Troubleshooting LED Trailer Turn Signals That Work Only When Brake Pedal is Pressed  

Question:

Expert Reply:

I suggest you test both your vehicle wiring and trailer lighting separately to find the cause for the lighting issue you're having. Separate testing tends to make it easier to isolate the problem.

To test the vehicle's wiring you will use a circuit tester like # PTW2992. The linked video illustrates use of the tester. Have a helper sit in the Frontier to activate the lighting functions one at a time while you apply the grounded tester's probe to the appropriate pins on the wiring connector. This will tell you if the correct signals are being sent to the trailer. If not, you could have a bad relay or a harness that is not fully seated together. For reference I linked our main page for all wiring products for the 2017 Frontier.

In 4-pole and 7-way wiring the stop and turn signals are combined and sent along the same circuit and the same wire on each side of the trailer. In a 4-pole setup the green wire carries RIGHT stop and turn signals and the yellow wire carries LEFT stop and turn signals. Please refer to the linked photo showing a trailer-side 4-pole connector.

To test the vehicle, ground the tester to the exposed ground pin (which connects to the white wire) and apply the tester probe to the contact for the green wire, first while the right turn signal is activated and then with the brake light activated. The turn signal should deliver an on-and-off blinking signal at the tester; the brake pedal being pressed should result in a steady signal on the same contact. Repeat the process on the left side yellow wire. If you detect appropriate signals on the vehicle's connector pins then you can move your attention to the trailer side.

The trailer can be tested using the truck battery or a spare 12V battery if you have a fully charged one around. A set of jumper cables and two lengths of wire can make it fairly easy to send direct 12V battery power to the trailer circuits. Connect the negative battery terminal to the trailer ground and connect the positive terminal to the pins on the trailer plug one at a time to see if the lights activate with this direct power input. If the lights do not function your most-likely issue is the main trailer ground or the ground at the lights themselves (if they use a mounting screw or stud to make the ground connection).

One of the most common causes for weird lighting behavior is a weak ground connection. This tends to occur more on aluminum trailers (which do not establish a ground connection as well as a steel-frame trailer) but it can happen on steel frames too.

expert reply by: Adam R

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