B+E (Towing a trailer driving test)

When something goes wrong with the lights on a trailer, people automatically assume the problem is in the trailer. However, reality tells us something different. A bunch of the time, the problem is in the tow vehicle. If the “I’ve seen this problem before” approach doesn’t fix the problem, start with the tow vehicle. First of all, make sure all the lights are working correctly on the tow vehicle. Once this is verified, go to the connection at the tow vehicle and test there. Prove to yourself that the tow vehicle is putting out the correct voltages on the correct connector pins. Again, I am not a big fan of using a voltmeter for this task, I prefer to use a bulb with two test leads. If you prefer, you can also purchase a little LED (light emitting diode) tester that plugs into the tow vehicle jack (receptacle). Besides providing a load to draw enough test current, this tester quickly show when too many lights are coming on. For example, the running lights come on when the brakes lights come on.
A typical tow vehicle jack is an in-line four-pin jack. It has three female pins and one male pin. The female pins carry the voltages for the different lights, the male pin is the ground or return for all the circuits. Starting with the male pin and working toward the other side of the jack, the correct voltages and wire colors are:
White – Ground
Brown – Running (tail) lights
Yellow – Left turn/brake light
Green – Right turn/brake light
To test the voltages at this jack, turn on the running lights and then connect your test bulb between the white wire pin and the brown wire pin. The test light should come on. Connect your test bulb between the white wire pin and the yellow wire pin. The test light should stay off. Connect you test bulb between the white wire pin and the green wire pin. The test light should stay off.
Turn off the running lights, then repeat this basic test approach for each turn signal and the brake lights. Remember, always connect one test lead to the white wire pin (ground) and see what is on the other pins. The test light should only come on when you are on the correct pin. Also keep in mind, when testing the brakes, the test light should come on at both the yellow and green wire pins.