Mid-Drive: Speed Sensor Install and Troubleshooting

How To / Technical Support / Voltbike.com

Speed sensor install 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

WARNING
Installation and removal should only be done by a qualified
person. Incorrect installation or failure to follow this warning may result in
personal injury or death, and/or loss of property.

Intro

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.

Install

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.