Troubleshooting Trailer Lights Not Working on Gooseneck Trailer

Updated 09/17/2025 | Published 12/24/2018

Question:

The lights on our trailer quit working when we changed trucks. All the connections have been checked, ground/turn/tail lights/etc. - all good. We did discover the trailer is not grounding thru the ball as we assumed so we tied into the ground wire coming from the breakaway battery and attached it to the trailer. This worked except that the breaks locked when we attempted to move the trailer. We unhooked the added ground and am now back to beginning - no lights. We have pulled other trailers a 5th wheel with this same truck and everything worked, just not with this gooseneck.

asked by: CATHY C

Expert Reply:

If the truck wiring is working with other trailers then the issue is definitely on the trailer side. Although the hitch ball does provide an extra ground source, you do not want that to be the main ground for your trailer wiring. You should have a main ground wire somewhere that is connected to a clean, bare-metal surface. If this is not the case then that is the culprit and you need to fix it. The ground wire can be found and traced from the pin in the 5 o'clock position on your trailer side 7-Way (see attached) and should be grounded to your trailer frame. The breakaway battery won't be the cause of your trailer lights not working as it is only connected to the trailer brake circuit.

Also check out the 7-Way connector to see if there is any corrosion on it. It is very odd that all of your lights would stop working at the same time on the trailer. If the 7-Way or the wiring from it has been damaged then that would make more sense. If all of that looks good then the next step is to add 12V power to each of the pins on your trailer. I have attached a picture of the pn-out for a typical 7-Way connector. Using a 12V battery (like what is in your pickup) ground the negative terminal and then use the positive terminal to see which functions are working properly.

I have attached some wiring helpful articles for you to check out. Let me know if this troubleshooting doesn't fix the problem and I can help you further.

expert reply by: Jon G

click to enlarge

Rjs

11/29/2024

I have a load trail gooseneck. Dump trailer that has light issues. When you step on the break, the right taillight and brake light. Come on, but the left one does not, but when you turn the lights on, everything works fine. Could you please give me a suggestion? White, what might be wrong?

Jon G.

12/2/2024

@Rjs Sounds like you've either got a bad ground on your left brake light assembly, or a bad connection that you'll need to check and make sure it's secured and without damage. Trailer tail/running and brake lights are on different circuits which is why you're seeing one function work and the other function doesn't on the same light assembly.

Richie

9/15/2025

How to check the power cord that plugs into your bed to the trailer with multimeter or is it better to use a test light

Jon G.

9/16/2025

@Richie A 7-Way tester, like the Curt # C73NR, is definitely the easiest way to go, but a multimeter tool like part # PT89ZR can be used by setting your multimeter to the "volts" setting and then putting the black probe into the ground pin slot and using the red probe to check the functions of the other 6 pins.

Richie

9/16/2025

@JonGno power to the brake controller

Jon G.

9/17/2025

@Richie To the brake controller? Or from the brake controller? I'm guessing it's from the brake controller since we're talking about the socket in your truck bed. Did you try applying the brakes with the manual override while you tested it?