Having the motor cut out while you’re riding can be annoying. Although in most electric bikes, it would be an error code or software issue, they all tie into the same problem of sensor faults and hardware defects.
So in the case of an electric bike or converted bicycle, which doesn’t have a very sophisticated onboard computer, why does the electric bike keep cutting the power to the motor?
Although the battery might be functioning properly, you might still encounter intermittent power supply issues to the motor if there are any loose connections or sensors (from the brakes, derailleurs, throttle, or rear wheel) giving false readings.
However, despite having the same setup on your E-bike, you might still be unable to figure out the issue. Therefore, I’ve created this guide which helps cover all the hardware-based reasons that might be causing the motor connection to shut down.
Why Is My Electric Bike Motor Cutting Out?
Generally, it would always be something that has to do with loose wires, connections, or shorted circuits. Nonetheless, it doesn’t hurt to specify the most common E-bike problems that cause the motor inhibitor to turn on.
So for that reason, I’ve listed some specific issues below. If you diagnose them, you will determine what’s going wrong with your motor unless it’s an error code (see how to fix ebike error code here) or software glitch.
Loose Phase Wires
If there’s no power transfer to the rear hub motor (also front hub or mid drive), the first place you should look is where they draw electricity from; hence, the three-phase motor connections that connect to the three copper coil wires.
If you see loose connections, melted or burned wires, corroded connectors on the controller side, and if not, smell smoke coming from the motor, it’s most definitely a shorted wire or lack of contact between the motor connection and the power cables.
The wires might also be shorted out at the motor end. Unfortunately, it’s not easy to diagnose without removing the motor enclosures, so you’ll most often be limited to a visual inspection.
That said, if you can access the phase cables on the motor’s side, don’t forget to use a multimeter and test for continuity of the three cables. Here’s how:
Attach one probe of the multimeter to one end of a color-coded wire
Attach the other problem to the other end of that same wire
If the multimeter doesn’t beep or gives out a higher resistance (around 60Ω), you’ve got an issue with conductivity.
Also, you can check for a short between the phase wires by testing the continuity between each color-coded wire. If there’s a short circuit, it might give out a beep or a lower resistance.
Shorted Out Phase Wires
Speaking of short circuits, when two phase wires come into contact, it would also cause the motor to cut out and start lagging. A situation like this would immediately disrupt the power transmission resulting in the bike coming to a complete stop.
If this happens to you, you can disable the motor by disconnecting the hall sensor cables. After which, the motor disengages magnetically, and you can pedal manually until you can get it back to the garage or to a repair shop.
Whenever the thumb or twist throttle sends input signals to the controller, the controller will provide power to the motor via the phase wires. Unfortunately, if your throttle control is not working properly (either due to a short or loose connection, it could interrupt the operation of the motor. (Although, in the case of throttles, it would accelerate the motor instead of making it stop.)
Nonetheless, you should check the throttle and see if it gets stuck or doesn’t work. If you’ve been using it for a while, try realigning it or replacing it with a newer model.
Loose Battery Prongs
Another issue that’s very common among E-bike users is the battery prongs coming loose while riding. It can be annoying, and although the battery pack might not fall off the bike so easily, the motor and controller (when not receiving steady power) will disengage the motor and sometimes cause damage. (Due to surge currents, short circuits, etc.)
So if you check the battery and fix all the other loose wire connections but still find the motor or display screen cutting out when you hit a bump or install the battery, it might be due to the battery case or its prongs.
If the battery case or mount is damaged or won’t get connected properly to your bike frame, try getting a replacement.
Battery Management System Cutting Out
Every battery pack has a BMS or Battery Management System to protect the E-bike battery and maintain efficiency. Regardless, whenever the controller or charger draws too much power or creates a fault (short circuit, extreme battery voltage), the BMS will stop the power flow. (Resulting in intermittent power cut-outs.)
Apart from that, when you have abnormal temperatures or one or more cells that are dead, the BMS will shut down the electrical system.
So if you feel like the battery is getting too hot, it’s best to let it cool down and pedal manually. Also, you should never ride while it’s freezing outside; very bad for the batteries.
Unfortunately, repairing a battery is not very straightforward. It requires a lot of tools and experience dealing with electronics. Therefore, it’s best to keep a spare battery. If you need to do a BMS reset, you can check our article about how to do an ebike BMS reset.
And most often, the BMS would start giving all these errors could be because of a failing battery. So if your battery is old or built upon recycling batteries (from old cells), it might not be able to maintain a charge when powering the motor.
So although the display works but your motor cuts out as soon as utilize pedal-assist or throttle, you could be dealing with a dead battery (or a soon-to-be one).
The fastest way to diagnose this is by replacing the battery. If all the problems you had before get resolved as soon as you switch to a newer battery pack, then without a doubt, it’s the previous E-bike battery that was at fault.
However, if the issue keeps happening, and you’ve checked the motor and other wire connections, there might be a fuse or circuit breaker blown/tripped off. On top of that, the battery pack might be undersized for that particular model of e-Bike.
Defective Battery Charger
If the battery is not charging properly, experiencing constant power cut-outs, and shows a low voltage, the issue might not even be your batteries at all (lower-quality batteries or otherwise); it might also be a defective battery charger.
