24x_Crank_SensorCrank shaft position is one of the four major inputs to an ECU. To best judge Spark, & Fuel timing, It is critical, the ECU understands where in 720degrees & how fast the Crankshaft is spinning.
Common CKP sensors include Magnetic, Hall, and Optical devices. Most technicians will favour a Scan Tool for testing these components, and there is nothing fundamentally wrong with this, but there is a caveat.
Many times now, especially on newer engines, the ECU is looking for a satisfactory (positional) relationship with a CMPS (Camshaft Sensor). If for example, your timing chain has stretched, or the idler pulley has failed, the two critical components will not be in phase. This is generally likely to set a CKPS improbable DTC.
Bosch_CrankAngle sensor copyTherefore, the qualified technician, will reach instead, for his/her 2-4 Channel Oscilliscope. With both patterns now displayed on the screen, the operator should be able to detect any slop or play in the valve train by monitoring the relation ship between the two sensors.
Further, It is important that any Magnetic sensors have sufficient Amplitude to trigger an ecu event, and that the sensor is wired correctly. A reverse wired sensor will produce a reversed pattern sending a less observant technician around in circles as the ECU triggers spark & fuel timing incorrectly, if at all.
Hall Sensors are not infallible either, but they do produce a very simple switched scope pattern. The Duty Cycle will reflect the design of the chopper plate or wheel, and only the frequency will increase with RPM increases.
Optical sensors are not so common, and they are the most likely to give problems as the LED emmitters & sensors slow down with age. It is even a little known fact that some OE Optical crank trigger devices factor in this ageing to their software to account for delays in timing as the components degrade. Optical sensors will give a scope pattern very similar to the Hall Sensors. I.E a regular switched, or square wave pattern, with a fixed Duty Cycle, and an increasing frequency correspondent to RPM.