Electric Trailer Brake Magnet Short Circuit Indicator on Brake Controller Comes On After Moving
Question:
My electric trailer brakes have a short only when moving. I know this because my controller has a red LED to show when the system is shorted. I did run all the wiring and found 3 places the insulation was worn off and fixed that but that did not totally fix the problem. Now I have to be moving before it will show a short. I have inspected all 4 magnets and wiring but they seem to be in good shape. What is my next step?
asked by: scott
Expert Reply:
You said you checked the brake magnets. When inspecting them did you notice if the windings inside the magnets are showing? If so this is most likely the reason that the short is showing up on the controller when the vehicle is moving. This would mean that the magnets would need to be replaced.
If you are seeing the short condition after you have been towing for a bit then the problem most likely lies inside a magnet. Brake magnets work best when they are warm and, if there is a problem, the problem will show when they warm up and may not show up when cold. This is due to heat expansion inside the magnet.
You can check the brake magnets with an ohm meter to determine if they are bad. You will need to get the magnets warmed up and then check the resistance in the magnets. A 7 inch brake magnet should have 3.8 to 4.0 ohms of resistance and 10 and 12 inch brake magnets should have 3.2 to 3.5 ohms of resistance. If the resistance is not as specified the magnet needs to be replaced.
You can also check the magnet with a 12 volt battery. Sever the magnet wires from the trailer wiring. Connect one wire from the magnet to the ammeter and the other ammeter probe to the positive post of the battery. Connect the second wire from the magnet to the negative battery post. If the amperage is greater than specified below the magnet needs to be replaced. Also, when the negative battery wire is disconnected, if there is amperage on the ammeter the magnet is shorted and will need to 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
Finally, if the magnets check out to be good, you will need to troubleshoot the brake controller. Sever the blue wire at the back of the brake controller, leaving enough wire to reattach it if needed. Now test the brake controller for proper function. You should get voltage on the blue wire when the brake pedal is depressed and when the manual lever is activated. If the LED short indicator does not appear with the blue wire severed then the problem lies in the wiring or magnets.
In your situation I believe you will find that one or more magnets are in need of replacement.
Product Page this Question was Asked From
Replacement Magnet Kit for 12" Electric Trailer Brake Assemblies - 5,200 lbs to 7,000 lbs
- Accessories and Parts
- Trailer Brakes
- Electric Drum Brakes
- 5200 - 7000 lbs
- Brake Magnets
- 12 Inch
- etrailer
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Bruce
7/22/2020
"Also, when the negative battery wire is disconnected, if there is amperage on the ammeter the magnet is shorted and will need to be replaced." What this really means is your ammeter is bad or you did not make the connections only between the magnet, battery, and ammeter per the description. There can be no current flow if there is no connection to the negative terminal of the battery.