Testing Trailer Brakes that are Getting Weak on a 8-1/2 X 24 Foot Cargo Trailer

Updated 03/09/2010 | Published 03/07/2010

Question:

I have a 8.5x24 cargo trailer that I use for my construction company. The brakes are getting weak. I have my brake activator turned all the way up and when i hit the brakes I can barely feel the trailer brakes kick in. Is that normal? If I need to change them, what would be my best choice? I would rather have more brakes than not enough. Thanks.

asked by: Jerry C

Expert Reply:

It sounds like the magnets are getting worn and are not supplying the enough force to the brake assembly when the brakes are applied. The first thing I would do is use a voltage tester and check the brake output wire on the trailer while someone uses the manual knob in the cab. If you are not getting full power, you will need to test the brake controller for proper function.

If you are getting full power with the manual slide fully engaged, then you will need to remove the hub and check the brake magnets, brake shoes, and associated parts. If the magnets are wearing oddly, or the windings are showing then you will need to replace them (see link). If there are no signs of wear, then you will need to test them to make sure they are still good.

To test the Brake magnets with an ammeter 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, and if it 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 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 (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

You should also check the brake pads for wear. Another good idea would be to grease the trailer bearings while you have things apart. We also offer grease and parts to complete this maintenance(see links).