How to Install a Pedal Assist Sensor: Complete PAS Installation Guide

Installing a pedal assist sensor (PAS) lets your e‑bike motor engage when you pedal, giving you a natural riding feel without needing a throttle. The sensor detects crank rotation and sends a signal to the controller. Getting it right comes down to mounting the sensor disc cleanly on your bottom bracket and routing the wiring so nothing rubs or pinches. This guide covers the two common sensor types—disc‑style and spoke‑magnet—and walks through installation on square‑taper, ISIS, and Hollowtech bottom brackets.

Before You Start: What to Check and What to Have

Start by confirming your sensor kit and bottom bracket are compatible. Open the kit and verify the inside diameter of the magnet ring matches your spindle diameter—typically 17 mm for square taper, 19 mm for ISIS, or 24 mm for Hollowtech. If the ring won’t slide over the spindle, you cannot proceed with that kit without a spacer or a different spindle.

Tools you’ll need:

• PAS sensor kit (disc‑type or spoke‑magnet)
• Bottom bracket tool specific to your crank type (spline tool, crank puller, or 8 mm Allen key)
• Pedal wrench (left pedal is reverse‑thread)
• Zip ties (small, black, UV‑resistant)
• Electrical tape or heat shrink
• Side cutters
• Isopropyl alcohol and a clean rag
• Multimeter (for wiring verification)

Stop and escalate if: You cannot remove the bottom bracket cups without excessive force—a seized bottom bracket risks damaging the frame threads. Also stop if the magnet ring doesn’t fit the spindle; forcing it on can crack the magnets or damage the sensor. In either case, take the bike to a shop. A professional has the tools and experience to extract a stuck bottom bracket without damaging the frame.

Shut the battery off and disconnect it before starting. Remove the chain from the chainring if possible—it keeps grease off your hands and makes crank removal easier. Clean the bottom bracket shell thoroughly with isopropyl alcohol. Any grease or grit on the shell prevents the sensor ring from seating flush, and even a 1 mm gap can cause the sensor to misread cadence or rub against the frame.

Step 1: Identify Your Crank and Bottom Bracket Type

The sensor ring mounts around the bottom bracket spindle, and the removal method depends on your crankset. Look at the spindle shape:

• Square taper – A square‑ended spindle with a visible cutout. Requires a crank puller to remove the crank arms. The bottom bracket uses threaded cups.
• ISIS – A splined spindle with 10 splines (looks like a star pattern). Also needs a crank puller for the arms and a splined tool for the bottom bracket cups.
• Hollowtech (Shimano) / outboard bearing – A hollow spindle with pinch bolts on the non‑drive crank arm. No crank puller needed. The bottom bracket bearings are pressed into the frame cups, and the crankset slides out after removing the preload cap.

If your kit uses a spoke magnet instead of a disc ring, you attach a magnet to one chainstay and zip‑tie the sensor near it. This method works for simple cadence‑only systems but is less precise. The disc‑style sensor is more reliable for aftermarket conversions because it reads rotation directly at the spindle rather than from a spoke that can shift or hit the chainstay.

Step 2: Remove the Crank and Mount the Sensor Ring (Disc‑Type)

The sensor ring slides onto the bottom bracket spindle between the frame and the drive‑side crank arm. You need to remove the crank on the drive side (right side) first, then the bottom bracket if the ring doesn’t fit over the existing spindle.

Square Taper and ISIS

1. Use a pedal wrench to remove both pedals (left pedal is reverse‑thread—turn clockwise to loosen).

2. Use a crank puller to pop both crank arms off. Insert the puller tip fully and turn the handle until the arm separates from the spindle. Don’t force it—if the puller strips the threads, the crank arm is ruined.

3. Use the correct bottom bracket tool to remove the cups. For square taper, a Park Tool BBT‑22 or equivalent works. For ISIS, use a compatible splined tool. Turn the drive‑side cup counterclockwise and the non‑drive side clockwise (some thread directions vary—check your bike’s manual).

4. Clean the spindle and frame shell with isopropyl alcohol. Slide the PAS sensor disc over the spindle with the magnet ring facing outward (toward the crank arm). The ring must spin freely on the spindle. Reinstall the bottom bracket cups and torque to the manufacturer’s spec—typically 30–40 N·m for square taper.

Hollowtech (Shimano) / Outboard Bearing

1. Remove the left crank arm pinch bolts with an 8 mm Allen key, then slide the arm off the spindle.

2. Use the 8 mm Allen key to loosen the preload cap on the non‑drive side. Turn it counterclockwise and remove it.

3. Pull the entire crankset out from the drive side. The non‑drive arm comes off completely, exposing the spindle.

4. Slide the PAS ring onto the spindle from the drive side before reinstalling the crankset. Thread the drive‑side spindle through the ring as you reinsert the crankset into the frame.

