No Tail Lights On Boat Trailer When Parking or Headlights of 1995 Ford Truck Are On

Question:

I have a 95 ford 3/4 ton pickup. Am hooking up a boat trailer. I am using a 4 prong flat plug. White to ground, Green right turn, Yellow left turn, and brown, taillights. All lights work except the taillight when the headlight switch is on. I checked the fuses, all ok. New plugins on boat trailer and pickup. Solid connections on all wires. Any ideas? Thanks a bunch.

asked by: Ed

Expert Reply:

It sounds like you have a common ground problem. When the headlights are on, the highest amperage draw is placed on the wiring system and if you have a bad or loose ground, you will have problems with lights. You will want to inspect to make sure all the light assemblies have a good ground connection to a clean, rust and paint-free metal surface. You will also want to make sure the trailer and vehicle side connector have a good ground connection.

I would recommend unbolting the ground connections, and cleaning up the metal or relocating the ground connections to a clean metal surface.

If you are using a plastic license plate holder on the left side tail light, this could also be causing a ground issue and you will want to make sure the mounting bolts are making good contact with the trailer frame.

I have linked an FAQ on wiring which also has information on ground problems.

expert reply by: Reno H