Trailer Turn Signals Work But Not Running Lights. If still no resolution it is time to look for a blown fuse in the trailer. It s very possible that a weak ground connection can be just strong enough to allow sufficient current for some lights to pass but not enough for everything. First check the trailer wiring harness on your tow vehicle with a circuit tester like part 40376 and make sure all of the functions are working correctly without the trailer connected.
If your signal lights and stop lights are working in the truck then the problem has to be one of the following. First check the trailer wiring harness on your tow vehicle with a circuit tester like part 40376 and make sure all of the functions are working correctly without the trailer connected. With the running lights on there may simply not be enough power to also activate the turn signal lights. Take a look at the tow vehicle lighting fuses.
If still no resolution it is time to look for a blown fuse in the trailer.
The most likely cause for the turn signals not to work when the running lights are on with your trailer is a bad or weak ground. If the blown fuses continue it might be the result of the circuit being overloaded. If the issue is not a fuse or relay then you can test the vehicle side trailer connector using a circuit tester such as 40376. If it is not working correctly you will most likely have to replace the taillight converter. With the running lights on there may simply not be enough power to also activate the turn signal lights.