Is the controller bad? Test it, then match a replacement
Motor & controllerHub-motor and generic e-bikes
The controller is the box between battery, display, sensors and motor. It fails from heat, water, a shorted hall wire or a stalled motor. This flow shows the symptoms and tests that pin it down, the swap logic that proves it, and exactly what to match when buying a replacement.
Warning: Disconnect the battery before opening a controller. Capacitors inside can hold a charge briefly. Never connect phase wires to battery voltage, and never plug a 36V controller into a 48V+ battery.
What you need
- Multimeter
- Small screwdrivers, Allen keys for the controller box
- Torch, magnifier
- The old controller's label (photograph it: voltage, current, model, connector layout)
- Optional: a known-good controller with the same connectors for the swap test
LED patterns and what they mean
| Logic side alive, drive stage or start-condition problem | |
| Controller input stage, fuse, or key-switch wire | |
| Blown MOSFET or capacitor | |
| Shorted MOSFET | |
| 5V regulator dead | |
| One drive phase weak (partial MOSFET failure) or a hall input dead | |
| Thermal cut-off; controller mounted where it cannot shed heat |
Step by step
1. Symptoms that point at the controller
Note what happens: nothing at all; display on but no motor; stutter; cut-out under load; motor runs by itself; wheel stiff when plugged in. Compare with the table above.
If OK: Symptoms match a controller pattern. Continue with the tests to prove it.
If not: Symptoms are speed-sensor, brake or PAS related: those flows first. The controller is the last thing to replace, not the first.
Note what happens: nothing at all; display on but no motor; stutter; cut-out under load; motor runs by itself; wheel stiff when plugged in. Compare with the table above.
If OK: Symptoms match a controller pattern. Continue with the tests to prove it.
If not: Symptoms are speed-sensor, brake or PAS related: those flows first. The controller is the last thing to replace, not the first.
2. Input: is the controller receiving battery voltage?
Battery on. Measure at the controller's thick red/black input wires (or its main connector): full battery voltage should be present. If the controller has an ignition/key wire (thin red, often on the display connector), check it also carries battery voltage when the display is on.
If OK: Battery voltage at the input and on the key wire. Continue.
If not: No voltage: harness, fuse, key switch, or battery. Not the controller yet.
Battery on. Measure at the controller's thick red/black input wires (or its main connector): full battery voltage should be present. If the controller has an ignition/key wire (thin red, often on the display connector), check it also carries battery voltage when the display is on.
If OK: Battery voltage at the input and on the key wire. Continue.
If not: No voltage: harness, fuse, key switch, or battery. Not the controller yet.
3. Smell, look, feel
Battery off. Open the controller case if possible (usually two end caps). Look for bulged capacitors, brown burns on the board, melted phase solder joints, water marks, a cracked MOSFET. Smell it. Feel the heatsink side for a warped or discoloured area.
If OK: Clean board, no smell. Continue.
If not: Any of these: the controller is dead. Photograph the label and go to step 8 to match a replacement. Find the cause: water, overheating (mounted inside a sealed bag with no airflow), or a shorted hall/phase wire that will kill the next one.
Battery off. Open the controller case if possible (usually two end caps). Look for bulged capacitors, brown burns on the board, melted phase solder joints, water marks, a cracked MOSFET. Smell it. Feel the heatsink side for a warped or discoloured area.
If OK: Clean board, no smell. Continue.
If not: Any of these: the controller is dead. Photograph the label and go to step 8 to match a replacement. Find the cause: water, overheating (mounted inside a sealed bag with no airflow), or a shorted hall/phase wire that will kill the next one.
4. 5V rail
Battery on, display on. Measure red-black on the hall connector, the throttle plug and the PAS plug. All should show about 5V (some 4.3-5.2V).
If OK: 5V present. Continue.
If not: 5V missing on all: the controller's regulator is dead. Missing on one plug only: that plug's wire. Note that a shorted hall sensor in the motor can pull the 5V rail down; unplug the motor's hall connector and re-measure. If 5V returns, the motor's hall wiring is shorted, not the controller.
Battery on, display on. Measure red-black on the hall connector, the throttle plug and the PAS plug. All should show about 5V (some 4.3-5.2V).
If OK: 5V present. Continue.
