Speed sensor installation and troubleshooting

For Voltbike models:
with mid drive motors Bafang M600 / M620

Tools/ parts required:
T20 torx safety bit tool / or flat head screwdriver
4 mm hex key
T10 torx screwdriver

Introduction

Mid-drive motors use a detachable speed sensor unit and magnet mounted to the rear wheel spoke to calculate the bike’s speedometer reading and allow the motor and throttle to function normally. 

Failure of the speed sensor or an incorrect magnet position may result in the motor not functioning, inconsistent speedometer reading, no throttle function, and may cause random shutdown of the mid drive motor while riding. When troubleshooting mid-drive motor problems the speed sensor is typically the first test that should be done. Symptoms may include a speedometer that randomly changes speed ( 0-99 km/hr) and may shut the motor down when a value of 45km/hr or higher is displayed on the screen. An inconsistent speedometer reading can also prevent the thumb throttle from working as it requires a consistent speed of 10km/hr or higher to be active. The torque sensor can still activate the motor when pedaling but random and persistent shut down of the motor may happen if the speed sensor unit has failed, is unplugged, the cable damaged, or the magnet missing or misaligned.

Test

Turn the bike off and check the sensor mounting bolt is tight. It is located on the inside of the non-drive side chainstay under the small rubber cap. The bolt ( 4mm hex tool) should be snug and the sensor aligned forward. Next check the magnet is tight on the spoke, and is in proximity of the sensor unit as the wheel turns and passes the sensor block (not the mounting bolt). To ensure a good reading the magnet may need to be turned at a slight angle or moved to a spoke on the drive side of the wheel. A poor sensor reading may be caused by the magnet being too close or too far away from the sensor. 

To confirm the sensor/magnet is functioning correctly: turn the bike on, and spin the rear wheel forward by hand while safely lifting the rear wheel off the ground.

The display speedometer should show a consistent and accurate speed. Once above 10km/hr the thumb throttle can also be tested. If speedometer reading is not shown with the spoke magnet in proper position then the sensor plug and cable connections should be checked. 

Note: Variations of speed sensor cables

Some speed sensor units have a separate plug/cable that connect it to the motor plug. This quick connect plug is located just inside the chainstay hole of the frame. Carefully pull on the cable to determine if this style of cable / plug has disconnected.
Other speed sensor/cables may plug into a wire harness (for the rear light), or use a long single cable that plugs directly into the motor next to the power connector. To check the motor plug connection, use a T10 Torx screwdriver to remove the four small screws that fasten the non-drive side plastic cover in place. Ensure the proper style of replacement cable is used. 

Installation

Error code 21 indicates the speed sensor has likely failed. If the sensor plug, cable, and position have all been checked than a new sensor unit likely needs to be installed. Carefully remove the old sensor unit at the chainstay and at the plug connection.
Route the new cable through the chainstay hole and connect to the cable/motor plug or cable harness. This may require removing the motor’s side cover. Test the new sensor as noted above. Calibration of the motor is usually not required after a new speed sensor installation.