My Bafang BBS01 mid-drive electric which was fitted originally to my Bacchetta Giro 20 ATT and is now fitted to my Giant Defy 1 road bike has registered the dreaded Error 21 which means a speed sensor fault. Replacing the sensor with a new Bafang BBS speed sensor from EM3EV has not fixed the problem so here is what I have done to fix the problem.

Error 21 on the Bafang BBS Means …

First up the Bafang BBS Error 21 means a speed sensor fault which in turn “kills the Bafang C961 display”. This means no data is displayed and there is no ability to change assist levels once the error is registered. Thankfully the motor continues to work and because there is delay before the Error 21 is registered there is time to set the desired assist level. So all up there is not a total loss of functionality, more a loss convenience.

In my case I did try a replacement Bafang BBS speed sensor without any luck and I suspect I have a problem at the controller end (the speed sensor is “hard-wired” so to speak into the motor). Fixing the problem at the motor is a bit more involved as the controller needs to removed and the wiring or connection issue sorted if possible. An alternative fix is a using a reed switch speed sensor.

Reed Switch Speed Sensor Fix for Bafang BBS Error 21

The Bafang BBS Error 21 speed sensor problem was recently raised in an Endless Sphere thread. From that thread discussion I got the idea of swapping out the Bafang BBS speed sensor for a simple reed switch speed sensor commonly found on low-end bicycle computers.

As it was I had a speed sensor extension which I no longer use following the switch over to the Giant Defy 1 so I had something to experiment with. Not having an old bicycle computer with a reed switch I ordered a cheap (read under $4.00) bicycle computer from eBay that came with a reed switch. In my case this was described as a TIAU Bicycle Computer. It looks like it comes with what BMS Battery sell as a reed switch sensor, a King Meter Speed Sensor.

The Bafang BBS speed sensor cable is a three-wire configuration: red, white and black and the TIAU bicycle computer sensor switch is a two wire cable: black and black. I joined the white and black wires from the Bafang cable to the twin wires from the TIAU sensor, hence leaving the red wire unconnected. Preliminary testing indicates that TIAU speed sensor is performing as expected and I am getting a speed reading on the C961 display and no Error 21.

Wiring using the speed sensor extension cable whilst giving more length than I need does give me a joining point and the option to down the track to go back to a Bafang speed sensor if required.

I feel that I have been able to tidy the wiring up sufficiently to give this repair a decent run to see how it works. I also made use of some self-amalgamating tape to waterproof the connection. With winter coming on I guess I will find out if this taping has done the trick or not.

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