When the charger is not working (not showing an indicator light or not charging the battery), it might have a blown fuse or a burnt-out circuit board. So replace the fuse on the charger (either on the plug itself or on the side of the enclosure) and see if the charger works.
You can try recharging the battery pack using a different (but compatible) charger, and if the battery gets fully charged and works on your eBike without an issue, the previous battery charger might have been at fault.
Most often, repairing/replacing the electrical components on battery chargers is not easy and requires experience dealing with electronics. So you most likely won’t be able to ride your electric bike until you get it repaired.
Also, the battery charger port or the plug on the charger might be damaged, so don’t forget to check those electrical components.
Similar to how the CPU and motherboard are the “central nervous system” of the computer, the controller is the primary processing unit of the electric bike. So even if you don’t use a smart E-bike with an onboard computer and smartphone app integration, you still would need to use a controller to power the motor.
So like all the other electrical components, an E-bike controller is also susceptible to short circuits and total malfunction due to the wired connections or improper battery management.
Testing a controller is hard, but what you can do first is to perform a visual inspection for any tripped-off circuit breakers (in the off position), melted or burned wires, smoke, and mechanical damage within the controller.
After that, you can safely disconnect the controller and test the internals for short circuits using a multimeter or eBike tester. To properly connect an ebike controller, you can check our guide about that.
Also, don’t forget to check the programming wires (such as cruise control, ignition, and learning wires) and make sure they haven’t been tampered with or damaged.
Defective Pedal Assist System
If you find that the motor doesn’t work when you switch to pedal-assist (mostly on mid-drive) electric bicycles, the problem could be due to the pedal sensor.
Different types of E-bikes have different pedal assist systems, with each increased level of pedal assist; drawing more power from the batteries. However, in this case, it might not be due to lower quality batteries or mismanagement of the controller. Sometimes, it could be a fault within the sensors themselves.
Replacing/Diagnosing the cadence sensors or torque sensors on an original eBike might be difficult; you might have to take it to a bike shop in order to get the sensors repaired, but if you are checking the sensor on a converted eBike (converted using a DIY kit), you might have better luck since the sensors are not built-in to the frame.
If you have a DIY ebike with a pedal sensor, ensure the pedal assist magnet position is properly installed, and there’s more than 3mm spacing between the encoder/magnet and the sensor.
Also, check for any damages in the bottom bracket or front sprocket chainring, and readjust the sensors and sensor wires. (Check out this video to learn more.)
Broken Speed Sensor
Speaking of sensors: the motor might also be cutting out because the rear wheel speed sensor is not functioning properly. It might be sending exaggerated signals into the controller, making it think you are riding faster than you actually are, in which case, the eBike will shut down the motor in order to limit the speed.
So if the speedometer shows improper values:
Try reconnecting the speed sensor
Checking the spoke magnet for any damages
Cleaning the sensor of any mud or dust
Make sure there’s enough space between the sensor and spoke magnet
Clean and dry the spokes
Also, if you feel like the sensor is not working in your favor, you can derestrict your eBike by removing the speed limiter or unplugging the speed sensor, or hooking it up using the “Handy” method. Take note, however, that the speedometer will not display the correct speed.
Faulty Brake Levers
In most conversion kits and stock electric bikes, the controller uses brake sensors to detect the operation of the brake levers. This brake motor inhibitor is used to prevent the motor from rolling while the rider is trying to slow down using the brakes; hence, it is an added safety measure.
Unfortunately, when there are mechanical adjustments or faulty components within the brake lever sensors, the controller will detect a false signal and try to disengage the motor. As a result, the E-bikes disconnect intermittently, even if you didn’t pull on the brake handles.
So when diagnosing this issue, you can try disconnecting the brake cables from the controller and trying to ride the bicycle. If these brake sensors are the culprit, you will have to replace them or realign the lever magnet.
Faulty Shifting Sensors
Different E-bike models, especially mid-drives, include shifting sensors (that go in between the rear derailleur and gear shifters) that allow the controller to pause the motor while shifting gears.
The system does this because powering the motor while shifting can put undue stress on the power transmission system, especially the bicycle chain.
Like all the other sensors we’ve talked about, this shift/gear change sensor can also malfunction and cause intermittent power cuts to the motor.
Also, if you’ve set up a custom ignition system (or power switch for the E-bike), it might also be causing the entire system to shut down either due to a loose connection or broken switch.
You can diagnose this issue by disconnecting the ignition switch from the controller and shorting out the ignition cable; it will bypass the ignition and always keep the switch closed.
The buttons on your display screen or controller can get stuck, and in some controllers, the buttons could give signals to the controller, trying to reprogram or shut it down.
This issue can glitch the controller and prevent it from driving the motor. The best way to diagnose this is to remove the display cable (along with any other control wires that go into the display) and see if the motor runs normally.
Having an electric bike cutting out the motor while you are pedaling or accelerating can be annoying, and depending on the severity of the fault, you might have to spend hundreds of dollars to get it fixed. Nonetheless, it will always work in your favor if you carry a spare battery pack and a couple of extra tools to fix a loose connection or burnt-out wire.
Andrew Strider is an electric bike enthusiast. He currently owns 5 electric bikes and is an active member of his local electric bike club where he is able to test many other models/brands a few times a month.
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