Recently I was at a friend’s house helping her clean up after her basement flooded. Upon leaving she was showing me her trailer that she was building to haul her kayak and stuff. It was really impressive, and I was jealous. It was made of steel, she had someone weld rafters to haul the kayak, leaving room underneath for supplies, bags, and whatever you want for the trip. She also repacked all the bearings and painted it herself. I was impressed. But she noted that she couldn’t get the lights to work at all.
I assumed that since the lights and the connector to the vehicle were brand new, then they ‘should’ be ok from the factory. The trailer she had previously worked fine, so I also assumed that the wiring inside the vehicle was fine (for now). Since I’ve never grown up with trailers, I asked her for a schematic. She has the above 4-wire system with no side or rear markers.
I looked through the schematic. She used self-soldering connectors connecting the wires to the new lights. I tried to pull them apart and they were tight. Since she was telling me about them and how to use them, I assumed that the connectors were fine. In troubleshooting connectors that I cannot pull apart, like wire crimps, I sometimes take two small pins (like the ones you hold fabric together while sewing) and gently pierce the wire insulation and connect an electronic meter to test continuity. It really works fantastic.
Since nothing worked, it had to be something that was common to all the lights; improbable that all the wires are bad. So I looked towards the ground. I checked the white ground wire to the trailer, and noticed that she shaved off paint to make a good connection. I also checked the ball and socket from the vehicle to make sure the inside was unpainted (which it was). Even though both lights were affected, I went for each individual light.
Since the trailer was all steel, she was able to bolt the posts from the light straight to the trailer. The cool thing is that the bolts that connect the light to the chasse IS the ground for the lights. Then it hit me — she spray-painted the rear of the frame including where the bolts make contact.
Paint does crazy things. Sometimes paint can act as a conductor, but most of the time it acts as an insulator. Once she disconnected the lights and scraped the paint away from under the washers, the lights lit right up when connected to the car. If this trailer was made of non conductive material like wood, then you would have to individually connect the posts of the light to the white ground wire.
Now if this still did not work, my focus would be more on the connections on the vehicle considering first the fuses, then connectors using the pin trick I talked about earlier. Now that the lights work, she will be well on her way to enjoying time on the lake this spring and summer.