If not: 5V missing on all: the controller's regulator is dead. Missing on one plug only: that plug's wire. Note that a shorted hall sensor in the motor can pull the 5V rail down; unplug the motor's hall connector and re-measure. If 5V returns, the motor's hall wiring is shorted, not the controller.
5. Shorted MOSFET test
Battery unplugged. With the motor's phase wires plugged into the controller, spin the wheel by hand. Then unplug the phases and spin again.
If OK: Wheel spins the same in both cases (geared hub: freely; direct-drive: light even drag). No shorted MOSFET. Continue.
If not: Wheel is stiff or lumpy only when plugged into the controller: a MOSFET is shorted. Confirm with the meter on ohms between each phase wire and the controller's battery negative and positive: a low reading (under a few ohms) on any phase is a shorted MOSFET. Controller dead.
Battery unplugged. With the motor's phase wires plugged into the controller, spin the wheel by hand. Then unplug the phases and spin again.
If OK: Wheel spins the same in both cases (geared hub: freely; direct-drive: light even drag). No shorted MOSFET. Continue.
If not: Wheel is stiff or lumpy only when plugged into the controller: a MOSFET is shorted. Confirm with the meter on ohms between each phase wire and the controller's battery negative and positive: a low reading (under a few ohms) on any phase is a shorted MOSFET. Controller dead.
6. Thermal cut-out
If the bike works but stops after 10-30 seconds of hard climbing and recovers after a rest: feel the controller case after the cut-out. Check where it is mounted (inside a foam-lined bag, under a battery with no airflow, or wrapped in tape).
If OK: Controller cool, cut-outs are not thermal. Check the battery under load instead.
If not: Controller too hot to hold: it is over-driven or cannot shed heat. Remount it with the heatsink side exposed to air, or replace it with a higher-rated unit.
If the bike works but stops after 10-30 seconds of hard climbing and recovers after a rest: feel the controller case after the cut-out. Check where it is mounted (inside a foam-lined bag, under a battery with no airflow, or wrapped in tape).
If OK: Controller cool, cut-outs are not thermal. Check the battery under load instead.
If not: Controller too hot to hold: it is over-driven or cannot shed heat. Remount it with the heatsink side exposed to air, or replace it with a higher-rated unit.
7. Swap test: the proof
Fit a known-good controller with the same voltage, connector layout and display protocol. Connect halls, phases (colour to colour first), display, throttle, brakes, PAS. Wheel in the air, power on, throttle or pedal.
If OK: Motor runs on the swap controller: the original controller is confirmed dead. Keep the swap or buy a matched one.
If not: Swap controller also does nothing: the fault is not the controller. Go back to battery, motor (halls/phases) and sensors. If the motor runs backwards or stutters on the swap, the phase/hall combination is wrong (see step 9).
Fit a known-good controller with the same voltage, connector layout and display protocol. Connect halls, phases (colour to colour first), display, throttle, brakes, PAS. Wheel in the air, power on, throttle or pedal.
If OK: Motor runs on the swap controller: the original controller is confirmed dead. Keep the swap or buy a matched one.
If not: Swap controller also does nothing: the fault is not the controller. Go back to battery, motor (halls/phases) and sensors. If the motor runs backwards or stutters on the swap, the phase/hall combination is wrong (see step 9).
8. Matching a replacement: what must match
Read the old label and match: 1) Voltage: 36V, 48V, 52V or a 36-48V dual-rated unit; never lower than the battery. 2) Current: same continuous amps (15A, 17A, 20A, 22A, 25A, 30A) or slightly higher if the motor and battery BMS allow. 3) Sensored/sensorless: a sensored motor (hall wires present) needs a sensored or dual-mode controller; sensorless controllers work on any motor but start rough. 4) Display protocol: the display must speak the controller's language: KT (LCD3/LCD8), Bafang UART, Lishui, S866/S830 UART, or CAN. Buy the display and controller as a pair if unsure. 5) Connectors: 9-pin Julet motor plug, or separate hall (5/6-pin) and phase (3 bullet) connectors; brake 2/3-pin; throttle 3-pin; PAS 3-pin; display 5/6-pin. 6) Physical size: it has to fit the frame box. 7) Features: regen braking, torque-sensor input, light output, only if you use them.
