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Trailer Brakes and Wiring Installation

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How to Install Trailer Brakes and Wiring on a Trailer Without Them Previously

Today we are going to show you how to install brakes on a trailer that does not have existing brakes. It is actually pretty easy to do. It is just very time consuming. There are a couple of things you want to look out for before you start. You want to step behind your trailer, and look behind the existing hub you have. Make sure you have a flange like this on the axle. It is this 4-bolt flange on here; if you have that on your axle you are pretty much home free. This size flange has four bolts. You will see it on 3,500- pound axles like we have here. You also see it on 2,000- pound axles. On a 3,500 -pound one you would probably use a 10- inch drum and on a lighter axle, 2,000 pounds, you probably would use a 7- inch size. Also you notice we have the hub missing. We have to take that off because the hub and drum assembly is all in one piece. It is not like the automotive application. You have to take off the old hub, discard it, and start off fresh.

For this install we are going to install the backing plate. Basically it slides on like so, then it gets bolted onto 4 places. A couple things you might want to take note of is that there is a left and right side to it also an up and down. The magnet always stays pointed toward the ground. And if you did not have a sticker on your brakes that tells you which left and right is, just look for this bow right here. The actuating arm or bow always points to the front of the trailer, so that is a good reference point for you. At this point, let us go ahead and start bolting on our hubs. Works down in a criss cross pattern and make sure you have them evenly tightened. And at this point, you probably want to go ahead and do the same thing for the other side. One thing to note while you are adding the hubs to a trailer - take a look at the spindle. This spindle we have here, for example, we have what is called an EZ Lube spindle. It has a zerk fitting on the end. What happens is grease travels through the middle of the spindle and comes out this hole here. If you have that it is great, but the thing is when you get a new hub you want to make sure what kind of seal it comes with. Most of the hubs will come with a regular single- lip seal. You can see the just one lip right here. It has been around for ages and works great. However if you use this with an EZ Lube Spindle, what happens is grease gets pulled in and applies pressure to this. So what you want to do actually is get what is called a double- lip seal. As you can see here it has a lip on the inside edge here along with the lip on the outside too. Again, this is double protection that keeps grease in and has a little bit extra pressure. You can also tell it is a double- lip seal because you can see a little spring in there. Every double- lip seal out there is going to have that spring in it. So you have an EZ Lube Spindle or double check. Just to be on the safe side, go ahead and order a double- lip seal for your hub. You can see where our hub is sealed right on the edge of the spindle here – I guess you could say it is the bearing or seal surface. This is where you are inside bearing rides right here. [I am going to leave this out for a second.] As you can see here there is the hole where the grease comes out, so it is actually between this seal and this bearing.

When it is all put together this whole area is going to fill up with grease. Of course it cannot come from over top because your race is here. What happens is the grease comes up and fills up here first. Then comes through your rollers and actually into the middle of this cavity here. The whole thing gets packed full of grease. Once that is packed full of grease, the grease will actually travel through the rollers of the outside bearing and then come out through here and out by the nut. Once you have the grease full up to the nut you know that your hub is fully packed and is really ready to go. It is completely waterproof at the same time actually. At this point we can go ahead and slip on our hubs. We have our spindle lightly greased and we have our bearings already packed, and let’s go ahead and put them together. Some spindles like this EZ Lube spindle come with a special tang washer here. If yours has just the plain old regular spindle then you just have a cotter pin going through. In this case we have the EZ Lube spindle so we will go ahead and assemble it. And there will be one extra step involved. Make sure you take out any play. A little bit is fine - just a hair is all you need. Get it to stop then bring it back one notch. Then flip this little edge up into place like so. If you had a regular spindle we would be finished by now but since we have the EZ Lube spindle we will go ahead and use that feature to fully pack the bearings and bring out the grease to both bearings. Installing our hub is complete now. The next step is actually to adjust the brakes. From this point what we need to do is go ahead and adjust our brakes. Basically what we are going to do is just tighten up the drum until it stops turning. Then back off 10 clicks, and then you should have just a very fine light drag. What we do now is go ahead and start running the wires. We are going to follow the factory harness on back to the drivers side wheel. Then we are going to follow the axle on over to the passengers side wheel. Once you are by the axle here go ahead and leave a little loop for working room. Then take the rest of this and then make sure it is tied to your passenger side wheel. We will go ahead and at this point zip- tie the wire to the axle. At this point you have your wire set across the axle the way you want it so lets go ahead and pull the slack up toward the front of the trailer. We will go ahead and make our connections to the wires here. Make sure you note that one wire goes to ground and one wire goes to positive. Again it does not matter the magnet does not care – you just have to make sure that one goes to each one. We will go ahead and split our wire here. To make our connections there are a variety of different ways to do it. There are different types of connectors You can use the quick splice connectors like we are going to use here. You can also use butt connectors also. Basically it is personal preference. For clarity or ease of use we are going to use the quick splices here. At this point you can tape them up or apply sealant to them want. Again, its personal preference how you want to do it. Then once you have those sealed up, then you can just go ahead and zip- tie these to the axle and keep them out of the way so they do not get smashed.

