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Trailer Brake Controller Wire Testing

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How to Test Wiring on a Brake Controller for a Trailer


Today we are going to cover a few notes on testing an electronic brake controller. What we are going to cover here is going to apply to virtually all brake controllers. Basically, if you run into a problem where you think your brake controller is not working, well, it could be a the brake controller or it could be something in the truck itself, in the connector or c it actually could be problems in the trailer. What we are going to show you today is how to look for problems that will either rule out the brake controller or make sure that it is the brake controllers fault. Just like any electronic devices, they seem to have a limited lifespan some will work for years and some will not. But this way you can also verify that it is working good.

All right, the steps involved to test a brake controller are going to be pretty easy. You need one definite tool you need a light tester, like we have one here. Basically you ground it onto the body of the vehicle and you use this to probe the wire you are going to test. And it is best to use an incandescent bulb because a lot of times voltmeters do not put out sufficient load to actually activate the brake controller itself, because brake controllers are activated by the magnets that are on the trailer, and thus a load. So you can replace that load with a simple incandescent light bulb. So one of the first things you want to check if you suspect your brake controller is pretty simple. Go ahead and make sure you have power going to the brake controller.

So we will our ground our tester, and the black wire is going to be positive so we will go ahead and test it. Okay, we know we have got power going in, so that means you have a good 12 volts going into it. The next thing you want to double-check is the red wire. That is typically the signal that is coming off the brake switch. Again, the same thing applies. So go ahead and put your tester on that and then hit your brake switch. And if you have got signal going to it, then that part of the circuit is fine, too. If you do not have power going, then obviously your brake controller is not going to work when you hit your foot brake. Now the last thing you want to do is go ahead and test the blue wire. Now the trick is, with this, you want to go ahead and disconnect this blue wire from the rest of the vehicle. The reason for that is that it eliminates any problems with the connector on the back of the vehicle or the trailer itself. That way you verify you have output coming from the brake controller itself. So you test that, and then press the manual override. And then if you have a time delay brake controller you can do the same thing with the foot brake, kind of an additional little test you can do.

If all that is working, then your brake controller is fine, and there is something else usually down the road from that. So basically the next you want to check is to make sure your blue wire is connected to the harness securely, and then go out to the back of the truck to the rear connector of the truck. Again you probably want to have somebody manually activate the brake controller and make sure you have current from the blue wire out the end of your connector out back. Now if that is all fine, then you know there is something definitely wrong with your trailer, and you have to start troubleshooting from there.


John M.

8/20/2021

I am having an issue with my brake controller. Its a draw tite controller. It sends 12 volts back to the plug when the trailer is not hooked up. Once the trailer is hooked up it will not send 12 volts. The light on the controller barely lights up. I have replaced the plug on the truck and just installed a new plug and wire kit on the trailer also. If I unhook the brakes from the wiring the controller puts out 12 v. Once the brake is hooked back up it won't. We hooked the magnet from the first wheel up to 12volts and it works. The only thing I can figure is the controller won't push out 12v when under load. Does this make sense that the controller is bad? Thanks for any replies.

Victoria B.

8/25/2021

I agree, it does sound like the controller doesn't want to send power to the brakes when actually connected to the brakes pulling power. Before replacing your brake controller, I would check the ground for the brakes as well as the ground for the controller. If the ground is loose or has some corrosion, that could be causing this failure when the brakes are connected. If you find this, reattach the ground to a clean, bare metal spot and see if this solves your problem before replacing the connector.

John M.

8/25/2021

@VictoriaB Thanks. I replaced my controller and it fixed the issue.

Scott S.

8/18/2021

I have a 1997 Ford F-350, I have 12v on red wire when pressing the brake pedal and manual override and nothing when sitting there. I have 3.5v on blue wire with nothing being actuated, when I press the brake it flashes to 5v then back to 3.5v, it also doesn’t have the red light to indicate the trailer is there anymore. I have replaced trailer battery, cleaned grounds, replaced all brake assembly’s not to long ago. Controller issue or look somewhere else?

Victoria B.

8/25/2021

With nothing being actuated, you should not have any power pull on the blue wire. I would check your brake controller ground to make sure it is still connected to a clean, bare metal spot on your vehicle. If the ground is still good, then I would go through the process outlined in the linked article to really determine where the issue on the blue wire is - the brakes, the trailer connector, or at the controller.

Dave C.

7/26/2021

2016 E450 Motorhome, worked fine when I left on trip, now no power on red wire ? So you are saying it comes off of brake switch ?

Victoria B.