If OK: All seven matched: order it.
If not: Cannot match the display protocol: buy a controller and display bundle (KT is the most common generic pairing). Cannot match connectors: order a controller with bare wires and reuse your old plugs, or buy an adapter set.
Read the old label and match: 1) Voltage: 36V, 48V, 52V or a 36-48V dual-rated unit; never lower than the battery. 2) Current: same continuous amps (15A, 17A, 20A, 22A, 25A, 30A) or slightly higher if the motor and battery BMS allow. 3) Sensored/sensorless: a sensored motor (hall wires present) needs a sensored or dual-mode controller; sensorless controllers work on any motor but start rough. 4) Display protocol: the display must speak the controller's language: KT (LCD3/LCD8), Bafang UART, Lishui, S866/S830 UART, or CAN. Buy the display and controller as a pair if unsure. 5) Connectors: 9-pin Julet motor plug, or separate hall (5/6-pin) and phase (3 bullet) connectors; brake 2/3-pin; throttle 3-pin; PAS 3-pin; display 5/6-pin. 6) Physical size: it has to fit the frame box. 7) Features: regen braking, torque-sensor input, light output, only if you use them.
If OK: All seven matched: order it.
If not: Cannot match the display protocol: buy a controller and display bundle (KT is the most common generic pairing). Cannot match connectors: order a controller with bare wires and reuse your old plugs, or buy an adapter set.
9. Phase and hall combination on the new controller
If the new controller makes the motor run backwards, stutter, or draw high current: the phase order and hall order do not match. With the wheel in the air and low throttle, try the phase wire combinations (there are six) until the motor runs smoothly forward with the lowest current. Some controllers have a self-learn wire: short the two learning wires briefly with the display on and the wheel free, then unshort when the motor runs the right way.
If OK: Smooth, quiet, correct direction at low current: done.
If not: No combination runs smoothly: a hall sensor is bad, or the controller is sensorless-only on a motor that needs sensors. Recheck the hall test.
If the new controller makes the motor run backwards, stutter, or draw high current: the phase order and hall order do not match. With the wheel in the air and low throttle, try the phase wire combinations (there are six) until the motor runs smoothly forward with the lowest current. Some controllers have a self-learn wire: short the two learning wires briefly with the display on and the wheel free, then unshort when the motor runs the right way.
If OK: Smooth, quiet, correct direction at low current: done.
If not: No combination runs smoothly: a hall sensor is bad, or the controller is sensorless-only on a motor that needs sensors. Recheck the hall test.
What you now know for sure
- Not the controller: No input voltage, or a swap controller does not help. → Battery, harness, sensors, motor.
- Controller: input or 5V regulator: Battery voltage in, no display and/or no 5V on plugs (with the motor's halls unplugged). → Replace controller.
- Controller: drive stage: Shorted MOSFET (wheel stiff when plugged in), burnt smell, phases low-ohm to the battery rails. → Replace controller; find the cause (water, stall, hall short).
- Controller: thermal: Cut-outs after sustained load, case very hot. → Remount for airflow or upgrade current rating.
- Wrong replacement: Runs backwards, stutters, or display shows nothing on the new unit. → Fix phase/hall order or match the display protocol.
Manuals and video
Videos found on YouTube and checked automatically; they are other people's work, not ours.
Frequently asked
Can I use any controller with my hub motor?
Any controller of the right voltage with the right hall/sensorless mode can drive the motor. The display is the constraint: it only talks to controllers using its protocol (KT, Bafang UART, Lishui, S866, CAN). Buy the controller and display as a matched pair unless you know the protocol.
Why did my new controller die the same way as the old one?
Because the thing that killed the first one is still there: a shorted hall wire in the motor, water in the motor connector, or a battery BMS that lets the controller pull more current than it can handle during a stall. Test the motor halls and phases before fitting a replacement.
Can a bad controller damage the motor?
A shorted MOSFET puts DC through a winding and can overheat it if you keep riding, and a controller running the wrong phase/hall combination draws high current and heats both. Stop riding as soon as the wheel feels stiff or the motor stutters.
Still stuck? Take a photo of the display and show it to a local e-bike workshop. Bring the bike's system name (e.g. Bosch Performance Line CX) and the exact code; that saves diagnostic time and money.