Testing Electric Trailer Brake Magnets with a Multimeter

Published 01/06/2010

Products Featured in This Question

Electric Trailer Brake Assembly - Self-Adjusting - 12" - Right Hand - 5,200 lbs to 7,000 lbs

(351)

$68.18

Digital Multimeter with 42" Long Test Leads - LED Display

(3)

$22.17

Question:

lost brakes on my 38 foot goose neck trailer and cant seem to get them to work use to hear the brake magnets when i applied the brakes can i hook a 12 volt battery directly to the magnet wires to test them or is there a ohm reading or some other way of testing the electromagnets.

asked by: Robert A

Expert Reply:

The brake magnets can be tested in the manner you have suggested, just make sure that the trailer is not connected to the vehicle when performing this test. You would not want to short out the brake controller or worse. 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 test the Brake magnets with an ammeter,item # PT89ZR, you would connect the ammeter between the positive terminal of a 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 (see 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 12-13 Volts
1 Magnet 3.0 - 3.2

10 and 12 Inch Brake Drums
Max Amps at 12-13 Volts
1 Magnet 3.2 - 4.0

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 (see below) then 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