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.

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

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.

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

Updated by: Raymond P.

Last updated: 7/3/18

Questions and Comments about this Article

Richard M.

I recently purchased an RV that has electric brakes. One of the four wheels would lock up when braking. I took it to the dealer and they said the magnet had broken off, but when I asked what they thought the root cause was they could not answer. All of the other brakes seem to be functioning properly, and I inspected all of the wirings myself before sending it to the dealer and everything checked out. I suspect the magnet failed/had too little resistance and held too tightly to the drum. With my limited knowledge, I do not see another explanation. Am I way off or is there another plausible scenario? 95682

Reply from Jon G.

I think you're right on the money. The brake magnet either failed or it was adjusted too tight and ended up breaking off. 70703

Reply from Richard M.

@JonG Thank you, Sir! That really helps to put my mind at ease. 70704

David C.

I have 7000 pound axles. I replaced all the brakes complete, 12" drums. all the wiring was replaced. Even run 4 awg wiring on trailer to have a good solid ground. The brakes will not hum. They will work but will not lock up the brakes. 2 different controlers that are new. what could be causing this problem. 76468

Reply from Chris R.

It sounds like you've pretty much ruled out everything on the trailer side. You mentioned that you tried a couple controllers on the tow vehicle, but have you inspected the trailer wiring on that side? I would first just visually inspect the tow vehicle's 7-Way for any debris or corrosion inside the pins that could be preventing a solid connection with the trailer. Then check any wiring behind it that you can gain access too, looking for pinched or exposed sections that need to be replaced. Also, how are you actually testing the brake controller? Have you driven around with it or are you just testing at a stand still? 62401


can you shop test trailer brakes with out hooking up to truck? maybe a 12v battery wired to trailer pigtail brake and ground and listen for magnets to hum and lock up wheels when trailer is jacked up. 69944

Reply from Jacob H.

You are correct. You can test the brakes on a trailer by lifting the trailer wheel off of the ground and connecting the 12V power to the brake wire. This will allow you to see the brakes activate and tell whether or not you need to adjust the brakes. 57065



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