Troubleshooting Non-Functioning Trailer Tail Lights

Question:

I have a 5 prong plug which has yellow and green for the turn signals. There is also a green/brown and yellow/brown which I assumed to be brown for tail lights. When I plug into said portal there are no tail lights i.e. when headlamps are on. When I run green/brown together with green the side lights do work with the turn signal but still not with the headlamps. My problem is getting the tail lights to work with headlamps on.

asked by: Carl S

Expert Reply:

It sounds like you may be using a 5-pole wishbone flat trailer connector such as # A25W5B. This has dual brown running light wires to give you a separate wire for each side of the trailer.

You can disconnect the trailer and vehicle and test each one individually; this will help isolate the cause for the problem. You can test the trailer directly with a 12V battery (use the vehicle's or a spare) to see if the running lights behave properly. If the running lights work when powered directly in this manner, then move on to test the vehicle's wiring. If the running lights don't work when powered directly in this manner, then inspect all the wiring from the connector to the lights looking for signs of cracked or worn insulation and repair or replace as needed. Make sure all connector contacts are clean and free of dirt or discoloration.

You'll use a tester like # PTW2993 to test the output from the vehicle's harness while a helper activates the various lighting functions. Please see the linked video. If the vehicle is generating the correct signals, then you can focus on the trailer. If it does not, then check the main ground wire for the vehicle wiring harness to ensure it is firmly attached to a clean bare metal surface. Clean or relocate that grounding point to ensure a solid contact. Sometimes a weak ground will let certain lights work while others do not.

expert reply by: Adam R