We will do our drivers side now, and what we are going to do is install our ground on this part of the frame here. Actually you can do it everanywhere you want, but we are going to hide it back here and make it a little nicer, and then run our ground wire on up to the front. So we will just go ahead and open up the sheath again. We will then split our wire here and put two ring terminals on there. Now, what we did was we ran our wire and mixed it in with the loom up – that makes it look nice and neat. We are going to put our 7- pole connector on next. First you slide on the cover; then you strip these wires and add ring terminals to them so the connector fits inside. Now well go ahead and add our 7-pole connector here. Now, its tempting to do it color for color because you’ve got the red-yellow-green-brown-blue-black on here and they match up to here, but theres actually two different codes. Say, for instance, sometimes red on here is actually left turn, but on the trailer it is actually yellow, and so forth down the line.. The only common ones you have is white for ground wire and white on the trailer, and blue will be used as our black wire. And the black wire, is actually used on here is actually for the 12- volt power lead. We will start off with the ones that match up and that would be the white. Then well go right next door and do our brake wire. And again, on a trailer it is black on this one but its going to be blue on the 7- pole. Left turn on our trailer is yellow, but the left turn on our 7- pole is actually red. We have green here and that’s our running light and on the trailer it is brown. And then green is right turn on our trailer and that is brown on our 7- pole. Next we just go ahead and slipput our connector back in with the little screw. Then were going to fix our loom and were done. And then tighten the screw. With that our install is complete all we have to do is test it.



Any professional using scotchlocks on anything, let alone something as critical as brakes, should be ashamed of themselves.

Etrailer Expert

Mike L.


I agree. Soldering wire connections and using heat shrink tubing to protect those connections is definitely the way to go. However, some folks do use the scotch locks. Their long term reliability is questionable, but they DO work in the short term.



If this is a brake installation on a trailer wo existing brakes, shouldnt there have been a breakaway switch installed also?

Etrailer Expert

Jon G.


You are correct that there should also be a breakaway kit installed when electric brakes are added to a trailer that doesn't have brakes. I have linked our selection of breakaway kits, some helpful articles, and a sample installation video that can be referenced.



What about if the trailer has existing light wiring but you are adding brakes? How do you join the existing light wiring with the new brake wiring into the new 7-way plug? Thanks.

Etrailer Expert

Rachael H.


The easiest way is to use a junction box like # 38656. You can then route the trailer existing wiring to the junction box and add the necessary wiring from the 7-way as needed. I have also attached a link to one of our help articles that details how to add brakes to a trailer that did not previously have them.



So many steps are skipped or glossed over. No cleaning of back plate to ensure a good seat of brake assembly. No greasing of back seal surface. It was just mentioned that he packed the bearings, but with no demonstration. Once assembled, why use EZ Lube to pack hub full of grease when bearings are packed already? A hub full of grease just churns when towing, getting warm hot and decreases tow vehicle mileage. There are many reports of the back seals blowing out. Hot grease has to go somewhere. Are the Scotch locks on brake wires waterproof? Just taping them up, does not make them waterproof. Connections will not last anytime with salt in the north and in coastal areas. Same for the unprotected trailer ground connection- terrible. Beware of expert advice.

Etrailer Expert

Rachael H.


Thanks for your opinion on this video yes there are many items that were referenced in this video but were not shown, and that was done to keep the video reasonably short. We do have another videos that show how to properly pack the bearings, and I have linked those for your reference. For this instance the bearings were greased and then topped off using the easy lube connection. You are correct in that there are many reports of the back seals blowing out if there is too much grease in the spindle, that is one reason that we do recommend packing the bearings by hand. When using the easy lube zerk fitting always it is recommended to use a manual hand held grease gun and not using a pneumatic, or battery powered grease gun so that you can feel the resistance, and not overfilling the bearings and hubs since it is easy to do with a pneumatic or battery grease gun. The connection of the brake wires can be done using the Scotch Locks as shown. Another option would be to use butt connectors and heat shrink, that would be a more secure connection. For this install it was just the installer's preference to use Scotch locks, and tape as necessary.

Rob A.


The wire between the brakes broke on my trailer and I need to replace it. What is the recommended wire for this? Thanks!

Etrailer Expert

Rachael H.


Usually 12 gauge wire is recommended for the brake circuit, and connecting your brake assemblies. However if you have a long trailer with multiple axles and brake assemblies, then 10 gauge wire is recommended. I have included links to the wiring options, and you can order these by the foot.

Info for these parts were:

Employee Zack K
Video Edited:
Zack K
Employee Jeff D
Installed by:
Jeff D

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