Bear with me. Last year coming through WV my truck and trailer brakes got very hot and we had quite the scare. The brakes on the truck got repaired and now onto the trailer. The trailer has 2 axles. It only had brakes on one axle. The rear axle has flanges for backing plates, so I added brakes to the other axle. I rewired both axles with 12 gauge trailer brake wire. The front axle wires come down the left side and piggybacks from left to right. The rear goes down the right and piggybacks from right to left. It is all brought together up front in a junction box. You also have an emergency brake battery on board the trailer. Battery power and ground enter the junction box from the truck (along with all others). Power is jumped to a mini charger for the E brake battery. One wire to the positive of the trailer battery, one to positive from the truck and the other goes to the breakaway switch. From the breakaway switch to the brakes. I got the brakes done and went to burn them in. BARELY any brakes. I turned up the brake control and I have nothing but the left front locking. Back home, I checked all wires and connections. Good. I jacked up the trailer and pulled the breakaway. Nothing but left front, and not fully locking at that. Truck connected or disconnected. I disconnected the trailer battery and tested it. 12.86V. I'll skip a few steps here, I ended up putting my jump box in place of the trailer battery. Truck still disconnected. Pull the breakaway, and I have all 4 brakes. Put the breakaway back in, hook the wire back to the truck and hit the brake control, all 4 brakes. Trailer disconnected, or connected I still have power and ground at the trailer connector on the truck. Which powers the charger for the mini battery. I have power to the charger, and it has a working output for the battery. Everything is working. Except for the battery that still has almost 13V, that I replaced with my jumpbox. I went to the store in utter disbelief. Bought a battery, put it in and the brakes work. I just don't understand. How does the EMERGENCY brake battery play any role in whether or not the applied brakes work? It works, so I'm happy. But I need to understand this. What the heck is going on here. Is this common knowledge and I'm a big dummy? 😆
Electric trailer brakes (drum)
Kefsn
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Bear with me. Last year coming through WV my truck and trailer brakes got very hot and we had quite the scare. The brakes on the truck got repaired and now onto the trailer. The trailer has 2 axles. It only had brakes on one axle. The rear axle has flanges for backing plates, so I added brakes to the other axle. I rewired both axles with 12 gauge trailer brake wire. The front axle wires come down the left side and piggybacks from left to right. The rear goes down the right and piggybacks from right to left. It is all brought together up front in a junction box. You also have an emergency brake battery on board the trailer. Battery power and ground enter the junction box from the truck (along with all others). Power is jumped to a mini charger for the E brake battery. One wire to the positive of the trailer battery, one to positive from the truck and the other goes to the breakaway switch. From the breakaway switch to the brakes. I got the brakes done and went to burn them in. BARELY any brakes. I turned up the brake control and I have nothing but the left front locking. Back home, I checked all wires and connections. Good. I jacked up the trailer and pulled the breakaway. Nothing but left front, and not fully locking at that. Truck connected or disconnected. I disconnected the trailer battery and tested it. 12.86V. I'll skip a few steps here, I ended up putting my jump box in place of the trailer battery. Truck still disconnected. Pull the breakaway, and I have all 4 brakes. Put the breakaway back in, hook the wire back to the truck and hit the brake control, all 4 brakes. Trailer disconnected, or connected I still have power and ground at the trailer connector on the truck. Which powers the charger for the mini battery. I have power to the charger, and it has a working output for the battery. Everything is working. Except for the battery that still has almost 13V, that I replaced with my jumpbox. I went to the store in utter disbelief. Bought a battery, put it in and the brakes work. I just don't understand. How does the EMERGENCY brake battery play any role in whether or not the applied brakes work? It works, so I'm happy. But I need to understand this. What the heck is going on here. Is this common knowledge and I'm a big dummy? 😆
I have 3 two axle trailers with brakes on all four wheels and I know the controller in the cap has a rheostat built in you cannot supply 12 volts to the wheels without locking them up, usually 6-9 volts all that is required. My 24 ft enclosed trailer is the only one with a disconnect and it has a 9v battery. Most open trailers only have brakes on one axle when I built my 16 and 18 foot open trailers it only added about $50 to the build for two axle braking.
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read this Keith---
Will Trailer Brakes Work Without Battery : Major 6 Reasons
If your trailer's brakes stop working Without Battery, don't PANIC! Here are six major reasons why they might not be functioning and what you can do about it...
www.shootingstar-rvresort.com
I was just pulling the disconnect for testing purposes. I never applied voltage directly to the magnets. It does have a 12V 5.0 AH battery in it. Same size it's always had. I just don't remember having problems like this the last time it needed a battery.
I guess the main reason I'm confused is that the battery I took out still has voltage. I do not have a way to load test this little thing.read this Keith---
Will Trailer Brakes Work Without Battery : Major 6 Reasons
If your trailer's brakes stop working Without Battery, don't PANIC! Here are six major reasons why they might not be functioning and what you can do about it...
