Troubleshooting Trailer Turn Light Signals  

Question:

Helpful Expert Reply:

Trailer wiring is set up to have running lights on their own circuit while the trailer's brake lights and turn signal lights share a common circuit, one on each side of the trailer.

In standard 4-pole wiring the RIGHT brake and turn light signals are carried on a green wire; the LEFT side signals are carried on a yellow wire. You can see this in the photo I linked for you which shows a trailer-side 4-pole connector.

The best way to test things is to disconnect the trailer from the vehicle wiring and directly apply 12V power to the pins on the trailer connector. This method of testing completely removes the vehicle from the equation and makes it easier to isolate the problem.

You can use your car's battery (or any well-charged 12V spare battery you might have around the garage) to test the trailer's lighting circuits one by one. Connect the battery ground (-) to the ground pin on the connector and then one at a time connect the battery positive (+) to the pins. You will then be able to see if the trailer lights work. If they do, then the issue may well still be on the vehicle.

If the trailer lights do not work with this direct battery power, then you will need to 1) check all bulbs to make sure none are damaged; 2) check the light bulb sockets for signs of corrosion; and 3) check all wiring for the problem lights all the way from the light itself forward to the trailer connector looking for cracked or worn insulation or wires that rub on metal parts of the trailer. Repair or replace any damaged wiring, lights or bulbs.

If you have a circuit tester like # PTW2993 you can also test the pins on the vehicle's connector. If the signals are not present on the car's connector, then your wiring still has an issue such as a bad turn signal relay.

expert reply by: Adam R

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