How to Test Electric Trailer Brake Magnets to Determine if They Need to be Replaced


How do you measure a magnet in order to decide if it needs to be replaces? Thanks

Expert Reply:

I would check the magnets for wear before going on to test them with a meter. If the magnets show abnormal wear or if you can see the windings beginning to show through the friction surface of the magnet then they will need to be replaced.

To further test your magnets, first make sure the trailer is not connected when performing this test. If the trailer is connected, you run the risk of shorting out the brake controller or various other electrical components.

To test the brake magnets with an ammeter, connect the ammeter between the positive battery terminal of a separate battery and one of the magnet wires. Then, connect the other magnet wire to the negative battery terminal. Check the amperage, if the amperage is greater than the specified amount in the chart below, replace the magnet. If the reading is less than the specified amount, then the leads are bad and the magnet should be replaced.

Electromagnet Specifications:

7 inch Diameter Brake Drum
Max Amps at 9-10.5 Volts Max Amps at 12-13 Volts
2 Brakes 4.7-5.6 6.3-6.8
4 Brakes 9.5-11 12.6-13.7
6 Brakes 14.3-16 19.0-20.6

10 and 12 Inch Brake Drums
Max Amps at 9-10.5 Volts Max Amps at 12-13 Volts
2 Brakes 5.6-6.6 7.5-8.2
4 Brakes 11.3-13.3 15.0-16.3
6 Brakes 17.0-20.0 22.6-24.5

Next, disconnect the negative wire from the brake magnet wire and connect it to the base, housing, of the brake magnet. If there is any amperage, the magnet is shorted and will need to be replaced.

Finally, using an ohmmeter, check the resistance between the brake magnet wires. If the resistance is not within the the specified range below, the magnet will need to be replaced.

7 inch brake magnet resistance should be 3.8-4.0 Ohms
10 and 12 inch brake magnets resistance should be 3.2-3.5 Ohms

I have included a link to the trailer brake magnets that we currently offer for your reference.

expert reply by:
Michael H

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Questions and Comments about this Question

Much easier to have someone hold the manual brake button on the controller or step on the brake pedal while another person checks to see if a screwdriver sticks to the magnet. No meters, or disconnecting anything required. If the screwdriver wont stick, the magnet is defective.

comment by: Greg T - 4/11/2013

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