Troubleshooting Malfunctioning Trailer Brake Magnets

Updated 06/23/2014 | Published 06/20/2014

Products Featured in This Question

Replacement Magnet Kit for 10" Electric Trailer Brake Assemblies - 3,500 lbs

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$28.99

Brake Adjustment Spoon for Drum Brakes

(218)

$8.08

Question:

I have a question about trailer brakes. My brother purchased new brake assemblies a while back I believe from etrailer that included the shoes / magnets / etc. These are 10inch drums. The brakes functioned fine but seemed to be less effective after a while - didnt really tow with them that often but no real complaints...then we changed tow vehicles. We had an issue with a brake controller that seemed to be putting out 12v constant to the brakes - didnt know it and as youd expect brakes arent working now. Weve replaced the brake controller and now seems to be working okay from truck to trailer, however, still not able to get brakes to work. Weve checked the wiring all the way back to the axles and it appears to be all intact and functional - weve isolated the wiring from the brakes at the hub attachment point and weve got 12v on the wiring coming from the truck when brake pedal is depressed - weve checked the resistance of the brake magnets and they seem to be within acceptable range - we can hook a battery charger to the individual wheel magnets and they function properly but when hooked together with other trailer wiring we are not able to get the brakes to work. Also, when the brakes are hooked up to the trailer wiring as normal and the pedal is depressed our voltage reads zero - weve got correct voltage when the brakes are not hooked up but once connected theres no voltage reading. Hopefully this makes sense - Im thinking just need to replace the magnets but they function fine when hooked direct to a power source. Brake controller seems to be operating okay but Im open to suggestions. Thanks

asked by: Seth C

Expert Reply:

You'll need to do a full troubleshoot. With the trailer unplugged, use a voltmeter on the brake pin on the trailer connector while the manual brake switch is actuated. If you have a time delayed controller, you should also get voltage on that pin with the brake pedal pressed. Next, plug the trailer in, and check the voltage on the output circuit as close to the magnets as you can get. With the gain on the controller turned all the way up, you should get around 12 volts at the magnet when the manual switch on the controller is actuated. If you don't get voltage at the magnet, examine the brake output circuit on the trailer. Make sure the wire is in good shape, with no loose connections. Make sure one lead from each magnet is connected to the output circuit, and the other is grounded to the trailer. It doesn't matter which wire is to power and which is grounded.

If all this checks out, look at the faces of the brake magnets that ride against the inside of the brake drum. Are the magnets worn to the point where the copper wiring on the interior of the magnet is visible? If so, it's time to replace the magnet. You should also make sure the brakes are properly adjusted.

To adjust the brakes, you'll need a brake adjusting tool, like part # W80630. Use the tool to tighten the adjuster down to the point where you can't turn the wheel by hand, then back it off by about 10 clicks, or until there's just a slight drag of the shoes against the brake drum. Repeat on all brake assemblies. I've linked you to a video showing how to adjust the brake assemblies. Simply fast forward to the 1:50 mark.

If you check out all the above and still don't find anything, please get back with me, and we'll look into some other possibilities.

expert reply by: Mike L