Measuring an inductive crankshaft sensor

Sensor information

Type: Crankshaft sensor inductive
Power supply: -
Signal type: Frequency varying
Signal level: ±0.25 V minimum up to ±60 V

Workings of the inductive crankshaft sensor

The inductive crankshaft sensor is constructed of a permanent magnet with a coil around it. The magnetic field strength changes when a magnetism sensitive object passes trough the magnetic field of the magnet. This changing of the magnetic field induces a voltage in the coil. The polarity of the induced voltage depends on the direction of the moving object, moving away or towards the sensor. This sensor doesn't need a power supply.

In most cases the object used to influence the magnetic field is a disk or ring with evenly distributed teeth, mounted on the crankshaft or flywheel. When the crankshaft is rotating, the teeth are passing the sensor and the pattern in which they are placed is visible in the crankshaft sensor signal. Every period of the signal is a tooth passing the sensor. The frequency and amplitude of the signal depends on the rotation speed of the crankshaft and the amount of teeth on the disk or flywheel. The disk or ring usually has one or two teeth missing or a tooth that is bigger than the rest as a reference point so the ECU can determine the position of the crankshaft. The ECU uses the crankshaft position to determine when to inject the fuel and when to ignite it.

Hint:

With a RPM I/O the crankshaft sensor signal can be converted to a signal that shows the course of the engine speed.

Connecting the lab scope

Correct functioning of the crankshaft sensor can be checked by measuring the following signal voltages, see figure 1:

Channel Probe Voltage Range
1 Red probe Signal at the positive side of the sensor 8 V 1
Black probe Signal at the negative side of the sensor
Measuring diagram
Figure 1: Measuring diagram
Measuring a working crankshaft sensor
Figure 2: Measuring a working crankshaft sensor

The lab scope is connected to the crankshaft sensor via a Measure lead TP-C1812B and Back Probe TP-BP85. The lab scope is set to normal scope mode with the trigger-timeout at infinite. When a one-shot measurement is started with these settings, the measurement is performed when the engine is cranked.

  1. When the engine is cranked the signal voltage of the crankshaft sensor can be measured in the 8 V range. The signal voltage can be higher when the sensor is measured on an running engine. In this case the range should set to 80 V with the range button Range up down.

Measuring

Figure 3 shows a waveform of a crankshaft sensor of an engine during cranking. This signal can be downloaded and used to correctly set up the lab scope or as reference signal.

Download crankshaft sensor inductive measured during cranking

Download crankshaft sensor inductive measured idling egnine

Lab scope measurement of crankshaft sensor

Figure 3: Lab scope measurement of crankshaft sensor

The crankshaft sensor signal voltage on channel 1 (red) shows amplitude changes during cranking. The changes are caused by the changes in engine speed during each compression stroke of a cylinder. The change in engine speed can also be seen reflected in the frequency variation of the signal. Clearly visible are the reference points caused by two missing teeth.

The signal amplitude must be 0.6 V at minimum for the ECU to recognize it. In the beginning of this measurement the amplitude is 0.7 V which is just enough. The signal amplitude varies with the engine speed: it is higher at higher rpms. The exact amplitude is not important because the ECU uses the flanks to determine the crankshaft position.

Lab scope measurement of crankshaft sensor on idling engine

Figure 4: Lab scope measurement of crankshaft sensor on idling engine

Figure 4 shows the signal of a crankshaft sensor of an idling engine. To make the amplitude difference clear, the measurement has been performed with the same settings as used for the previous measurement. The graph is zoomed in on the first quarter of the measurement to be able to see the signal periods. Also during idling the amplitude variations caused by the compression strokes are visible.

Diagnosis

Signal values may differ on different types of engine control units and crankshaft sensors. Consult ATIS for information on specific engine control units and crankshaft sensors.

The following signal deviations can indicate a problem:

  • No signal:
    Cause: back probes have no connection (perform a connection test), sensor loose, sensor defective
  • Signal shows more noise than example signal:
    Cause: signal wires damaged, poor connection in connector terminals, sensor loose, sensor defective
  • Signal shows an offset:
    Cause: scope is not set to AC coupling: AC
  • Signal has faulty pattern:
    Cause: cam or disk damaged

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Disclaimer

This document is subject to changes without notification. All rights reserved.

The information in this application note is carefully checked and is considered to be reliable, however TiePie engineering assumes no responsibility for any inaccuracies.

Safety warning:

  • Before measuring, check that sources of dangerously high voltages are switched off or shielded from contact. Voltages considered to be dangerous are voltages over 30 V AC RMS, 42 V AC peak or 60 V DC.
  • Keep a clean working environment when doing measurements.
  • This measurement and procedures are a examples / measuring suggestions and are no prescribed protocols.
  • TiePie engineering can not anticipate the safety actions that need to be taken to protect persons and appliances. Before starting a measurement, check which safety measures need to be applied.