Step by Step: How to Rewire a Trailer with Lights and Electric Brakes 2


Check out a step by step guide to rewiring a trailer end to end with new lights, wires, and a junction box. LOTS of pics included.

I semi-permanently borrowed my Dad’s trailer a couple of years ago and haven’t given it back to him since. Thanks Dad πŸ˜‰ The problem is he got it when I was about 10 years old and the wiring is sketchy at best. It sat in a field for 10+ years and has been chewed on by mice and cows. I briefly tried to diagnose the lights before realizing how bad the wiring actually was. So began the project of rewiring the whole trailer end to end.

Parts needed

LED Light kit

I started by going to Amazon and grabbing a new LED light kit. This is literally all you need if your trailer doesn’t have brakes. Don’t mess with bulbs anymore, LED’s are just as cheap now and these things are BRIGHT!

Butt connectors

The above trailer light kit comes with a bag of t-splices and wire nuts… THROW THEM AWAY. Then go buy some real heat shrink butt connectors that won’t come apart at the most inopportune time possible. These are good to keep in the shop anyways because you will need them again.

Junction box with RV plug

If you trailer has brakes, I highly suggest you get a junction box like this. Feeding the wires into the tiny RV plug adapter and tightening all the screws sucks, this avoids all of that and gives you a professional looking adapter to plug into your truck.

TIP: Be sure to pay attention to the FUNCTION, not the color in the junction box. The RV plugs come with different color wires than what you would expect, but each terminal is labeled with the function which is helpful. Took me a minute to figure this out.

Tools needed

You really don’t need many tools to do this. A basic set of hand tools and wire cutters/crimpers should cover most of it. I got a little fancy and busted out my welder, but it wasn’t necessary.

I bought a new tool for this project that ended up saving A LOT of time. These strippers automatically grab the wire and strip off the insulation with one squeeze. There is an adjustable backstop to make it consistent if you like to strip off more or less of the insulation.

Remove the old wiring

I was cautious about this at first until I realized how messed up the existing wiring actually was and that there was no point in trying to diagnose it further. Most importantly the insulation on the wiring gets old and starts crumbling when you try to work with it. It just isn’t worth it.

This isn’t very complicated and you will be able to figure this out with a good diagram. Just grab your cutters and go after it!

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
I didn’t take a very good picture but there is 20 years worth of electrical tape and wire nuts all wrapped up in this connector.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
This was the trailer brake wire! Believe it or not, it still worked.

Getting access

Safety first people. Put the trailer on a hard flat surface, hook it up to a vehicle and/or chock the tires so it doesn’t move, and make sure the wiring is disconnected from the tow vehicle before you cut any wires.

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
I started by hooking up to the trailer and pulling the front (non braking) axle up on some oil change ramps. This boosted the trailer up a couple of inches so I had more room to work under it. I kept the trailer hooked up to the vehicle and parking brake on for safety.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
You can see here that with the front axle up on ramps the back tire is just barely off the ground. This gave me more room to access the wires on the back of the brake drums.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Good riddance!

How do I wire the trailer brakes?

The trailer brake light kits don’t come with wires for brakes, so we are on our own for figuring this part out.

I think the reason the brake wire isn’t included is because it depends on your trailer setup. In my case it is a relatively short 16′ utility trailer with only one braking axle. If you have a super long RV with triple axles and brakes on each axle, you will need a heavier duty wire.

I ended up using 12 gauge wire which is plenty and probably a little overkill for my setup. 12 gauge should also be sufficient if you have two braking axles. However if you have a really long trailer, or it is heavy duty in some other way (7k lb axles, skid steer hauler, etc.), you should probably be on the safe side and use something closer to 10 gauge wire.

I found that Home Depot carries some 12/2 braided wire (don’t use solid or romex type house wiring in automotive applications) that is labeled as ‘Irrigation wiring’. It has a really tough insulation and is made for being buried, so I bet it will be acceptable here. It was ~.70 cents a foot when I bought it, so I was able to get out the door for around $15.

Most diagrams I found just had a 12 or 10 guage wire running to one terminal on the brake solenoid and the other side attached to the frame for ground. I went a little overkill and ran two 12 gauge wires to the brakes, one for power and the other for ground both of which were hooked into the junction box.

Mounting the junction box

I wanted this junction box to be secure, so I got fancy with it and busted out my welder. This is not necessary though. Find a non-load bearing piece of steel then drill a hole in it and either put a bolt through it or use a Nutsert.

Don’t know what a Nutsert is? Check out my other article: What is a RivetNut / Nutsert tool and how do I use it?

