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What To Check When Trailer Running Lights Are Not Working


I installed all new wires yellow and brown to DS green and brown to PS all running to 7 pin junction box. I have blinkers but no running lights. Help


Expert Reply:

To start with, and this will sound very obvious so don't get mad, but you would be surprised how many times this fixes the issue, make sure your running lights are turned on. With that said, if they are indeed on and you don't want to beat me up, lets get into the more complicated stuff.

First and foremost, check your ground connections. I have found that running lights are quite sensitive to bad grounds and often this is all it is that's causing them to not work. I would also suggest not having any of the grounds stacked as this can cause one or more to not seat properly and it is always best to have each ground on its own.

If all your grounds look good, it is time to start checking wires for power. You will need a tester, like the # PTW2993, but I suggest starting this search at your junction box. You will want to make sure you are getting power to the post designated for the running light function and I included a quick diagram of how these are typically wired. If you are getting power to the running light post, skip to the next paragraph. If you are not getting power to this post, you will need to move up the line to the actual 7 way plug on your vehicle.

You will need to check the 11 o'clock position on the 7 Way on the vehicle as this is the one that handles the running lights. If you do have power at this port, there is a break somewhere in the 7 Way wire on your trailer and the easiest way to proceed will be to replace it with something like the Hopkins # H20046. If you are not getting power here, you will again need to move up the line to the fuse box in your vehicle as odds are, the one for this function has blown. If that fuse is good, or if you have an aftermarket 7 Way that was added on at some point, you have a break in the wiring some where and you will need to find and repair it. If your vehicle has a factory 7 Way, it will be better to take it to an auto shop to have repaired as it will be very difficult to trace.

If you did have power to the running light post, you will need to trace the running light wire back, testing as you go, as there is a break somewhere and it will need to be repaired.

In the off chance that non of this gets you back up and running, let me know what the year, make, and model of your vehicle is and if the running lights are LED or not. I may be able to glean some information from that and try something else.

7 Way Junction Box Wiring Diagram
7 Way Junction Box Wiring Diagram
(click to enlarge)
expert reply by:
Lazer S

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