E-bike troubleshooting

Ebike Pedal Assist Not Working

Sounds like this? I pedal and nothing happens, but the throttle still works · Assist kicks in several seconds late · Assist cuts out and comes back while pedaling steadily · Assist stopped after a bottom bracket or crank service · Assist works in one gear or at one cadence only · Speed readout shows zero while I am riding

Pedal assist depends on the bike knowing you are pedaling. That sounds obvious, but it is the whole diagnosis: something has to detect crank movement or pedaling force and report it to the controller. When assist dies, that reporting has usually stopped.

There are two main designs. A cadence sensor uses a ring of magnets on the crank spindle passing a small pickup, and it only knows whether the cranks are turning. A torque sensor measures the force you apply, usually at the bottom bracket or the rear axle, and delivers assist proportional to your effort. Cadence systems fail mechanically — a shifted magnet disc, a bent bracket, a widened gap. Torque systems fail electrically more often.

As with a dead throttle, the first fact worth establishing is whether the other input still works. If the throttle drives the bike normally and only pedaling does nothing, the battery, controller output, and motor are all proven fine.

Common causes

1. Assist level set to zero — Nearly every system disables pedal assist at level 0. Some displays also reset to 0 on every power-up, which makes this feel like a fault.
2. Cadence magnet disc shifted or damaged — The magnet ring slides on the spindle, loses a magnet, or gets cracked. Assist then works intermittently, only in one crank position, or not at all.
3. Sensor gap too wide or misaligned — The pickup has to sit close to the magnets. Bottom bracket work, a knocked bracket, or a loose zip tie moves it just far enough to stop reading.
4. Sensor connector unplugged or corroded — The pedal assist connector sits low on the frame, right in the spray from the front wheel, which makes it a magnet for water and grit.
5. Torque sensor failure — Torque sensors are sealed units and are not user serviceable. When they fail, assist usually stops entirely and a display code often appears.
6. Brake cutoff stuck engaged — Just as with the throttle, a brake sensor that thinks you are braking suppresses assist.
7. Speed sensor or spoke magnet knocked out of place — On systems that require a valid speed signal, a missing one can disable or heavily limit assist.
8. Damaged cable at the chainstay — The thin sensor cable runs along the frame near the chain and gets caught, chafed, or crushed.

What to check first

1. Raise the assist level above zero and pedal. Check whether the level survives a power cycle.
2. Try the throttle. If it drives the bike normally, you have narrowed the problem to the pedal assist circuit and its settings.
3. Watch the display while you pedal slowly. Many displays show a cadence or power indicator. Any movement in that reading proves the sensor is being seen.
4. Look behind the left crank with a flashlight. Find the magnet disc and the small sensor beside it. Look for a wide or uneven gap, missing magnets, a cracked disc, or a sensor hanging loose.
5. Spin the cranks backward by hand and watch the display. On many cadence systems nothing should happen in reverse, but any flicker of a power reading confirms the pickup is alive.
6. Trace the sensor cable to its connector. Unplug it, look for corrosion or water, reseat firmly, and try again.
7. Check the spoke magnet and speed sensor. Confirm the magnet is still on the spoke, is aligned with the sensor, and passes it closely. Check whether the display shows a speed while you ride.
8. Squeeze and release both brake levers, then retry. Rule out a stuck cutoff before you go any further.

When to stop and get help

• Stop if assist engages on its own without pedaling, or continues after you stop pedaling.
• Stop if the motor does not cut out when you brake.
• Stop riding if you hear grinding or knocking from the bottom bracket area, which suggests the mechanical problem is bigger than a sensor.
• Do not remove or bypass a brake cutoff sensor to restore assist.
• Do not force a magnet disc onto or off a crank spindle with tools. It is easy to crack the disc or damage the spindle.
• Do not attempt to open a torque sensor or drive unit.

What a repair usually involves

A shop confirms settings and brake cutoffs first, then determines which sensor design the bike uses, because the paths diverge sharply from there. For a cadence system they check magnet spacing, alignment, the physical gap, and whether the sensor produces a signal as the cranks turn. For a torque system they check supply voltage and the sensor’s output under load.

The parts that come out are usually inexpensive: a cadence sensor and magnet disc, a connector or short harness section, a speed sensor, or a spoke magnet. Torque sensors and drive units are the costly outcomes, and a careful shop confirms them rather than assuming.

Preparing an organized description before that visit matters because pedal assist faults are pattern faults. Assist that fails only in one crank position, only above a certain cadence, only after the frame is wet, or only since a specific service appointment each point somewhere different. Photos of the magnet disc and sensor area, along with a clear account of when the symptom started, remove most of the guesswork before anyone touches the bike.