Troubleshooting Trailer Lights That Go Off When Truck Lights Are Turned On

Question:

When I turn on my truck lights my boat trailer lights do not work. However the lights for the turn, brake and flashers do. My connecter is pretty beat and I would like to replace it. It is a 4 pole flat but has 6 wires running back to the trailer. The trailer is a 16 ft. 2003 EZ loader. Thanks for any help.

asked by: Bill L

Expert Reply:

The problem you're having with trailer lights not working when you turn on the truck's lights suggests a grounding issue. Running lights, because they stay on continuously, draw more power than stop and turn lights that are on only briefly. This is why a weak ground connection can result in some light functions working while others do not.

I suggest you check the main ground wire for the truck's wiring harness, typically this a white wire. Make sure it is tightly affixed to a clean bare metal spot on the frame. Choose a spot that is free of primer, paint, dirt, grease, etc. You may be able to just sand the current grounding point to get it good and clean before re-attaching the ring terminal. Apply dielectric grease like # LT37534 to keep the connection free of dirt and moisture.

If this does not correct the issue then next check the main ground wire on the trailer in the same way. It too needs to make a clean bare-metal-to-bare-metal connection at the frame. Each trailer light's ground wire (or grounding post) also needs to have a solid ground to ensure proper operation.

If you want to replace the trailer 4-pole wiring harness you can use 35-foot wishbone harness # A35W42B or 20-foot # A20WB. These are 4 pole connectors that have dual running light wires.

expert reply by: Adam R