Troubleshooting Trailer Stop and Turn Lights

Question:

my 2008 f 150 when i hook any trailer up i have no turn signal or brake lights on the trailer but i have running lights. but all the lights work on other vehicle when you hook the trailer lights up so it has to be my truck. so i took a light tester an put it to the pins to see if fire is there blinkers and brakes all have power to the pins. please help me

asked by: James

Expert Reply:

On a 4-pole trailer connector like # 40185 brake and turn lights are carried on the same wire; yellow for the left brake and left turn light and green for the right brake and right turn light. Running lights for both sides of the trailer are carried on the brown wire.

Since you tested your truck connector with a circuit tester like # PTW2993 and found that the correct signals are present on the pins for the yellow and green wires when you activate the brake and turn light functions, this tells us that the problem is potentially in the trailer's connector or ground.

You can use a spray on electrical contact cleaner and a small brush to clean the contact surfaces. Even slight greenish or whitish discoloration indicates corrosion that could cause an intermittent connection. You can also check that the truck's harness white ground wire is secured to a clean bare metal surface. A weak ground connection at the harness can cause strange behavior from the trailer lights.

expert reply by: Adam R