Attachments
You know... Unless I've got a bad connection on one of the battery wires. I did peek at them. Maybe I didn't pay close enough attention.
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just what I was gonna say, during switching things around sometimes things kinda fix themselves LOL--I've never owned a trailer that required anything but the controller in the truck. Is your controller built into your truck or is it an add on?
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AND, things that "fix" themselves always make me nervous
You ain't kidding! 😂 Factory controller. Part of what I left out in my diagnosis, I have a brake force meter that plugs into the truck. It is powered on by the truck etc. It's scale is 1-10. With my truck turned up to 10, it ALMOST reaches 10 on the meter. So I believe the truck is ok.
Yeah, they tend to unfix themselves. LOL
First for me is to add brakes to the second axle. They are cheap and easy to install. Is the brake controller working? Is it adjusted properly? Check and adjust the brakes at each wheel the same as car drum brakes. If only one brake is tight, it's highly possible that only that one will work. Test them. Have someone in the truck manually turning on the brakes. At each wheel you should hear a faint buzzing noise. That is the PWM, pulse width modulation supplying power to the brake coils. The main reason they work with full battery voltage is because they aren't really made to work that way, back to the PWM controller. It only gives limited power to each brake. The battery gives the full 12 volts. The new battery? Probably dumb luck it works. The break away switch supplies a full (non modulated) 12 volts to the brakes.
To make a long story short, adjust the brakes, then adjust the controller.
To make a long story short, adjust the brakes, then adjust the controller.
The brakes were adjusted before any of this was noticed. And the controller is working. I have a brake force meter to prove that. Once the battery was replaced, the brakes worked great. I think I had a bad connection at the battery. Because the one I pulled still has 12.75V.
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For those reading... 90% of most two axle trailers only have one set of brakes on one axle and is stupid cost saving deal when you buy a trailer they really need them on ALL axles IMO. Adding the second set to the other axle doubles your breaking as well as takes the load/wear off the other as well as your truck brakes. These are drums and need adjustment as they wear or checking adjustment in the least. What I have found in working on few hundred basic trailers with brakes is they wear out then don't engage at all and never get adjusted.
The battery is just for the breakaway switch. So it applies 12v to mag for a few mins to stop the trailer if it separates from truck. The battery is not need for normal operation and brakes will work with our with out it. On the truck there is a manual override lever to slide to apply 12v to the brakes. If you have the gain down it will not apply full 12v to the brakes. The overide will. So if you have gain down and just hit brakes on truck then that maybe way they don't work completely. Also other factor is grounds. The truck and trailer need to be grounded better then just the hitch. It needs the white wire grounded to trailer and verify it on truck. So that can be an issue as well. You can check that axle is grounded as well at each drum.
The battery is just for the breakaway switch. So it applies 12v to mag for a few mins to stop the trailer if it separates from truck. The battery is not need for normal operation and brakes will work with our with out it. On the truck there is a manual override lever to slide to apply 12v to the brakes. If you have the gain down it will not apply full 12v to the brakes. The overide will. So if you have gain down and just hit brakes on truck then that maybe way they don't work completely. Also other factor is grounds. The truck and trailer need to be grounded better then just the hitch. It needs the white wire grounded to trailer and verify it on truck. So that can be an issue as well. You can check that axle is grounded as well at each drum.
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gonna have to rewire my trailer brakes completely--seems those who borrow it don't care what they run over so all the wires are broken going to both axles AND now have a bent ft axle and a bent fender.. Had to replace both ft axle tires as the insides were worn slick and belts showing. End of loaning to just anyone. Either neighbor is welcome to it but no one else. Hoping my local machine shop can straighten the axle.
Which is why I am baffled as to why they wouldn't work until I replaced the battery. The brake force meter tests the "integrity" of ground on the truck also. And power and ground is supplied from the truck, through the wires. Not the hitch. That's something I've never done. Always through the harness.
Just like a chainsaw. NO ONE BORROWS IT.
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Depending on how its wired it may have needed connected to battery to even work hard so say without physically seeing it. The breakway switch is what sends 12v from battery to the brakes. But again hard to troubleshoot something in words.
As far as ground main issue I see is guys get a good ground static but are grounded via the hitch and it bounces around and grd fails. I usually tell people to disconnect trailer completely just use wire harness that test grd 100%.
I drew a diagram. It's a little hard on the eyes. LOL! If you can follow all my lines. This is how it is wired.
![Image]()
Please don't take this as me debating with anyone. I want to understand why. If something is wired wrong, or in the truck still somehow. And I want to fix it if it needs a fixin. Or if this is normal for the way it is wired. But I don't think so. Thank all for input. Again the brakes are currently working, I would like to understand what electrically was going on.
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