Why Would Trailer Lights Work With Ground Wire Not Attached Then Stop When Connected

Published 06/15/2016

Question:

Hi, Bought a used trailer which had a 4-pin flat extension harness spliced/taped together from trailer harness to reach the tow vehicles connector and the trailer lights were working fine. I decided to solder/heat shrink the 4-pin flat extension harness wires the correct way. I noticed when I took the tape/splice connectors off the wires the white ground wire was just taped with the others and not grounded to ANYTHING or was any of the wire exposed to even possibly make a ground connection by chance…but the lights still worked? I soldered/heat shrank all of my connections 100 correctly, plugged the connector into my vehicle and grounded the white wire to the coupler and everywhere else I could fin a good ground and now I have no lights at all? How is it possible that the original owner didnt have the white ground wire grounded and the lights worked? I have my 12v from the vehicles connector pins for L/R/brake and also read continuity from the vehicles ground pin to the coupler…I just dont understand how in the world it worked without the ground wire and now I ground the white wire and it doesnt work? Going back outside to tackle the issue…in the meantime any explanation/advice would be appreciated. Thank you,

asked by: Joseph S

Expert Reply:

Some trailers will actually end up grounding through the coupler, to the ball of the ball mount, to the hitch, and then to the frame of the tow vehicle. It sounds like that is what you trailer was doing. Did the lights ever flicker when you hit bumps? That's a tell-tale sign that the ground is being made through the ball mount.

Did you have the trailer on the ball of your ball mount when you tested the trailer again? That could still be the way the trailer likes to ground and why it wouldn't work. What a lot of people do is run a ground wire back from each light assembly to the main ground harness to bypass grounding through the frame. You could try moving the ground attachment that you made back to the frame of the trailer instead of the coupler as well.

But your best solution is going to be to run a ground wire back from each assembly.

expert reply by: Jameson C