How to test trailer lights with a multimeter?

As for testing trailer lights, the LED devices are useful only to prove the tow vehicle lights are ok. They are no help for diagnostic purposes if they don't work. As for the trailer, I use flat four connector with all three power leads connected together. read more

Place the negative (black) meter tester lead on the frame of your trailer to establish a ground. Place the positive (red) test lead against the bottom of the bulb socket. Do not allow the test lead to touch the bottom and side of the bulb socket at the same time because this can cause a short. read more

Then you have the other 2 wires for brake/turn signal. As mentioned above when testing the turn signal the voltage will be pulsing. Start at the truck with trailer not connected. Turn the lights on and make sure you have 12v/test light illuminated on 1 of the wires (that will be running lights). Now try the left turn signal and find the pulsing wire. read more