plug in tester. On the truck, when the parking lights are off, both directional signals tester flash. When the tail lights are on, the test light is on, but neither directional test lights work. When the trailer is plugged in, tail lights on, the outer filament burns, the inner doesnt. When the left directional signal is activated the outer filament flashes. The right side, the outer filament burns with the tail light on does nothing with the directional on. With the trailer plugged, the lights on, they go off when I step on the brake. The inner filament doesnt burn in any case. New bulbs. Seems like there is something odd with the truck, but also the rh trailer signal. Is the outer filament supposed to be brake light/directional and the inner filament the tail light?

asked by: Rick H

Expert Reply:

The type of lighting issue you describe sound like a grounding problem and it could be on the vehicle, trailer or both.

What happens with a weak ground connection is that one or more light functions may operate properly until you turn on another additional function. Once the weak ground's current-passing capacity has been reached some functions will stop working. The classic example is that the trailer brake and turn lights work great, until you turn on the vehicle headlights. This activates the trailer running lights which adds enough more power demand to cause things to malfunction.

What I suggest is finding on both the vehicle and the trailer the main ground wire for the trailer harnesses. This is usually a white wire with a ring terminal. This needs to make solid contact with clean bare metal. You can sand off that grounding point or make a new one at another spot but be sure the ground wire makes bare metal to bare metal contact.

Doing this on both the vehicle and trailer ought to correct the issue.

You can prevent corrosion of the new grounds by applying dielectric grease # 11755 which will block water and dirt.

expert reply by: Adam R