Troubleshooting Trailer Lights That Come On When Truck is Turned Off

Question:

I bought a used job trailer and plugged it into my truck which is new and without the truck running or the lights turned on the trailer lights are on when plugged in ? Any thoughts would be very helpful

asked by: Chad M

Expert Reply:

I recommend starting by testing the connector on your truck with a circuit tester like # PTW2979. This will help you quickly narrow down the possible locations of the issue. Have a buddy sit in the cab and run through the lighting functions while you test the appropriate connector pin for power. You should only get power at a specific pin when the corresponding function is activated (power at the left turn/brake light pin when that function is on, etc).

If you find that you are getting power at the pins without the corresponding function activated then there is something going on with the truck. If this is the case check the wire connections just behind the connector to make sure they are secure attached and there aren't any exposed or pinched sections. Also visually inspect the connector itself for any debris or corrosion that needs to be cleaned out. Since it is a new truck this likely isn't a problem but you will want to check to be sure.

If everything tested correctly at the connector then you know the issue is on the trailer side.

On the trailer inspect the connector for debris/corrosion, cleaning out as needed. Make sure that none of the trailer light wiring is connected to a separate trailer mounted battery that might be powering the lights.

Lastly check all of the ground connections on the trailer, both with the main harness ground and those for all of the individual light assemblies. It's possible that the there is a weak/bad ground that's allowing power to backfeed from the 12V circuit when the truck is turned off. Make sure the ground wires (or mounting hardware if your light assemblies don't have a separate ground wire) are attached to clean, bare metal surfaces that are free of any dirt or rust. Since it's a used trailer it might be worth it just to remove all the grounds, sand down the trailer frame surface, and reattach them just to be certain of their connection.

If you still are unable to find and resolve the issue please let me know and we can continue to troubleshoot. I have also attached a couple articles on trailer wiring that you might find helpful.

expert reply by: Chris R