Troubleshooting Cargo Trailer Lighting That Works Until Headlights Are Turned On

Question:

I have a 24 ft cargo trailer when I hook up the lights it takes a while for the lights to turn on I figure it is because the brake away battery needs to charge.Then they work fine until I turn the truck headlights on, only the center 4 diodes work in the tail lights not all 10 turn signals and running lights only brakes all 10 light up same on both sides. Do you have any suggestions.

asked by: Roger C

Expert Reply:

The trailer lighting issue you described could be caused by several possible problems but it would not be related to your breakaway battery's charging requirements as this is fed by a different circuit.

The easiest items to check and correct are bad grounds. If the vehicle or trailer main wiring harness ground is not affixed to a clean bare metal surface it may not be able to pass enough current to operate all of your trailer lights. Because things work okay until you turn on your headlights (which also activates the running lights) I suspect the vehicle's ground wire is the problem. A bad ground will let some functions work while others do not.

Remove the white ground wire and clean off that frame surface with sandpaper to remove rust, grease, primer or paint that could be interfering with the required metal-to-metal contact between the frame and ring terminal. If that does not correct the issue then repeat this process on the trailer ground wire.

You can also just disconnect the trailer and test the vehicle socket using a circuit tester like # PTW2993 to see if the signals are reaching the socket. If so, move downstream in the wiring to find the problem. Check for corrosion on the connector contacts, inside and out, and clean or replace as needed. You can refer to the linked article for additional help.

Once you get your connections sorted out you can protect them with dielectric grease # 11755.

expert reply by: Adam R