7/27/2021

The red wire coming out of the back of the brake controller is typically connected to the stoplight switch on the tow vehicle. In your E450, the brake signal wire was pre-ran by the factory so it may not be visibly spliced into the brake switch. The red wire should ONLY have power when the brake pedal is pressed. If you are not getting power on the red wire when you press the brake pedal, you will need to check the connection between the brake controller red wire and the pre-ran brake signal wire on your 2016 Ford E450 to make sure the wires are still securely connected.

Jeremiah R.

6/12/2021

I have a similar issue on my 2010 Nissan Armada. Purchased the connector and Curt Triflex controller through etrailer. With an RV travel trailer connected as soon as I apply the brakes in the tow vehicle the trailer brakes lock up and the controller flashes the signal for a disconnect trailer. Tried with two different trailers with same results. I tested the controller and found the blue wire has 12v constant, red wire is near 0 and then 12v when I press the brake or slide the manual control. Reading this thread it appears the controller is bad, I'll replace it this week and try again next weekend. Thank you for the video and notes!

Kenneth T.

5/26/2021

i am trying to install a brake controller in my truck and im having all sorts of problems. I have a 2018 Chevy Colorado Z71 V6, Brake controller is a Hopkins agility. After hooking everything up and connecting my trailer my trailer brakes activated and i couldnt do anything to release it unless i disconnected the controller or pulled the trailer brake fuse from the fuse panel. Think there may be an issue with the connectors i cut all the wires to hard wire them in place but received the same issue. The truck has four wires tucked away to be used for the controller: Black=ground, Red=Battery, White/Blue=stoplamp, and Blue=7way plug. The controller has the basic four wires: White=ground, Black=Battery, Red=Stoplamp, Blue=Controller signal. I have connected them inthe format Truck>Controller: Black>White, Red>Black, Blue>Blue, White/Blue>Red. As far as i can tell i have hooked everything up correctly so i started trouble shooting the wires indivually. First thing i notice is when using a multimeter on the White/Blue(stoplamp) it has a constant .50v and when i press the brake it shoots to 12v. Second i wanted to test the controller so i hooked up the Black>White and Red>Black to power the controller and provide it a ground. I left the blue wire(controller side) disconected and tested it with a multimeter. The blue wire coming out of the controller is sending a constant 12v, The blue wire on the truck going to the 7 way is read it 0v. Question 1: I am now at the conclusion that the controller is bad because i am under the assumption the the blue wire coming out of the controller should not send any voltage unless the red wire is connected aswell and it receives a signal from the vehicles brake pedals or the blue wire would receive a signal if the stop switch on the controller is pressed. Am i correct with how i hooked up the wires and my conclusion the the controller is bad? Question 2: The White/Blue wire on the truck side is the stop lamp switch and should not receieve any voltage unless the brake pedals are pushed. It does shoot up to 12v when pushing the brake pedals but the constant .50v is what confuses me. Do i need to worry about this .50v or is this amount nothing to be concerned about? Since its not reading at 0v, does this mean i have a short somewhere along this wire? If so, how would a novice go about finding and correcting that issue? I traced the white/blue wire back to this big plastic multiwire connector but i have no idea how to discconect wires from it or test the different connection slots. Thanks for your time!

Etrailer Expert

Jon G.

5/26/2021

I think you may have a bad controller. The blue wire output from the controller should only be showing 12V power when the brake pedal is pressed or the manual override is used. Since it's showing constant 12V power that means something in the controller is telling it to send all of the power. I don't think that 0.50V power on the stop light switch is the culprit but if you have a buddy with a brake controller you might be able to hook their controller up to see if you see any difference.

Kenneth T.

5/26/2021

@JonG I went ahead and traced the stop light wire to see if I could find the culprit. The wire comes from the brake stop lamp and then is spliced into 2 directions. 1 part goes up into the dash through the factory taped wires that I couldnt trace and the other part is were I was connecting the brake controller. I cut the connection that is going further up into the vehicle and using the multimeter that is the part that was bring in that constant .50v, not sure what it's for or from. Now I am just connected from the vehicles stop lamp switch to the controller and reading at 0v unless the peddle is pushed. Also went and bought a new brake controller and just hooked everything back up and all appears good. Only concern now is what to do with that wire that was feeding .50 volts to the stoplamp switch. I left those wires temporarily secured so I could go back in and reconnect it just in case. Later today I'll reconnect it to see if the .50v causes any issues. I see alot of people bring up cruise control wires when splicing into the stop lamp wire so with that .50v wire disconnected I tested my cruise control and everything operates as normal.
Etrailer Expert

Jon G.

5/27/2021

@KennethT Glad to hear it sounds like you got your problem solved! That extra wire might go to your 3rd brake light or something on your truck.