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.

Questions and Comments about this Video

Jerry 11/01/2012

If all else is well such as power, ground, brake signal, should a brake controller send voltage from the controller through the red wire to the brake switch?374

bob pope 01/26/2013

where dies one get the tester your useing.912

Reply from Patrick B 02/07/2013

If you are referring to the circuit tester, it is part 40376. Its the circuit tester we use in our install shops. If you are asking about the brake controller testing box, the large aluminum box, that is made by Tekonsha and supplied to dealers and installers like us. I am not sure if it is available for resale. 579

Joe C 12/26/2013

Times the power adjustment light will get reder times nothing. Times put the override switch over nothing happens. But the brakes on when brake is applied. BAD SWITCH YES OR NO2463

Reply from Patrick B 12/28/2013

I would be happy to help you get your brake controller problem figured out, but I will need some more info first. What model brake controller is it? What is the year, make and model of the vehicle it is installed on? Finally, is the controller plugged into a factory port on the vehicle, or was it spliced in?1787

rogseil 04/21/2014

I own a 2003 Chevy Suburban with factory installed trailer wiring and trailer plug. all functions work except the trailer brakes. I located the blue brake wire at the fuze box under the hood. there is a connector in this wire I pulled apart and did a continuity check from the rear plug contact to one side of the wire under the hood. checked good with the brake controller plugged in. I checked for power on the other side of the connector with truck power on and off. No power at that side of the blue brake wire. Am I missing something real obvious here? I also checked all fuzes indicating trailer use. All checked good for continuity. Any help would be appreciated3659

Reply from Patrick B 04/23/2014

Did you also make sure the red wire is connected under the hood? The blue wire under the hood is a direct 12V power feed to the rear of the truck. The red wire under the hood is what actually powers the brake controller. I have added a link to a tech article that walks through the whole process of installing a brake controller on 1999-2006 GM trucks and SUVs. Check it to see if there are any missed steps. If you've got everything done or I misunderstood, let me know.2769

Helpful Links


Dave F 08/22/2014

Where can I get a tester like the one you are using in the video?6423

Reply from Rachael H 08/22/2014

The circuit tester # 40376 is available for purchase from our site. I have included a link below for your reference. 5312

Tim Nelson 10/05/2019

Is there supposed to be power to the controller when the ignition is off? I installed the Primus 90160 with the Toyota wiring harness into my 2011 Tundra and everything works fine I am just wondering about the red indicator light when the ignition is off. Thanks.62684

Reply from Rachael H 10/15/2019

The Primus does have power to it all the time even if there is not a trailer connected. That is what the red indicator lights are conveying. The brake controller has power. This is absolutely normal for this controller and will not cause any issues with draining your battery. 53231

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