New Boat Trailer Running Lights Do Not Work when Plugged Into Truck

Question:

turn and brake lights work no running lights.new boat trailor truck has 4 prong plug. 1 white 1 brown 1 green and 1 yellow wire trailor has 4 prong plug but 2 white 2 brown 1 green and 1 yellow wire

asked by: John B

Expert Reply:

The 4-Way on your trailer sounds like it has a wishbone harness and is actually a 5-Way and not a 4 way. A wishbone harness will have 2 wires coming from the running light pin. The running light pin is the one right next to the ground pin. Everything should still function the same though. The 5th circuit is normally for a reverse electric lockout.

What you will want to do is first test the vehicle side connector using a circuit tester such as # PTW2992 while some one sits in the truck cab and runs through the light functions. If the running light circuit is not working on the truck side trailer connector then it will need to be repaired.

Make sure that the truck and trailer side connectors are clean and free of corrosion. I have seen this issue before and the problem was crud in the vehicle side connector. You can also check to see if any tow package fuses are blown. Use the owners manual to find their locations.

Since the trailer is new it does not seem likely that the trailer is the problem. I have linked a help article showing how to test vehicle side wiring harnesses.

expert reply by: Michael H