Troubleshooting Gooseneck Trailer Lights Not Working

Question:

So I’m having lighting issues. According to my father 6 months ago the lights all worked fine I went to hook up to his gooseneck and had zero marker lights on brake lights or electric brakes working but the inside dome lights on the trailer are working. At one point we did have all lights working and then nothing we replace the 7 wire plug made sure each wire went back in the exact spot tried for 6hrs and got nothing for lights still. What and where does a guy begin to look I’m lost

asked by: Jordan

Expert Reply:

Troubleshooting a lighting issue like yours is easier if you test the vehicle and trailer separately.

The vehicle connector can be tested for correct light signals by using a circuit tester like # PTW2993 or # PTW2992 on the ground and each of the individual circuit contacts. Please refer to the linked photo to see which contact to test for each light function. If signals are not present when you activate the function check the vehicle fuse box for blown fuses, check the main ground for the 7-way to ensure it is solidly attached to a clean bare metal spot on the truck's frame and also check that there is no corrosion on the contacts, within the socket or in the connector that plugs in to it.

Test the trailer directly with a fully-charged 12V battery such as the one in the truck or a spare if you have one. Use two lengths of wire to connect the battery terminals to the ground and circuit contacts on the trailer plug. Please refer to the other photo of a male trailer plug to see which contact to test for each light function. If lights do not work with this direct battery power input then first check the trailer main ground wire just as you did for the truck.

Some trailer lights use their mounting hardware to make the ground connection and sometimes corrosion can develop between those mating surfaces on the trailer and lights; this will prevent operation. Similarly if your trailer lights use a separate wire for ground make sure this is tightly attached to a clean bare metal spot on the trailer frame.

The dome light that works may be powered from another location such as an on-board trailer battery or if it is a highly efficient LED type it may still function even with a weak ground since such lights use very little power.

expert reply by: Adam R