Is it Normal to Have Residual Magnetism on an Electric Brake Magnet When No Power is Applied

Question:

Ive got a hot wheel problem due to brakes dragging on one wheel. The bearings are ok, and brake adjustment is ok. With no controller connection and battery on trailer disconnected, if I hold a screwdriver up to the magnet it will stick ... not super hard like on a speaker magnet, but enough to be noticeable. Is this normal?

asked by: Greg G

Expert Reply:

The magnet on an electric trailer brake assembly, such as # 23-27 from Dexter, is an electro-magnet (not a permanent magnet) that should exhibit magnetic attraction only when 12-volt power is applied to it. Even after extended use there should not be residual magnetism, certainly not enough that the magnet would hold the weight of a screwdriver.

It sounds like there must be some 12-volt power reaching the brake assembly. This would cause the always-engaged brake pads to heat up the assembly. I suggest you fully disconnect your trailer 7-way from the tow vehicle, and also one at a time completely remove your trailer batteries (if you have both a breakaway battery and an auxiliary power battery for the trailer). If the brake loses magnetism after removing one of the batteries you will know which one is leaking voltage to the brake circuit.

Carefully inspect the brake circuit wiring all the way from the trailer 7-way to the subject brake assembly; look for any cracks or worn spots in the brake wire insulation.

You can test the brake with a multimeter like part # PT89ZR. You can also test the magnet itself. Please refer to the linked article for step-by-step testing instructions.

expert reply by: Adam R