How to Determine Function of Trailer Wires for New Running Light Install

Updated 07/20/2015 | Published 07/18/2015

Question:

I remove the lens cover from the old assembly and see a white wire loop and a green wire loop. These have metalprongs pushing into their insullation to mke the connection. The new units have one black wire into the back of the lamp housing and the other end of it bare. There is also another prong connection in the back of the housing that I am not sure what if anything attaches to it. Please help as I am trying to replace multiple of these units both Yellow and Red.

asked by: Brian

Expert Reply:

If you are unsure what the functions of your trailer lighting wiring are you can use a circuit tester like part # PTW2993 to test the wires for function while you have the vehicle taillight functions applied.

If there is only one wire on your new light assemblies they are most likely single illumination lights that power through the one wire and ground through the mounting assemblies.

Most likely the white wire of your old assemblies is the ground wire and the green is the power wire. You would need to run the ground to the install hardware of the new light so that the circuit would complete.

expert reply by: Jameson C