Crankshaft Position Sensor

Discussion in 'Engine' started by mercman1969, Nov 7, 2008.

  1. mercman1969

    mercman1969 Hardcore MB Enthusiast

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    What role does this play within the engine and what components does it interact with

    W163 STILL running lumpy and spoke AGAIN to garage today and confirmed it was the above they changed last time it was in
     
  2. Number_Cruncher

    Number_Cruncher Hardcore MB Enthusiast

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    >>What role does this play...

    It tells the engine management computer where the engine is in its cycle.

    Typically, it is a magnetic sensor which is placed very close to some machined teeth on the crank, or sometimes on the flywheel; it produces an electrical waveform as each tooth passes.

    As the camshaft rotates at half crankshaft speed, the output from the crankshaft position sensor isn't enough on its own to fully define the exact position of the engine in its cycle - it can be one full turn of the crank wrong.

    This ambiguity is usually overcome by the use of a camshaft position sensor which works on similar principles, but in a much coarser manner.

    The output from the crankshaft position sensor allows the engine management unit to calculate;

    crankshaft position,
    crankshaft angular velocity (rpm),
    crankshaft angular acceleration.

    Crakshaft position and angular velocity are used to determine the timing of events like sparking and fuel injection.

    Crankshaft velocity is used to allow basic mapped values of settings like ignition advance and basic fuelling amounts to be revocered from the ECU's memory. There basic mapped values are then modified by feedback sensors like the knock sensor and the lambda sensor.

    Crankshaft velocity is also an input to functions like idle speed control, and overrun fuel cutoff.

    Crankshaft angular acceleration can be used as part of (really quite complex) misfire detection algorithms, and is also used as part of adaptive algorithms where small changes in fuelling and ignition timing are tried, and their effect upon crankshaft angular acceleration is used to continually optimise the power output of the engine.

    I'm sure there are many more places where these signals go in the engine management computer, but, it's a start.

    Practically, these are a small capsule with many turns of very fine wire within them. If (when!), the wire breaks, you can get odd behaviour, when perhaps, the engine is OK when cold, but when the sensor warms up, and the broken wire ends expand apart, the sensor fails.

    Owing to the fundamental need for timing information, the crankshaft position sensor is one of the few sensors which can stop a modern engine dead, and it rarely leaves a trouble code in the computer's memory. It can be checked either by an oscilloscope to look at the waveform, or by interrogating the ECU live data, and finding out whet engine speed the ECU thinks the engine is turning at!

    Sorry about the long post!
     
  3. mattkh

    mattkh Member

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    Hi
    I am guessing, it must be telling the engine ECU the tdc so that the ignition timing can be adjusted.
     
  4. Dieselman

    Dieselman Banned

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    It does that by a metal dowel inserted into the flywheel at the appropriate place.
     
  5. Dieselman

    Dieselman Banned

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    It can also be checked with a simple voltmeter, although this won't give absolute accuracy, but the voltage will rise as the revs do.
    These sensors either work or don't as they are passive coils so can't give minor fluctuations.
     
  6. Number_Cruncher

    Number_Cruncher Hardcore MB Enthusiast

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    >>These sensors either work or don't

    That's not quite my experience of them, I've seen them being responsible for many intermittent and heat soak related faults, where, for example, the engine will start OK cold, and run OK, but the sensor will not produce enough output under hot cranking to start the car again.

    This low output from low crankshaft speeds has been partially overcome in more recent sensors which are active, i.e., powered.
     
  7. OP
    OP
    mercman1969

    mercman1969 Hardcore MB Enthusiast

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    Cheer guys...phonic wheel then?

    I only ask as dealer changed thiss and stated it had cured the rough running...it hasnt
    joined A1 tonight put my foot down to accelerate...got to about 30mph and then nothing, i mean nothing------it was although the brake had been applied for a couple of seconds and actually threw me forward against my seatbelt....then a huge rise in revs to 6000....seemed like gearbox has switched into 1st gear....and off it went
     
  8. Dieselman

    Dieselman Banned

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    I appreciate heat being a problem but they don't give generally spurious readings during a cycle.
    Hot starting problems is a classic test of a CKP.
     
  9. Number_Cruncher

    Number_Cruncher Hardcore MB Enthusiast

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    >>the engine ECU the tdc

    Oddly (or, logically, depending how you view it!), the TDC position is one which is usually not explicitly sensed.

    More typically, the ECU needs to know accurately a position way ahead of TDC, say, 60 degrees BTDC. Then, with a knowledge of crankshaft speed, and acceleration, the ECU calculates a time delay before the spark, a short time delay is a very advanced spark, and a long time delay a very retarded spark. Even a retarded spark will be happening before top dead cente, so, all the action is over before the engine gets to TDC.
     
  10. OP
    OP
    mercman1969

    mercman1969 Hardcore MB Enthusiast

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    So what would you guys recommend?
     
  11. Number_Cruncher

    Number_Cruncher Hardcore MB Enthusiast

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    What is your car doing?

    Is it running roughly all the time?, or is just intermittent cutting out?

    Is the cutting out you mention above random?

    When it cuts out what do you see on the dashboard?

    Has anyone read the ECU memory? What fault codes were found?
     
  12. OP
    OP
    mercman1969

    mercman1969 Hardcore MB Enthusiast

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    Car is running lumpy intermittently

    At times particulary at cold start up it ides a little roughly then as you go to accelerate it coughs and slutters - then grab a fisful of throttle and after a 2 or 3 sec lag the revs fly around and off you go

    It isnt exclusive to cold start.............there is also alot of radio interference which leads me to think it could be a lead or coil breaking down
     
  13. Number_Cruncher

    Number_Cruncher Hardcore MB Enthusiast

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    Yes, it sounds much more like a coil or HT problem than a crankshaft position sensor problem. What diagnosis has your garage done?, or is it a case of repair via poke & hope?
     
  14. OP
    OP
    mercman1969

    mercman1969 Hardcore MB Enthusiast

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    They have said they had done an oscilliscope diagnosis and also there was a STI out from mercedes regarding the crankshaft sensor..........they reckon the sensor was intermittantly 'flatlining' (I would have thought it would simply fail)
     
  15. Number_Cruncher

    Number_Cruncher Hardcore MB Enthusiast

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    That's good, it sounds like the crankshaft position sensor was replaced following some reasonable logic, and it wasn't a pure guess.

    It would be interesting to see if the output from the sensor is OK now, especially while the engine is running badly.
     

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