Trailer Running Lights Only Work when Interior Lights are Turned On

Published 09/24/2015

Question:

Hi All, I recently installed the Hopkins 7 pin on my Featherlite enclosed. thanks for the guidance. I also confirmed pin mapping versus the old connector with a multimeter. Unfortunately, it looks like I have some other wiring issues on the trailer end. To isolate the variables, I have disconnected everything so I can test each line using a 12v battery. At the moment it is the running lights that are causing me grief. I have connected the white wire to 12v battery negative. I have the brown running light wire connected to 12v negative. No other wires connected at this time. What is occurring is: - No lights at all UNLESS I turn on the toggle switch for the two 12v interior lights. - With interior toggle on I get power to the front marker lights. The rears do not light up and only show about 2 volts with meter. Where the white wire enters the trailer it connects to the aforementioned toggle switch. The other side of that toggle switch has a blue wire going directly to the interior lights. I would have thought that if white = ground that it would simply be attached to the trailer frame / chassis. The brown wire enters the trailer and then is tapped with black wires going to the marker lights. It runs all the way back to the tail lights. It is not even clear to me how the white wire is powering the interior lights. There is no second wire to those, the fixtures simply attach to the frame. Hope this gives enough info to provide some insight.

asked by: Mike I

Expert Reply:

If the white wire is the ground then what you might be experiencing is a short to ground. But the main issue here is that the interior lights need to be wired from the switch to the battery independently of any of the exterior lights.

After you have isolated the interior light wiring from the external light wiring the problems may heal or it could reveal additional wiring problems. Be sure that all of the lights are grounded to clean and corrosion free bare metal surfaces.

Try to inspect the trailer wiring as best as you can and fix any broken or pinched wires.

For the interior lights, the power wires will go from the lights to the appropriate terminal on the toggle switch. From the correct terminal on the switch you would then run a wire to the positive battery terminal. And if there is a ground terminal you could route it to the negative battery terminal which would work best. The exterior running lights should not be wired into this system and I do not recommend grounding anything else with the interior light wiring.

expert reply by: Michael H