Bafang M420 Help

Apollogk said:

Hi everyone,

I was hoping for some help with a Bafang M420.

Basically everything turns on and seems to work correctly, but the pedal assist does not kick in, whether on 1 or 5. The display shows it constantly using 0 watts of power too. Adding to the confusion is the fact that the bike will move when it is set to walking mode. This confirms the motor and the hall sensors are ok in my experience.

I have pulled the motor apart and observed the following:

- Sensor wires all connected, no broken wires or loose connections
- No error codes
- Magnetic speed sensor on the wheel is fine and shows a seemingly accurate speed
- All functions on display seem to work
- Battery shows full charge both on display and on the battery's meter itself and voltage across terminals is correct

I feel like given that the walking mode works and the motor clearly engages, it has to do with the PAS sensor, but I know nothing about the PAS sensor in this motor and have struggled to find out much online so far.

Does anyone have any ideas about what could be the cause or how I might go about fixing it?

Thanks in advance

Click to expand...

Hi, I've got exactly same problem. Curious if you were able to find a solution? My suspicions are about torque sensor.

J'ai eu le même problème. J'ai changé le capteur de couple par un plus récent acheté sur greenbike
. Il fonctionne parfaitement maintenant, mieux qu'avant !
No wory to change it , there is a tutorial video on greenbikes youtube channel about that

J'ai aussi un M420

So, a small report upon replacing the torque sensor.
The new one was ordered from greenbike and delivered in ca. 3 weeks along with the chainwheel nut tool. The standard head for Shimano bottom brackets is not deep enough to reach the nut, so had to order this one as well. Also consider ordering noseclip pliers as they save a lot of efforts while removing C-clips.

NB: the nut has left thread, and is sitting on thread sealant. It didn't want to come off at all until I heated it with hot air gun.

Then, follow YouTube manual from greenbike which is pretty much self explanatory except the chainwheel nut moment.

I found lots of stains inside the motor, the water somehow get inside through the seals. Seems the sensor was damaged by water, as it has had white corrosion marks as well. The original gasket is useless as it's made of paper and doesn't survive when you open the motor. The entirety has had to be cleaned and regreased as well.

NB: there's nothing to break on the sensor's axle, and it is enough to have just the torque sensor hub replaced.

NB: the thermal pad under the controller PCB is of some low quality crusting into pieces, so was replaced as well.