How to Diagnose a Bad Electric Trailer Brake Magnet β Test and Replace
Test each magnet's resistance with an ohmmeter: 10" and 12" magnets should read 3.2β3.5 ohms. 7" magnets should read 3.8β4.0 ohms. Outside that range means a bad magnet. Also test for a short by checking resistance between the magnet wire and the magnet housing β any reading at all means it's shorted. Replace in pairs β both sides of the same axle together.
Symptoms of a Bad Brake Magnet
Before pulling out a multimeter, identify what you're seeing. Bad magnets produce specific, recognizable symptoms:
| Symptom | What It Suggests |
|---|---|
| Braking feels weak even with high controller gain | One or more magnets not pulling full amperage β worn, open circuit, or high resistance |
| Brakes work on manual override but not from brake pedal | Wiring/controller issue β not the magnet itself (see FAQ below) |
| One wheel noticeably hotter than others after towing | Magnet on that wheel may be dragging β shorted magnet or stuck lever arm |
| Brake controller shows "short" or overload error | Shorted magnet pulling too many amps |
| Brake controller shows "open circuit" error | Broken wire or failed magnet coil β open circuit |
| Braking suddenly worse after hitting a big bump or water | Wire broken at stress relief grommet β common failure point |
| Visible metal windings showing through magnet face | Magnet worn through β replace immediately |
Visual Inspection First
Before testing with a meter, pull the drum and look at the magnet face β the flat surface that contacts the inside of the drum. A healthy magnet has an intact friction surface covering the entire face. Replace the magnet if you see:
- Metal windings or copper wire showing through the face (worn all the way through)
- Deep grooves or scoring on the friction surface
- Uneven wear β one side of the face significantly more worn than the other
- Grease or oil contamination on the face (from a failed grease seal)
- Cracked or chipped friction material
Quick Field Test β The Screwdriver Method
No multimeter needed for this one. With the trailer hooked to the tow vehicle, have someone press the brake controller's manual override while you hold a large metal screwdriver near (not touching) the magnet face. The magnet should pull the screwdriver toward it with noticeable force. If there's little or no pull, the magnet is failed or not receiving power. This doesn't tell you why it failed, but it confirms the magnet isn't working.
Resistance Test β The Definitive Check
This is the most reliable way to test a magnet. You need a multimeter set to ohms (Ξ©). Disconnect the trailer from the tow vehicle first β you don't want to test through the brake controller.
- Disconnect the Trailer from the Tow Vehicle
Unplug the 7-way connector completely. Testing brake magnets while connected to the vehicle risks damaging the brake controller.
- Isolate the Individual Magnet
All brake magnets are wired in parallel, so measuring at the 7-pin connector gives you the combined resistance of all magnets together β not individual readings. To test a specific magnet, find and disconnect that magnet's two wires at their closest connection point (usually near the axle).
- Measure Resistance Between the Two Magnet Wires
Place the ohmmeter probes on the two magnet wires. Polarity doesn't matter β the wires are interchangeable. A good reading should fall within these ranges:
Brake Drum Size Good Resistance Range Bad β Replace 7" drums 3.8 β 4.0 ohms Below 3.8 or above 4.0, or "OL" (open) 10" drums 3.2 β 3.5 ohms Below 3.2 or above 3.5, or "OL" (open) 12" drums 3.2 β 3.5 ohms Below 3.2 or above 3.5, or "OL" (open) An "OL" or infinite resistance reading means the coil wire is broken inside β open circuit. A reading significantly below spec means the coil is shorted internally and drawing excessive current.
- Test for an Internal Short to Housing
With the magnet still disconnected, place one probe on either magnet wire and the other probe on the metal body (housing) of the magnet itself. If you get any resistance reading at all, the magnet is shorted internally and must be replaced. A healthy magnet shows no continuity between the wires and the housing.
Amperage Test β Advanced Confirmation
If you have an ammeter, you can verify the magnet is pulling the correct current. Connect the ammeter in series between a 12V battery positive terminal and one magnet wire, then connect the other magnet wire to the battery negative. A good 10" or 12" magnet should draw 3.0β4.0 amps at 12β13V. A 7" magnet should draw slightly less. Excessive amperage (shorted coil) or insufficient amperage (open or high-resistance coil) both indicate replacement.
Dexter's official recommendation is to always replace magnets in pairs β both sides of the same axle at the same time. If one magnet is worn or failed, the other has been doing extra work and is likely near the end of its life too. Mismatched magnets on the same axle cause uneven braking, which pulls the trailer to one side under hard stops.
Replacing a Brake Magnet
Magnet replacement does not require removing the entire brake assembly. The magnet unbolts from the actuator arm on the backing plate:
- Remove the Hub and Drum
Remove the wheel, pull the dust cap, remove the cotter pin and castle nut, slide the hub/drum assembly off the spindle. Set the hub down carefully to avoid damaging the bearings.
- Note the Magnet Wire Routing
Before disconnecting anything, photograph or note how the magnet wire is routed and secured. It runs through a stress relief grommet β this is the most common break point on worn magnets, and you'll want to route the replacement wire the same way.
- Disconnect the Magnet Wires and Remove the Magnet
The magnet is held to the actuator arm by a single bolt or retaining clip depending on the assembly. Remove it and pull the magnet out. Disconnect the two wires from the trailer wiring harness.
- Install the New Magnet
Route the new magnet wires through the stress relief grommet before securing the magnet to the arm. Connect the wires β polarity doesn't matter. Secure the grommet so there's no sharp bend in the wire at the entry point.
- Reinstall Hub, Drum, and Wheel
Reinstall in reverse order. Torque the castle nut per Dexter's spec, install a new cotter pin, seat the dust cap. Reinstall wheel and torque lug nuts.
- Adjust the Brakes and Allow Break-In
New magnets change the brake clearance β adjust the star wheel per the manual adjustment guide. Then plan for a break-in period β Dexter states more than 100 stops from 20 mph for new components to wear in. Braking will be slightly softer than normal until break-in is complete.
