How to Install a Pedal-Assist Sensor on an E-Bike Conversion

To install a pedal-assist sensor on an e-bike conversion, first identify the sensor type and controller connector, then remove power, fit the magnetic ring and pickup sensor on the crank side allowed by the kit, align them exactly as the maker shows, route the cable away from moving parts, and configure only the settings named in the kit manual. Test the system with the wheel and pedals controlled, using the maker’s low-risk procedure. Do not copy a sensor gap, magnet direction, connector color, or setting from another kit.

A cadence PAS sensor detects crank movement and sends a signal to the controller. Grin describes a simple sensor as detecting when the rider is pedaling, while a torque sensor measures pedal force and has a more involved installation (Grin, “Brompton Electric Conversion Kit”, retrieved September 1, 2026). This tutorial addresses a typical external cadence sensor, but the exact battery, controller, display, crank, bottom bracket, and sensor manual take priority.

Before you turn a wrench

• Confirm that the PAS sensor is compatible with the conversion controller and connector.
• Find the kit diagram for sensor side, magnet orientation, ring type, and cable route.
• Turn the e-bike off, remove the battery when the manual permits or requires it, and prevent accidental motor activation.
• Keep the sensor, ring, cable, crank, chainring, frame, and bottom bracket clear of rubbing.
• Never bypass a brake cut-off, BMS, fuse, controller limit, or safety interlock to make the sensor operate.

What you need to identify first

Write down the exact motor, controller, display, battery, PAS sensor, crank, and bottom-bracket models. A sensor that looks similar may use a different connector, signal arrangement, magnet count, ring diameter, crank-side orientation, or controller protocol. Check the kit’s wiring diagram and the controller manual before unplugging anything.

The electric bike conversion kit guide explains why a conversion is a system of compatible motor, battery, controller, display, brakes, and mounting parts. If you have not selected the donor bike yet, use can any bike take an e-bike conversion kit? first. PAS installation cannot correct an unsuitable frame, brake, wheel, or battery match.

Cadence sensor or torque sensor?

A cadence sensor usually uses a magnet disk or ring and a pickup near the crank. It reports crank movement, not how hard the rider is pushing. A torque sensor reads force through a dedicated bottom bracket or crank interface and may require different parts, calibration, wiring, and professional tools. Do not install a cadence ring in a torque-sensor location or change controller settings to pretend that one is the other.

Grin’s Cycle Analyst documentation says the setup must account for sensor type, pole count, and forward or reverse direction, and that direction depends on whether a sensor is installed on the left or right side (Grin, “Cycle Analyst V3.1 Official User Manual”, retrieved September 1, 2026). That is a controller-specific example of why the physical and software setup must match.

Step 1: Make the bike safe to work on

Move the bicycle to a stable, well-lit work area. Shift to a comfortable gear if the kit or bicycle manual calls for it. Turn the display and battery off. Remove the battery or disconnect the approved power plug only as the manual directs. Keep the removed battery dry and away from tools that could bridge its contacts.

Support the bicycle so the crank can turn by hand without the wheel, chain, or derailleur contacting the stand. Do not power the motor to test an uninstalled sensor. Keep fingers, loose clothing, cable ties, and tools away from the chainring and crank.

If the installation requires a pedal or crank removal, use the correct tool and the bicycle maker’s procedure. Mark the cable route with removable tape before removing existing parts. Do not force a stuck crank, bottom-bracket lockring, or retaining ring. A damaged thread or bent crank needs service before a PAS sensor is installed.

Step 2: Choose the approved sensor side

The sensor may install on the left or right crank, depending on the ring, frame clearance, connector lead, and controller direction setting. Use the kit diagram. Check that the ring can sit squarely around the spindle or behind the crank without touching the chainstay, chainring, bottom bracket, or crank arm.

Grin’s split-disk PAS instructions show a ring that closes around the exposed crank spindle and a working surface that faces inward toward the pickup sensor. The page also describes a spacer and a lateral clearance for that particular product (Grin, “Brompton Electric Conversion Kit”, retrieved September 1, 2026). Do not copy either dimension or surface direction to a different ring.

Some rings require a crank or lockring to be removed. Others split around the spindle. A bottom-bracket-mounted sensor may require a special tool and a different sequence. Follow the supplied drawing and preserve washers, spacers, dust seals, and lockrings in their original order.

Step 3: Install the magnetic ring

Clean the crank and spindle area so grit does not hold the ring out of plane. Install the ring in the orientation marked by the maker. The working surface, arrow, magnet side, or molded key may face a particular direction. If the ring is split, close it fully and confirm that it cannot rotate independently of the crank.

Keep the ring centered and square. It should not wobble into the pickup sensor as the crank turns. Reinstall the crank, pedal, lockring, washers, and spacers with the bicycle maker’s tightening procedure. Do not guess torque values or add threadlocker unless the manual says to use it.

Turn the crank slowly by hand. Stop if the ring contacts the frame, chainstay, chainring, cable, or sensor bracket. A small interference at the stand can become a jam under load.

Step 4: Position the pickup sensor

Attach the pickup to the bracket or frame location shown in the kit instructions. The sensor face must point toward the ring’s magnets, and the magnets must pass the sensing area in the direction the maker specifies. Center the sensor over the magnetic path without letting it touch the ring.

The required gap is not universal. Grin’s documented split-disk installation uses its own spacer and a specified lateral clearance, while eBikeling’s FAQ gives a different example for its kits (eBikeling, “FAQs”, retrieved September 1, 2026). Those figures belong to those products. If your manual is silent or unclear, ask the kit maker instead of choosing a gap by eye and declaring the system safe.

