Wiring A Trailer So That Turn Signal And Brake Signal Are Separated

Question

Question:

I currently have a single light on each side with 4 flat wiring. I want to install a second set of tail lights on my trailer. My goal is to have separate brake and turn signal lights. All four lights would function as tail lights, but one on each side would be the brake light and the other would be the turn signal. Is there a wiring adaptor that would do that? Thanks

asked by: David N

0

Expert Reply:

You will want to find lights that match the lights you currently have on your trailer. If you have incandescent lights I recommend either staying with incandescent or swapping out all of the lights to LEDs. LED lights draw much less power, so you may want to go that route. I have attached a helpful article that explains how the trailer wiring is run.

In order to separate the brake signal from the you will need part # 118158. This will take the signal from the vehicle that has the turn signal and brake signal combined, and split it into separate lines. I have attached a drawing that shows how the wiring should be done to accomplish this.

I have linked you to our product page for trailer lights you may want to check out.

expert reply by: Jeffrey L

0

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Bill W.

11/19/2025

I am trying to wire my boat trailer and very similar to situation that this other guy had, but I have standard four pin wiring harness with the standard LED lights for the back, but I am trying to also install side turn signals and side, running lights, though I have not done the side running lights yet the issue that I’m having is that when I hit the brakes, my brake lights go on as well as both of the side markers that are supposed to be used as side turn signals, I have that converter part number 118158 but I need to see a wiring diagram that works for having the side turn signals separated from the rear turn signals and brake lights, please send me a diagram so I wear it correctly. Of course, if I don’t hit the brakes and I just used to turn signals the side turn signals flash with the rear ones just like they’re supposed to. I saw a video on a guy on YouTube that made it work with that splitter but he also had a fuse black and a couple of relays if I don’t need to go through all that that’s fine but I need a diagram to understand how to wear everything so that my side markers, brake lights and side turn signs work like they should instead of working together. Thanks

Jesse M.

11/19/2025

@BillW Hey Bill, the issue you’re seeing is normal for a 4-pin setup since it combines the brake and turn signal circuits. The # 118158 converter can’t separate them on its own. To get independent side turn signals, you’ll need to switch to a 5-wire setup using a powered converter and relays or a fuse block, like in the video you saw. That’s the only way to isolate the brake lights from the turn circuits so each light works correctly.

William W.

11/19/2025

@JesseM that’s fine. I have a 2015 Ford F150 so I can do any of the wiring harnesses. I just don’t have electric brakes on my trailer but I have that converter and I have relays. I just need to see it in a diagram to make sense and understand how to wire it, so I can switch to a five cord wiring harness. I can do a seven it doesn’t really matter. I just need the diagram to understand how to wire it since my truck can fit any of the wiring scenarios. Please send me a wearing diagram that I can understand how to correctly wire everything so it works, YouTube videos are great, but somebody talking about it versus having something to see. I understand a wiring diagram a whole lot more than somebody telling me how to do it.

Jesse M.

11/19/2025

@WilliamW I am not seeing we have any diagram sadly but I can provide general instructions. Here’s the short version: You’ll keep your #118158 converter wired the normal way - white for ground, brown for running lights, green for right turn/brake, and yellow for left turn/brake. Then you’ll add two relays (one for each side) to split off the side turn signals. Use the green and yellow wires from the converter as the trigger wires for the relays, power each relay with a fused 12 V feed from the battery, and run the relay outputs to your side lights. That way, when you hit your turn signal, the relay sends power to the side light on that side, but when you hit the brakes, only the rear lights come on. If you also want the side markers to glow with your running lights, just tie the brown running-light wire into them with a diode or low-draw connection.

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Wiring A Trailer So That Turn Signal And Brake Signal Are Separated

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