Troubleshooting No Trailer Brake Lights or Turn Signals When Towed by Ford F-350

Question:

brake lights and turn signal not working on f350 truck fuses ck out good

asked by: Dean

Expert Reply:

The best way to troubleshoot your trailer lights is to unplug the truck and trailer and test each one individually. This will best tell you where the problem is.

Since your truck's fuses check out ok it may be that there is corrosion on either the truck or trailer connector. Inspect the contacts for signs of discoloration and clean as needed. Caig De-Oxit is a handy spray-on contact cleaner you can try. If you are using a harness like # 18002 that connects with quick-splice fasteners make sure they are fully seated together. If cleaning the contacts does not fix the problem, you can start testing upstream of the connector to see if signals are reaching it.

First, you can apply power to the trailer connector directly from a battery (the one in the truck or a spare) to see if the turn and brake lights work. If they do, then you know the problem is likely to be in the truck's wiring. If they do not work when you feed them direct power in this way then you'll need to check the trailer wiring from the connector all the way back to each light, looking for worn or cracked insulation or other damage, especially in spots where the wire has to bend sharply or where it may touch the trailer frame. Repair or replace any damaged wire as needed.

On the truck you can test the 4-pole connector using a circuit tester like # PTW2993. Have a helper activate the brake and turn functions while you apply the tester. Please refer to the linked photo for the function carried on each wire. The yellow and green wires should carry both stop light signals and turn signals.

expert reply by: Adam R