Secure the sensor so it cannot rotate under vibration. Use only the supplied bracket, adhesive, cable ties, or fastener method. Do not attach it to a flexible part that changes position as the frame moves. Turn the crank through a full revolution by hand and watch the ring pass the sensor without contact.

Step 5: Route and connect the PAS cable

Route the cable along a protected frame path, leaving enough slack for steering and suspension movement but not enough to reach the chain, crank, tire, brake rotor, or spokes. Keep the wire away from sharp edges and hot surfaces. Use smooth loops rather than tight bends. A wire that is pulled tight at full steering can fail before the bicycle is straight again.

Match the connector by the kit diagram, key, and model, not only by color. Inspect pins, seals, and the connector body for water, dirt, bent contacts, or damage. Connect it fully without twisting or forcing it. If the plug does not fit, stop. An adapter or repinned plug requires explicit maker instructions.

The conversion waterproofing guide covers connector caps, water exposure, and cable routing. Keep the PAS connector dry while the system is open, and do not use grease or sealant unless the connector maker approves it.

Step 6: Configure the controller or display

Some kits recognize a PAS automatically. Others require a display setting for sensor type, pole count, direction, start behavior, assist level, or a controller input. Record the original settings before changing anything. Change only the fields named in the manual and save one change at a time.

Do not increase current, speed, start sensitivity, or assist limits to compensate for a ring that is backward or a sensor that is too far away. Do not bypass brake cut-offs. If the controller has a direction setting, use the maker’s method to determine forward crank rotation and verify that assist stops when pedaling stops.

The e-bike controller guide explains the controller’s role in a conversion, but it cannot supply values for an unknown controller. If the display menu uses unfamiliar abbreviations, stop and consult the manual or a qualified technician.

Step 7: Test without creating a runaway bike

Use the kit’s own test process. Keep the drive wheel supported only when the manual permits and when you can prevent the wheel from contacting anyone or anything. Start with the lowest assist mode, keep hands and tools clear of the drivetrain, and use the brake cut-off if the kit includes one. Never test at full power or with a person holding the wheel.

A safer functional sequence is:

1. With power off, turn the crank by hand and check for contact, cable pull, or ring wobble.
2. Power the system on without a foot on the pedal, if the manual gives that instruction.
3. Confirm that no assist occurs when the crank is still.
4. Apply the lowest allowed assist and rotate the crank through a controlled, small movement.
5. Confirm that assist starts and stops according to the manual.
6. Switch the system off, inspect the connector and cable route, then secure all hardware.
7. Make the first outdoor test in an open, low-traffic area at walking speed, with both brakes working and a helper available if needed.

Some controllers have a start delay or require a full crank turn. Do not interpret every delay as a fault, and do not accept assist that remains on after pedaling stops. If the motor starts unexpectedly, cut power with the approved switch or battery procedure and diagnose before another test.

Troubleshoot a PAS that does not work

Start with power off and inspect in this order:

• Is the battery charged, installed, and compatible?
• Is the display or controller reporting a fault?
• Is the PAS plug seated in the correct port and undamaged?
• Is the ring the right way around and firmly attached to the crank?
• Does the sensor face the magnets and stay centered through a full rotation?
• Is the cable pinched, cut, stretched, or near a moving part?
• Does the controller setting match the sensor type, pole count, side, and direction?
• Is a brake cut-off active or misadjusted?

If assist works only when pedaling backward, the ring orientation or direction setting may not match the installation. If assist is always on, stop using the bike and inspect the ring, sensor, controller input, and brake cut-off. If the connector or controller is wet, power down and follow the maker’s drying and service instructions.

Do not open the controller or battery to chase a PAS signal unless the maker authorizes that service. The e-bike battery mounting guide and conversion battery-size guide cover different system checks that should be settled before blaming the sensor.

Installation mistakes to avoid

Assuming every sensor is universal

A three-pin plug, magnet disk, or sensor label does not prove electrical compatibility. Match the controller protocol, voltage, connector, pole arrangement, and physical fit.

Guessing the gap

A gap that works on one kit may fail on another. Use the manual or ask the maker. Do not allow the sensor to touch the ring while the crank turns.

Letting the cable reach the drivetrain

A cable can be pulled into the chain or crank even when it looks tidy while parked. Turn the crank and bars, compress suspension only as the bike maker allows, and check every moving path.

Testing with the bike ready to launch

Do not sit on the bike while changing settings or testing an unknown sensor. Keep the lowest assist setting, working brakes, and an immediate power-off path.

Ignoring the rest of the conversion

PAS only controls when the system responds to pedal movement. It does not secure a hub motor, correct a battery mismatch, make a brake adequate, or replace the torque-arm guide for a hub-motor installation.

When to use a qualified technician

Ask a qualified technician when the bottom bracket is seized, a crank or lockring is damaged, the sensor is internally routed, the connector is unknown, the controller has been rewired, the battery is damaged, or the motor responds unpredictably. Stop using a system with intermittent assist, an assist signal that does not stop, heat or smoke, a damaged cable, or water inside an electrical component.

An installed PAS should be boring: the ring turns with the crank, the sensor stays aligned, the cable remains clear, the controller sees the expected signal, and assist stops when the rider stops pedaling. Match the manual at each step and treat uncertainty as a reason to pause, not a reason to increase a setting.

Sources

• Grin, “Brompton Electric Conversion Kit”, retrieved September 1, 2026.
• Grin, “Cycle Analyst V3.1 Official User Manual”, retrieved September 1, 2026.
• eBikeling, “FAQs”, retrieved September 1, 2026.