Trailer Lights Do Not Work when Connected to Truck But Work with AC to DC Converter

Updated 09/09/2014 | Published 09/06/2014

Question:

I have a 10 year old enclosed 8foot trailer and some of the side lights were working intermittently or just not working. After a day of trying to fix, testing every possible combination with a meter, buying new adapter and dremel cleaning all connectors, and replacing light housings... Still problems.... So I just rewired the entire trailer. Yet the original problem showed up again. Tested everything again with a meter... So I cut and spliced on a 12v wall turtle and plug my trailer into the 110v creating 12v 1000miliamps, and Bamm! All the lights work great. Everything tests out fine running light, brakes, each turn signal all light up. So I plug it into truck and no lights. I have taken apart, tested and cleaned the trucks female connector and it all seems fine. Grounding of connectors and trailer all test fine. Where do I start next to find the issue? Or should I just install a new connector on the truck side? My truck has an old trailer brake under the dash that has been there since before I owned it, could this be the problem? I have a brand new seven pin to four pin adapter that has the led lights. The lights show fine until I plug in trailer and the LED for running light goes out BUT the trailer lights all work when plugged direct into my 12v turtle from 110v. Any help is greatly appreciated. I am enjoying your videos and help site but dont find anything specific. Thank you !

asked by: Mark S

Expert Reply:

First, when lights work intermittently that is an indication of a ground problem. It could also be a loose connection somewhere or a dirty/corroded trailer connector.

Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light.

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

It is also possible that the trailer lights draw more power than the tow package wiring on the truck is capable of. You can determine this by limiting the number of lights that are connected and then plugging in the trailer to the truck. If everything starts working then that was likely the issue. The fix is to use LED lights that draw much less power.

The truck side connector could be dirty or corroded on the pins or where the wires attach as well. And it could be the adapter that you are using. I have included a link to the right that explains some testing methods you can employ to determine if the issue is on the truck side or the trailer side.

expert reply by: Michael H