5. Reinstall the non‑drive arm and tighten the pinch bolts to 12–14 N·m. The ring should sit flat between the frame and the crank arm without rubbing.

Stop and escalate if: The crank arm won’t separate from the spindle after normal puller force, or if the bottom bracket cup won’t turn. Seized threads or a stuck crank can damage the frame or puller tool. A shop has penetrating oil, heat, and extraction tools that a home mechanic usually doesn’t.

Step 3: Position the Sensor Pickup

The disc‑style kit has two parts: a magnet ring (toothed or with embedded magnets) and a sensor pickup that bolts or zip‑ties to the frame.

• After the magnet ring is on the spindle, mount the sensor pickup on the frame. Most kits include a bracket with two screw holes. Align the sensor head so it sits about 1/8 inch (3–4 mm) from the passing magnets. The gap is critical—too far and the sensor won’t trigger; too close and it may rub or break.
• If your frame doesn’t have mounting bosses near the bottom bracket, use zip ties to secure the pickup to the chainstay or down tube. Make sure the pickup stays rigid—any wobble changes the gap mid‑ride.
• Rotate the crank slowly by hand. You should hear or feel a faint click as each magnet passes the sensor. If you don’t, adjust the gap. The ring should spin without making contact with anything.

For spoke‑magnet sensors, attach the magnet to a non‑drive‑side spoke and zip‑tie the sensor head to the chainstay. Position the magnet so it passes within 1/4 inch (6 mm) of the sensor head. This method is more prone to false readings because the spoke can flex or the magnet can shift over time.

Step 4: Route and Connect the Wiring

The PAS sensor wire runs to your controller or display—usually a three‑wire JST connector with red (+5V), black (ground), and green (signal). Routing mistakes cause most post‑installation failures.

• Run the wire along the down tube or chainstay, using zip ties every 4–6 inches. Leave enough slack at the crankset so the wire doesn’t pull tight when you turn the handlebars or flex the frame. A tight wire can snap a pin inside the connector.
• Keep the wire away from the chainring and derailleur. The chain can slice through the insulation in a few pedal strokes if the wire touches the chainring.
• Connect the PAS sensor plug to the matching port on your controller. Most e‑bike conversion kits label this port “PAS” or “Cadence.” If your kit doesn’t have a labeled port, check the manual for the pinout. You may need to splice the wires.
• If splicing, use solder and heat shrink or crimp connectors. Do not use twist‑and‑tape—it fails under vibration. Confirm polarity before powering on: red to +5V, black to ground, green to signal. A multimeter set to DC voltage can verify +5V on the red wire before connecting.

Step 5: Reinstall the Chain and Confirm Operation

Put the chain back on, reinstall the chainring if removed, and rotate the crank a few full turns to make sure the chain runs smoothly and doesn’t catch the sensor wire. With the battery reconnected and the display on, spin the crank forward by hand at a moderate pace (about one revolution per second). The display should show a cadence reading in RPM—typically 30–60 RPM for a slow hand spin. If the display shows 0 RPM, the sensor isn’t reading. Check the gap, the connector, and the polarity.

If the display shows RPM but the motor does not assist, the controller may need programming—consult the conversion kit manual. If the motor runs continuously without pedaling, the magnet ring is likely reversed (magnets facing away from the sensor). Flip the ring and retest.

Final Checks Before Riding

• Mechanical clearance: Spin the crank through several full rotations. Listen for rubbing or clicking. The sensor ring and pickup should be silent. If you hear contact, stop and adjust the gap or reposition the pickup.
• Wire security: Tug each zip‑tie gently to confirm the wire is locked in place. Make sure no wire loops hang low enough to catch on trail debris or the chain.
• Battery connection: Re‑seat the battery firmly and confirm the display powers on. Ride a few feet in a safe area to verify the motor cuts in within the first half crank rotation and stops when you stop pedaling.

A correctly installed PAS sensor will give you smooth, responsive assist every time you pedal. If you encounter consistent issues after following these steps, the problem is usually a loose gap or reversed magnet orientation—both easy to fix with the adjustments described above.

---

Related Articles

• How to Install a Pedal Assist Sensor on Your Electric Bike
• How To Install Your Robot Lawn Mower: A Step-By-Step Guide
• How to Adjust the Brakes on Your Rad Power Bike: A Step-by-Step Guide