How to Check an Electric Brake Magnetby Kate Klassen
A faulty brake magnet can severely impact the stopping power of a trailer. While some magnet problems will be apparent upon visual inspection, there may also be electrical problems that can impact performance. A compromised brake magnet can lead to weak or surging brakes, or cause the brakes to pull to one side. It is important to inspect and test the magnets each time the brakes are worked on to ensure maximum effectiveness.
Jack the trailer up high enough that each wheel turns freely. Secure the trailer with blocks under the frame at each end and on both sides to reduce the danger of the trailer falling on you should the jacks fail.
Remove the lug nuts and pull the tires and rims off of each axle.
Take off the grease cap and remove the cotter pin and castle nut. If there is a spindle washer, remove this too. Gently remove the outer bearing, drum, seal and inner bearing.
Locate the brake magnet. Take a straight edge tool and lay it across the top of the magnet. The edge of the magnet should be parallel to the straight edge all the way across. Any pitting or changes in the magnet's surface indicate abnormal wear and the magnet should be replaced.
Check the center of the magnet for copper coil. If any coil can be seen, the magnet is worn out and should be replaced.
Visually inspect the magnet for grease or oil residue. If any is found, replace the magnet.
Check the magnet for short circuits. Disconnect the leads and the strain relief so that you can pull the leads through the backing plate. Unclip the leads from the lever arm and disconnect the magnet. Connect the positive battery terminal to one end of the ammeter lead and the other end to one of the magnet wires. Use a piece of 16-gauge wire and connect the magnet housing to the negative battery terminal. The ammeter should not show a current reading. If it does, a short exists and the magnet should be replaced.
Keeping the magnet wire connected to the ammeter lead, take the other magnet wire and attach it to the negative battery terminal. Take the other ammeter lead and attach it to the positive terminal. An amp reading of 3.2 or more at 12 volts means there is a short circuit in the coil and the magnet should be replaced.
Check the brake lining, shoes and repack the bearings if needed while the wheel assembly is apart. Put the parts back on the axle in the opposite order of how you removed them.
Items you will need
- Trailer jacks
- Tire iron
- Bearing puller
- Straight edge tool
- Automotive battery
- 16-gauge wire
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