How to Test Electric Trailer Brake Magnets on Dual 6,000 Pound Dexter Axles

Published 09/23/2010

Question:

Ive replaced lots of shoes on automobiles with hydraulic systems. I worked as an auto mechanic in the 60s and early 70s. How do I evaluate the electric magnet in the hub? It appears that the best bet is to buy your backing plate with all components when its time to reline the brakes. I own a 2001 34 fifth wheel rv. Thanks for an informative site.

asked by: Kenn H

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 a 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
2 Brakes 4.7-5.6
4 Brakes 9.5-11
6 Brakes 14.3-16

Max Amps at 12-13 Volts
2 Brakes 6.3-6.8
4 Brakes 12.6-13.7
6 Brakes 19.0-20.6

10 and 12 Inch Brake Drums
Max Amps at 9-10.5 Volts
2 Brakes 5.6-6.6
4 Brakes 11.3-13.3
6 Brakes 17.0-20.0

Max Amps at 12-13 Volts
2 Brakes 7.5-8.2
4 Brakes 15.0-16.3
6 Brakes 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 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

If the magnets just need to be replaced, we carry various types and sizes of magnets. I have included a link to the ones we carry below. I have also included a video that shows, in part, replacing magnets. If other components are in need of replacement and you do not want to change out each part individually, it would be easier to go with an entire assembly like the 12 inch Electric Brake Assembly, # 23-105, you have referenced.