Electric brakes
Electric brakes use a magnet that grabs the drum when power hits it. Weak brakes are often wiring, magnets or adjustment.
The short version
- Check the brake controller in the truck and the brake wire at the plug.
- Measure current to the brakes with the controller applied. Low current points to wiring, a bad ground or a shorted/open magnet.
- Inspect magnets and shoes for wear. Adjust the shoes per the manufacturer.
- Grabby brakes: check the controller gain setting and look for grease or contamination on the shoes.
On the job
- Controller gain to max, manual lever pulled, clamp the blue wire at the plug: 4 brakes of 10/12 in should pull 15 to 16 A at 12 to 13 V.
- One wheel 0 A: ohm the magnet at the backing plate, 3.2 to 3.5 Ω (10/12 in), 3.8 to 4.0 (7 in). O.L = replace.
- Magnet to its own housing: any ohms = shorted, replace.
- Adjust: tighten the star wheel until the drum drags, back off until it just spins free. Replace magnets in pairs.
Numbers
- Magnet current: about 3A per magnet on common 10–12in brakes; check the axle maker's spec
- Magnet resistance: check against the axle maker's spec; open or near 0Ω is bad
- 2 / 4 / 6 brakes, 10-12 in, full gain: 7.5 to 8.2 / 15 to 16.3 / 22.6 to 24.5 A
- 7 in magnets: 3.8 to 4.0 Ω
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.