Trailer Running Lights Not Working When Using a 7-Way to 4-Way Adapter

Updated 12/04/2024 | Published 11/05/2011

Products Featured in This Question

Pollak Replacement 7-Pole, RV-Style Trailer Connector Socket - Vehicle End

(739)

$14.85

Hook Probe Circuit Tester

(65)

$13.35

Question:

After installing a new 4 way adapter for trailers with a 7 way adapter so that I have a 7 way female for a plug in, I now have no tail lights or parking lights. I checked the fuse box, and it seems to be fine. Any ideas??

asked by: Gus

Expert Reply:

It sounds like you have a 7-Way and are using a 7 to 4-Way adapter because the trailer has a 4-Way flat. If the running lights are not working with this set up, the first thing you will want to do is use a circuit tester, like # PTW2993 if needed, and test the vehicle side connector with no adapter installed in it.

If the running light circuit is not working at the connector, you know there is a problem on the vehicle side, and you will need to trace the wiring from the trailer connector to determine what the problem is.

If it is working, then you need to compare the pin location on the vehicle side connector to make sure it matches up with the running light pin on the adapter. On a vehicle side 7-Way, if looking at the face of the connector, the pin in the 11 o'clock position is for running lights. All the adapters we carry will have the pin for running lights in this location (1 o'clock position if looking at the face of the adapter).

If your 7-Way and the adapter do not match up, you will need to re-pin one or the other, so they do match. If you have found that everything is as it should be, plug in the adapter and then test the pins on the back side of it.

If the running light signal is not coming through, you know there is a problem with the adapter. Depending on the adapter, you may be able to take it apart and repair it. If the signal is coming through, then you will want to check the 4-Way on the adapter to make sure it matches up with the way the trailer connector 4-Way is configured. If they are different, you will have to re-pin one or the other.

If everything is as it should be then you know the problem is on the trailer side. Most likely it is a bad ground at the trailer connector or with the lights themselves. All grounds should be connected to bare metal surfaces. Also, if you have a lot of lights that use the running light circuit, it may be too much power draw for the system and that is causing the lights to not work.

expert reply by: Michael H

Gus

11/22/2024

I cleaned a terminal because it had green and white corrosion on it.Slammed in the adapter harder.Because it was really tight and then running light started working

Bryce D.

11/22/2024

@Gus Glad to hear that, Gus!

Greg K.

12/4/2024

the 4 pin flat that is on my trailer worked perfectly when using the 7 pin connection on my 2017 silverado but it will not work on my 2025 siverado and when I plug into the 2017 with the adapter the trailer lights work fine any ideas - is there a need for a different adapter for the 2025 silverado

Kate F.

12/4/2024

@GregK Hello Greg, fist off congrats on the new truck! What adapter are you using? With the truck being so new, I am going to have to do some investigating. They are constantly trying to "improve" the electrical systems in the vehicles, so it might not be compatible. Also, what kind of lights do you have on the trailer, incandescent or LED?