Troubleshooting Running Light Circuit on Trailer

Question:

Something has happened to the toyota wiring harness not through a fault of the company but all the lights on the wiring harness work except for the running lights to the trailer. My installer checked out the trailer lights and they are now okay. So what can I check or do to fix the problem? I really do not want to pay the installer to look at the harness and then have him turn around and say I need a new harness, that will be $75 than you very much. It has already cost me $75 to have him diagnose the cause of the problem which was that the company that made the LED trailer light had miswired the inside of the light and the brown wire coming out of the light was actually a yellow.

asked by: Jack

Expert Reply:

If the trailer lights are indeed now wired correctly, it sounds as if the problem exists with the converter on the vehicle. It is possible that having the one light miswired caused the converter to short out, but the trailer wiring should be examined, if the short is not located, you will keep shorting out converters until the short is located and repaired. Trace each circuit back from the trailer connector, looking for pinched or cut wires. Look for nicked or worn wire insulation that would allow the copper conductor inside the wire to short out on the trailer frame.

Use a circuit tester like part # PTW2993 to test the trailer connector. Turn on the vehicle headlights and use the tester on the pin fed by the brown wire. There should be power on this pin when the headlights are on. If not, test the converter. Trace the wires from the trailer connector, they will lead to the converter.

With the headlights on, you should have power on the brown wire on the INPUT side of the converter box, as well as power on the OUTPUT side. If there is power entering on the input side, but not exiting on the output side, the converter has shorted out and will need to be replaced.

I will link you to our trailer wiring FAQ, which shows how a trailer should be wired. I will also link you to a troubleshooting video that shows how to troubleshoot a converter box.

expert reply by: Mike L