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
First, I found a good spot on the frame underneath that left me plenty of cord to plug into the vehicle. I then cleaned off the rust with a flap wheel on my angle grinder.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Next I held the junction box up and marked where the two studs would be. I welded two 1/4″ bolts on (they are just tacked in this pic, I finished welding them later), but you could have just as easily drilled through for a bolt. Just make sure you don’t drill through a load bearing member that would be weakened by having a hole in it.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Here it is in it’s new home. I spray painted the frame black behind the junction box before bolting it on for good. I also used nylock nuts so it wouldn’t come loose.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Once I got the box ready for mounting, I brought it back up so I could pre-wire most of it without working on my back.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Once I got all of the connections made and the junction box mounted, I stretched the trailer light and brake wires out across the driveway to make it easier to string through all of the crossmember holes.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Picture of the wiring running through the frame to the back. Yes it ‘should’ be running through grommets but the wiring was this way for the first the first quarter century, so I think it will be OK. I did clean up the sharp edges on the holes with a step bit though.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
I mentioned earlier to wire based on FUNCTION, not color. For whatever reason the 4 wire trailer lights and 7 wire RV plugs differ on their colors for a couple of their functions.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Diagram of the 7 way RV plug functions and colors vs a 4 way flat plug.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Hookup at the light. Notice the ground wire just goes right to the tail light bolt.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Here is the connection from the brake power/ground to the brakes. The black one that goes into the axle tube goes to the brake on the passenger side.

Yes… I know I told you to throw away all of the wire nuts and there is clearly two in my picture above. The wires coming off the solenoid were REALLY short and I was afraid to pull off the wire nuts that had been on there for years for fear of creating a lot more work for myself. I’ll put this on the future to do list when I have more motivation.

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Here is a picture of the passenger side where you can see the wire coming out of the axle tube and hooking up to the wires from the brake solenoid. This is original and I didn’t have to touch this wiring.
Ugh, t-splices…but they are still working so I’m not touching them until I have to.

Wiring Function/Color list

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Once I tested to make sure all of the functions operated correctly, I screwed the cover on and was done with this part! Looks so clean…

Notice the picture above a wire comes out and attaches to the stud, it is a black 12 gauge wire but I put some white electrical tape on it for future reference. This is to give the junction box a ground to the trailer. The box is plastic and wouldn’t ground otherwise, It is not good practice to rely on the ball to ground the trailer back to the vehicle ESPECIALLY on a trailer with brakes. Please properly ground your junction box.

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Crikey! look at this 7 way RV plug in its natural habitat. What a Beauty!

Trailer Registration Holder

Here is a neat tip from when I used to be a fleet manager. Commercial trailers usually have a handy holder at the front to hold DOT and registration documents. These things are cheap and waterproof. A couple of self tapping screws and you never have to worry if you are driving the vehicle that has your trailer registration or not. This is especially useful if a lot of people borrow your trailer.

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Trailer registration holder mounted in front of the jack. When drilling through thick metal always pre-drill your holes before putting in your self tapping screws (or you will just snap them off).

Conclusion

Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
All finished up and ready to haul PJ to KS Rocks this weekend!
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
The pre-terminated junction box makes this part so easy and clean. I now have one weatherproof and compact wire instead of the birds nest it was before.
Step by Step: How to Rewire a Trailer with Lights and Electric Brakes
Don’t take up offroading if you don’t enjoy working on things. I spend WAY more time working on projects than I do actually driving them, but I love every minute of it.

This is a job that is totally DIY-able. A little bit bit sweat and some new parts from Amazon you can save yourself a lot of money. Don’t be afraid of it.

Hope this helps you with your project. Now YOU, go outside and work on something!


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2 thoughts on “Step by Step: How to Rewire a Trailer with Lights and Electric Brakes

  • Mike

    Brilliant article. Unfortunately on my last RV trip i didnt realise the electric hookup was dragging on the ground so for probably 50% of the journey I had no brake or indicator lights. Thankfully the actual brakes were still connected. I cut off the frayed section and added on a one foot section of new lead, colour to colour, 7 marettes on each side. My wife said it look like the corona virus. But it worked and got me home. I now have a new 8 foot lead to connect. I haven’t looked yet but I hope there is a junction box under the trailer/RV. Initially I was going to hook up colour to colour but you advised against this and I can see why but how do i test what each wire does? I am not on your level of expertise. Thanks for any help.

    • Paul Post author

      Sorry to hear that. Check out the table in this post where I listed out the common wire colors and their functions. You can hook up the wires without a junction box, I just suggest you make sure to use weatherproof butt connectors.