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Testing Trailer Brake Magnets for Proper Function

There are 4 different tests that can be done to check the brake magnets on your trailer. The first test you should do is to use a multimeter to check the amperage of your trailer braking system. The trailer brake magnets will draw amperage from the brake controller based on the output voltage sent by the brake controller. Performing this test will let you know if the magnets are functioning properly or if further testing is required.If further testing is indicated, you can test the individual brake magnets for proper amperage. Test each magnet for a short and test the magnets for resistance. Below we have outlined each of these tests for you.

Testing with Trailer Connected to the Vehicle

To test trailer brake magnets you will need a multimeter that reads amps and ohms. A good brake controller, like the Tekonsha P3, has test functions built in. To test with a multimeter, first you will connect the ammeter inline with the blue wire exiting the back of the brake controller or use the brake controller's diagnostic readings.Check the amperage. If the amperage is greater than the specified amount (see specifications below), replace the magnet or magnets. If the reading is less than the specified amount, the leads are bad and the magnet or magnets should be replaced.
Illustration of Connecting ammeter to blue wire between brake controller and trailer connector

Electromagnet Specifications

10 and 12 Inch Brake DrumsMax Amps at 12-13 Volts
2 Brakes7.5-8.2
4 Brakes15.0-16.3
6 Brakes22.6-24.5
7 inch Diameter Brake DrumMax Amps at 12-13 Volts
2 Brakes6.3-6.8
4 Brakes12.6-13.7
6 Brakes19.0-20.6

Testing Individual Brake Magnets

Testing individual brake magnets for proper function can be accomplished by severing the magnet wires and connecting the ammeter between the positive terminal of a 12-volt battery and one of the magnet wires; it does not matter which one. Then connect the other magnet wire to the negative battery terminal. Check the amperage. If the amperage is greater than the specified amount (see specifications below), replace the magnet. If the reading is less than the specified amount, the leads are bad and the magnet should be replaced.

Individual Brake Magnet Amperage Specifications

10 and 12 Inch Brake Magnets7 Inch Brake Magnets
3.2-4.0 Max amps at 12-13 volts3.0-3.2 Max amps at 12-13 volts
Connecting Ammeter between positive terminal on 12-volt battery to one of the magnetic wires

Testing Individual Brake Magnet for a Short

To determine if a brake magnet has an internal short, touch the base of the brake magnet to the negative post of a 12-volt battery. Then, connect one of the brake magnet wires to the negative lead of a multimeter and the positive lead of the multimeter to the positive post of the battery. If any amperage is detected, the magnet is shorted and will need to be replaced.
Touching base of brake magnet to negative post on 12-volt battery image

Testing Brake Magnet Resistance

Further testing can be done with an ohmmeter. Using the ohmmeter function on the multimeter, check the resistance between the brake magnet wires. If the resistance is not within the specified range (see specifications below), then the magnet will need to be replaced.

Brake Magnet Resistance Specifications

10 and 12 Inch Brake Magnets7 Inch Brake Magnets
3.0-3.8 Ohms3.8-4.0 Ohms
Testing with an ohmmeter image
Updated By: Dani S.Last Updated: 3/15/2021

Questions and Comments about this Article

Sparky A.

I used a battery charger as a voltage source. I got a lower current than expected, even though the DC resistance check out. Ohm's law seems to be violated in my garage. But then I realized the "DC" output of a batter charger is not true "DC" but is usually full wave rectifier waveform. Therefore the magnet exerted inductive reactance on my battery charger voltage source, reducing the current. I'm just saying, be advised that a batter charger does not produce a true DC voltage.

Matthew K.

I have two trailers and a brand new silverado 3500 hd work truck, On my dump trailer, I get " brakes connected" several moments later check trailer wiring. This goes back and forth on its own even with the truck just sitting there. Its been in the dealership twice but all the found was a blown trailer break fuse in the fuse box and a rubbed out nuetral between front of truck and connector. Need help on how to test the trailer for some mysterious issue. All trailer brakes appear to work.

Reply from Mike R.

@MatthewK - I had the same issue as you are describing with my RV. I too have a new truck but its 2021 Silverado 2500HD. After researching online for troubleshooting I had added 2 new grounds in addition to a new trailer 7 pin cable. What I found was not the trailer that was giving me the issues but when I finally check my trailer connection on my truck. Mine corrected itself after pushing on the wires inward on the back side of the connector. I have not had any other issues. Hope this help with yours.

Etrailer Expert
Reply from Les D.

Excellent troublshooting!


In your picture for Individual Brake Magnet Amperage Specifications, you have the negative of the multi meter connected to the magnet and negative battery terminal. But in your dictation you say to connect the positive of the multi-meter to the magnet and positive battery terminal. Which is it? I connected the positive of the multi meter in line with the battery and brake magnet, then connected the negative of the brake magnet to negative on the battery. I got no reading on a brand new brake magnet, what is the problem?

Etrailer Expert
Reply from Victoria B.

That section states than an individual magnet can be tested by "connecting the ammeter between the positive terminal of a 12-volt battery and one of the magnet wires." The positive of the multi-meter will be on the positive battery terminal. The negative of the multi-meter will be connected to one of the brake magnet wires. The other brake magnet wire will be on the negative batter terminal. It sounds like this is how you wired up your brand new brake magnet for testing. Since you didn't get any readout at all on the multi-meter, I would start by checking the battery's charge and that the multi-meter is working correctly.

Stephen B.

Some great testing ideas and specs from a company that I have purchased many repair parts for my fifth wheel.


Got brake problems,magnets are humming,shoes are adjusted with some drag,still no brakes,these are new brake assemblies,whats going on?

Etrailer Expert
Reply from Victoria B.

I have linked another help article that will walk you through a process of testing your braking system to help determine where the issue is originating. This is where I would start troubleshooting.


I just recently picked up a 16 foot flatbed trailer with brakes. Someone has removed the brake harness from the axle forward. Both wires coming out of the brakes are green. There’s one of these go to ground and the other one to the power source on my seven wire plug? With both of them being green does it matter which one goes where?

Etrailer Expert
Reply from Victoria B.

You are correct that either wire can be used to connect to either function. You will want to ground one wire and the other wire should be connected to the brake output wire on the 7-Way connector. You would not want to connect the brakes to the 12v power wire as that would cause the brakes to always be engaged.

David G.

i have 12volts going to front axle black and white wire but no brakes

Etrailer Expert
Reply from Victoria B.

Do you have signal on one of the wires going to the brakes at all times, or only when the brake pedal is pressed? Is there signal on both wires, or just one?

Mac E.

What if the code S.H. stays on when the trailer is disconnected? Does this indicate that the problem is with the truck wiring?

Etrailer Expert
Reply from Jon G.

Yeah that's what it sounds like to me. Look for any places where the wires might be damaged or exposed and fix them.

Reply from Mac E.

@JonG - Thanks for the prompt response, the hunt continues!

Jeff K.

The directions for determining the amperage across the blue wire does not apply to any brake controller that requires resistance (a load) to function. Example: Curt Triflex requires an electrical load be sensed before the controller will send a signal in the blue wire (signal = power) . A simple 12V test light shining brightly is a suitable indicator of signal.

Kenneth C.

Best troubleshooting guide on the web. Thx



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