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Electric Bike Spares, Parts & Accessories

PAS Sensors

PLEASE NOTE: This is a troubleshooting guide for ‘cadence’ PAS sensors, not ‘torque’ sensors, which function in a different manner.

Operation & Installation

A cadence PAS sensor consists of a sensor head and a disc with a number of magnets embedded within. Sometimes these two parts are contained within a single assembly, but more often than not, they are two separate pieces.

The magnet disc is installed on the bottom bracket spindle beneath one of the cranks so that it spins along with the crankset as the rider pedals. The sensor head is mounted in a fixed position adjacent to the magnet disc, usually secured beneath the bottom bracket securing nut. When the magnet disc rotates, the sensor detects each magnet as they pass.

In order for a cadence sensor to function correctly, both the sensor head and magnet disc have to be oriented correctly. Both parts are directional, so there are four possible combinations of sensor head and disc position. Only one combination will work, the other three will not. In addition to this, the magnet disc needs to pass the sensor head within a distance of ~4mm or less. It is recommended that you confirm the correct orientation of the sensor head and magnet disc by hand before fitting the parts to the bike.


Wiring

The majority of cadence sensors feature 3 wires - power (5V), ground and signal. 4 wire PAS sensors feature an additional ‘direction’ wire. Although I don't explicitly discuss 4 wire sensors here, testing and troubleshooting them is similar to dealing with a 3 wire sensor.

There is little in the way of standardisation when it comes to electric bike wiring, but some of the more common wire colour combinations for PAS sensors are as follows:

Power

Ground

Signal

Red

Black

Green

Brown

Black

Yellow

Red

Yellow

Blue

Please note that the exact wire sequence order might not match that listed above. The wire colours of the sensor and controller might also not match each other, but their functions must match in order for the sensor to operate correctly. An incorrect match could lead to damage of the sensor and/or controller, so care should be taken.


Testing, Troubleshooting & Fault Finding

With the aid of an electrical multimeter, there are a couple of basic tests that can be carried out to ascertain if the sensor is installed and functioning correctly. All testing should be carried out with the sensor connected to the controller and the controller powered on.

  • With your multimeter set to 20VDC, insert your multimeter probes into the rear of the PAS connector (sensor side) on the POWER and GROUND wires. You should see a voltage of ~5V. If you do not see 5V, there are a number of possibilities:

Possible Fault

Solution

The PAS sensor cable is not connected correctly to the controller.

Check that the PAS sensor wiring matches the controller side wire functions.

The controller is not powered on.

Connect a battery to the controller and turn on the power.

Both halves of the connector are not mated properly, preventing a number of terminals from making good contact.

Check that both halves of the connector are pushed together all the way and secure. Also ensure that the connector terminals haven't backed out of their housing.

There is a break in the 5V or ground wire somewhere between the controller and sensor head.

Try wiggling the cable to check for an intermittent connection. Ideally, use a multimeter to continuity test between the controller and rear of the sensor side connector.

The 5V regulator inside the motor controller has failed.

This requires that the 5V regulator IC is replaced. Depending on the controller, it might be more cost-effective to simply swap the whole unit for a new one.

 

  • Next, insert the multimeter probes into the rear of the PAS connector (controller side) on the SIGNAL and GROUND wires and rotate the PAS sensor ring. You should see the output signal alter between high and low as the ring is rotated and the magnets pass the sensor. If you do not see the output signal alter between high and low, there are a number of possibilities:

Possible Fault

Solution

The magnet disc is not mounted close enough to the sensor head.

The magnets should pass within ~4mm or less of the sensor head.

The sensor head is mounted on the wrong side of the bottom bracket spindle or is oriented the wrong way round.

Remove the sensor head and magnet disc from the bike and confirm the correct orientation by hand before refitting.

The magnet disc is mounted the wrong way round. If this is the case, the sensor will output a signal and the motor will spin when the pedals are turned backwards.

Remove the magnet disc from the bike, flip it over, then refit.

The sensor head is faulty.

This is hard to diagnose definitively and can only be confirmed using the process of elimination. Ideally you would try another sensor, or try the suspect sensor on a known good setup.


Additional Troubleshooting Tips

  • If pedalling backwards causes the motor to engage, simply flip the magnet disc over to reverse the required direction of movement.
  • Cadence sensors sometimes feature an LED that flashes each time a magnet passes the sensor head. This is handy for verifying that the PAS sensor is functioning correctly “in the field”. Illumination however, does not necessarily mean that the sensor is wired correctly. If the 5V power feed is incorrectly backfed through the signal wire, the LED will illuminate, but nothing will happen. If this is the case with a newly installed sensor, it might be worth checking if the 5V and signal wires haven't been accidentally swapped.