How to Test Brake Magnet Amperage on a Trailer with 4 Brakes

Published 12/14/2012

Question:

How can I test the unit to see if it is putting enough amps out for a four brake system?

asked by: Jim W

Expert Reply:

There are a few different tests that can be done to check the brake magnets individually and as a whole on your trailer. I would first recommend checking the magnets for wear before going on to test for proper amperage. 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.

Testing the magnets as a whole will involve using a multimeter, that can read up to 20 amps, to test the amperage on the brake output circuit from your brake controller when the trailer is connected to the vehicle. To test with the multimeter, you will connect the ammeter inline with the blue wire (brake output wire) from the brake controller. I have attached a diagram that shows how to perform this test. On a 4 brake system, the maximum amperage draw for 10 and 12 inch brake assemblies should be 15.0 to 16.3 amps and for 7 inch brake assemblies 12.6 to 13.7 amps.

Before you test the brake magnets individually, disconnect the trailer from the tow vehicle. Next, sever the brake magnet wires and connect the ammeter between the positive battery terminal of a separate 12 volt battery or 12 volt independent power source and one of the magnet wires. Then, connect the other magnet wire to the negative battery terminal. For 10 and 12 inch brake assemblies, each magnet should read 3.2 to 4.0 amps at 12 to 13 volts. For 7 inch brake assemblies, the magnet should read 3.0 to 3.2 amps.

If the amperage readings are greater than the specified amounts, replace the magnet(s). If the reading is less than the specified amount, the leads are bad and the magnet(s) should be replaced.

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 3.0 to 3.8 Ohms for 10 and 12 inch brake assemblies or 3.8 to 4.0 Ohms for 7 inch brake assemblies, then the magnet will need to be replaced.

expert reply by: John H