Rotation Sensors (CKP,CMP,VSS,ABS)

CKP, CMP, Speed, and ABS Sensors


What Are the Crankshaft, Camshaft, Speed, and ABS Sensors?

Known as rotation sensors, these are very important components in the electronic injection system, since they measure the rotation of the shaft where it is installed and inform the control unit (ECU). If a sensor has problems, the vehicle may not work.


What Is the Function of the Rotation Sensors?

Through the pulsed signal sent by the rotation sensor, the control unit calculates the rotation speed of the axle where the sensor is installed.


What Are Their Applications and Uses?

Rotation sensors can be:

  • Revolutions per minute (RPM) and crankshaft position (CKP) sensors;
  • Camshaft position (CMP) sensors;
  • Vehicle speed sensors (VSS);
  • Wheel speed sensors, used in anti-lock braking systems (ABS).


How They Work

The rotation sensor is always associated with a toothed disc or a disc that is alternately magnetized. Currently, these are the most used types:

  • Variable reluctance sensor
  • Hall effect sensor
  • Magneto-resistive element (MRE) sensor


Variable Reluctance Sensor

This sensor consists of a permanent magnet with a pickup coil over it.

Whenever teeth or marks pass by the magnetic sensor, the reluctance of the magnetic circuit decreases; otherwise, the reluctance increases.

Thus, when the wheel turns, the magnetic flux changes. This flow variation induces a variable voltage in the pickup coil – in the form of pulses, synchronized with the teeth passing by the magnet.

At the sensor terminals, it is possible to measure voltage pulses every time a tooth passes by the sensor. The voltage of the pulses depends on the speed of the spinning wheel.

  • Sensor: manter assim
  • Ímã Permanente: Permanent Magnet
  • Bobina: Coil
  • Entreferro (folga): Air Gap
  • Roda Fônica (roda dentada): Toothed Wheel


Hall Effect Sensor

It consists of a small piece of semiconductor material circulated by a direct current. The electrical resistance of the piece is sensitive to the presence of a magnetic field. Thus, as the magnetic field varies, so do the resistance and the current in the circuit, consequently. An electronic circuit inside the sensor converts current variations into voltage variations.

The figure below shows a hall effect-based rotation sensor, as configured for distributor engines. The magnetic flux that reaches the sensor varies due to the presence or absence of an impeller vane.

In the example, the impeller interposes between the magnet and the sensitive element. When rotating, the alternation between vanes and vaneless spaces generates the magnetic field variation necessary to produce the pulsed signal.

One should notice that, when it comes to Hall sensors, the voltage generated by the sensor does not depend on the rotation speed of the impeller or the toothed wheel.



  • Rotor: Impeller
  • Ímã: Magnet
  • Circuito Eletrônico: Electronic Circuit
  • Sinal: Signal
  • Aba: Vane
  • Janela: Vaneless Space
  • Elemento HALL: Hall Element


Magneto-Resistive Sensor

These sensors contain a disc of magnetic material, alternately magnetized, with magnets of different polarities.

An electronic circuit containing a magneto-resistive element is installed over the disc. The sensitive element can modify its resistance according to the intensity of the magnetic field passing through it. When the disc rotates, the magnets cause the magnetic field variation, which reaches the magneto-resistive element.

As a consequence, its resistance varies as well. An associated electronic circuit transforms resistance variations into voltage variations, thus generating the pulsed signal.


  • Engate p/ engrenagem (saída trasmissão): Gear Attachment
  • Circuito Eletrônico: Electronic Circuit
  • Ímã: Magnet
  • Engate p/ flexível velocímetro: Impeller
  • Disco magnetizado: Speedometer Attachment


Where Are They Located?

The location of a rotation sensor depends on its application:

  • Engine speed sensor: Associated to a toothed wheel, integrated to the crankshaft, or installed in the distributor.
  • Camshaft position sensor: Installed on the headstock or installed on a device mechanically connected to the camshaft – in case of cam-in-block engines.
  • Vehicle speed sensor: Associated to a toothed wheel installed at the transmission output or activated by the speedometer flex shaft.
  • Wheel speed sensor (ABS): Installed on the wheel hub, associated to a toothed wheel. In some rear-wheel drive vehicles, it is installed in the rear wheels drive shaft.


When the Rotation Sensor Doesn’t Work

  • When the defect results in a fault condition out of the sensor’s range (short circuit or open circuit), a fault code is written to memory and can be recovered using testing equipment (a scanner). In this case, the corresponding system enters a state of emergency. As for the wheel speed sensor (ABS), in this case, the system is deactivated.
  • When the defect results in a fault condition within the sensor’s operating range, the fault code is usually not recorded
  • The symptoms depend on the application. The most compromised are:
    • Engine speed sensors: The engine may not run or stop abruptly.
    • Wheel speed sensors (ABS): May cause erratic system operation.



A scanning tool is recommended to check parameters such as “Rotation”, “Vehicle Speed”, and “Wheel Speed” (ABS). Also, diagnosis may rely on recovering possible fault codes stored in memory.

  • In variable reluctance sensors, the coil can be checked with an ohmmeter. The generated signal can be verified with a voltmeter (not a pen-type) by rotating the corresponding axle at a certain speed.
  • Magneto-resistive and Hall effect sensors can be diagnosed with a voltmeter or a polarity test pen. The signal can be verified by slowly rotating the corresponding axle.


Can the “Check Engine” Light Indicate a Problem with a Rotation Sensor?

Yes, it can! It is very important to take the car to a mechanic shop as soon as possible to have it checked. Problems with this sensor may cause the vehicle to malfunction.



All MTE-THOMSON products are guaranteed for 01 (one) year against faulty materials or manufacturing defects. The guarantee is limited to the replacement of the faulty part; it cannot be extended to cover defects caused by misuse, neglect, accident, or wear and tear. We cannot accept liability for consequential loss or damage which is claimed to have resulted from the use of one of our products. More information at:


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