What to Check When the Light on a Trailer are Not Working  

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Expert Reply:

Lights not working on a trailer can be caused by quite a few different things. The first thing to check is all your grounds. A bad ground can cause any number of things to happen and is always a good place to start.

From there, you will need to check the towing fuses to make sure they are still in good condition. You will need to confirm which these are in your owner's manual. If one or more have blown, it will definitely cause the issue you are having. If all the towing fuses look good, you will need to check the plug on the back of the vehicle.

Using a tester, like the Circuit Tester # PTW2993, you will need to test the pins on the plug. If you have a 7 Way connection, the 2 o'clock position will be your 12-volt power, the 3 o'clock position will be the right turn signal and right brake signal, the 5 o'clock position will be your brake controller output, the 7 o'clock position will be your ground, the 9 o'clock position will be the left turn signal and left brake, the 11 o'clock position will be the tail light and running lights, and the center position will be the auxiliary circuit. If one or more of these are not working, you will need to then check the wires on the back of the plug to see if there is power going into the ports. If there is power at the back of the plug, you will need to replace the 7 Way with something like the Hopkins # HM40975. This would be a great replacement as long as the vehicle came pre-wired with the 7 Way plug. If it did not come pre-wired or if there was no power at the back of the plug, you will need to trace the wires back from the 7 Way connection as there is most likely either a break in one or more of the wires or the ground has become corroded or not secure. If this is the case, you can simply repair the wire and get back to towing, or you could rewire everything and use the Hopkins # HM40975-11998 at the end of the vehicle to give you a clean look.

If the lights still are not working, you probably have a break in one or more wires on the trailer and will need to follow them to find said break. That being said, you should always start this directly behind the plug that connects to the vehicle. These do go bad over time and are easily replaced. The Hopkins # HM20086 has built in test lights to make things easier if this ever happens again.

expert reply by: Lazer S

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Testing Trailer Wiring
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Testing Trailer Wiring
(click to enlarge)