How to Troubleshoot Trailer Wiringby K.K. Lowell
With nothing more than a test light and an hour or so, you can diagnose a wiring problem in a trailer. Before you start, make sure that the fuses in the tow vehicle are good. Check your owner's manual to determine if your truck has fuses for the trailer circuits, and that these are also good.
Verify the proper operation of the vehicle lights. Check that each contact in the vehicle trailer connector has power as you turn on the tail lights, left turn signal and right turn signal. Have a helper depress the brake pedal for you while you check the power to that circuit.
Visually check for broken wires on the trailer.
Repair any broken wires found by splicing the broken ends with a solderless butt connector. Replace any section of wire that has broken or missing insulation. (This is usually the problem when a trailer blows fuses.)
Connect the trailer to the vehicle and turn on the lights. Determining which light is not working will narrow your search for the problem.
Remove and clean the ground (white) wire connection on the trailer if the lights are dim or go out when the brake pedal is depressed. Another symptom of a bad ground is if all the lights flash weakly when a turn signal is on.
Check the bulb in a lamp if the problem is with one lamp. This is most easily done by replacing the bulb with a new one.
Remove and clean the ground connection at that lamp if a new bulb did not cure the problem. This may be a separate ground wire, or the ground may be through one of the mounting studs. A new lamp may be required if the problem is in a stud.