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Thread: Ever wonder what all those sensors actually do?

  1. #26
    Join Date
    May 2013
    Sydney, Australia
    My Cars
    1998 328i Convertible
    Aux fan temp sensor.
    Last edited by shogun; 01-14-2016 at 09:21 AM.
    "You must learn from the mistakes of others. You can't possibly live long enough to make them all yourself."

  2. #27
    Join Date
    Jul 2014
    Gig Harbor
    My Cars
    2011 328i & 1993 325is
    Is there a sensor such as the cam or crankshaft position sensor that reads and sends the rpm to the tachometer. I have a feeling my gauge cluster is fine but theres a fault elsewhere?

  3. #28
    Join Date
    Aug 2014
    My Cars
    1998 BMW 728i
    Quote Originally Posted by flyfishvt View Post

    Crank Position Sensor (CPS)
    - //-

    The CPS can get dirty or contaminated or it can fail altogether. Since the CPS is telling the DME when to fire the spark its easy to see that a faulty CPS can cause rough idle, misfire, loss of power, backfire and if it fails completely then the DME doesn't know what position the piston is in and the car will crank but it won't start.
    I beg to differ on this one - Test it for yourself....disconnect the Crank sensor - the engine WILL start.

    Correct info is :-

    A faulty sensor will send corrupt info to the ECU. << No Start >>

    A dead or disconnected sensor will allow the ECU to use stored reference values & combine with the Camshaft
    sensor readings << Laboured, but eventual Start up >>

    Congrats on an excellent write up
    I'm sure many peeps will benefit from the info !

  4. #29
    Join Date
    Aug 2014
    My Cars
    1998 BMW 728i
    For example...

    Quote Originally Posted by mamij View Post
    Hi Guys,

    I need a little help here with a few sensors.
    This is how my engine behaves:

    Crankshaft and Camshaft position sensor disconnected, engine only cranks and will NOT start.

    Crankshaft sensor connected but Camshaft disconnected, engine starts fine.

    Camshaft sensor connected and Crankshaft disconnected, engine STILL starts fine.

    I have heard and read that an engine is not suppose to start with the Crankshaft sensor disconnected.

    is it normal that an M52 engine can start with a Crankshaft postion sensor disconnected?

    I have taken resistance of the crankshaft sensor, I get ZERO reading in all combinations WTF?
    Camshaft sensor reads what it should.

    The reason why I have been checking this out is, I regularly get an error code on the Camshaft sensor, which I mentioned earlier reads fine resistance.
    So I am thinking I probably have a wiring problem somewhere,



  5. #30
    Join Date
    Jun 2013
    My Cars
    BMW 328ic 1998
    Does anyone know what the sensor that I have circled in red in the picture attached is for? It is attahced to the driver's strut tower, just above the ABS pump. It is a black plastic cylinder with tho wires coming to it: green/yellow and brown/yellow.
    Many thanks in advance!E36 drivers side under the hood.png

  6. #31
    Join Date
    Nov 2012
    Lawrenceville, Ga
    My Cars
    1997 Estoril M3
    I believe that is the ASC solenoid which you can strip and toss along with the ASC secondary throttle body and replace the solenoid with a 10 ohm resistor in the plug to keep the light off and be better off for it.
    Last edited by shogun; 03-08-2017 at 11:37 PM.

  7. #32
    Join Date
    Dec 2014
    Northland, N.Z.
    My Cars
    HMW M52 SC
    Just an added bit of Info,

    I am on my second failed Crank Position Sensor now, My car was leaning out and bogging down, no accelaration when given gas and suddenly loads of power and then bogg down again. Anyhow it lodged no codes(unlike last time). I tested the Crank sensor and whatdayaknow? Its out of spec. Guess it was not firing the plugs right, Its reading the same as the old sensor around 750 - 800.

    Just a way to test if your Crank or Cam Sensors are working right, if the symptoms show and there is not a code yet.

    values should be

    Crank Postion Sensor (CPS) - controls when the plugs fire (550 Ohm with 10% + or - tolerance)
    Cam Postion Sensor (CMP) - controls when fuel is injected. fuel injection (Have 2 Good ones here, around 15 Ohm).

    They start going off, then crazy to one day they just die, mine is on its way out.
    BMW just has parts laying around they decide to throw on cars for no reason.

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  8. #33
    Join Date
    Jan 2005
    My Cars
    11/88 E32 750iL+98 E36M3
    Technical information from Hella: Crankshaft Sensor

    Crankshaft sensors record the engine speed and crankshaft position. The fitting position is near the flywheel ring gear. More they are found on the engine block with the sensor ring bolted onto the crankshaft There are two different types of crankshaft sensors, a hall sensor and an inductive pick up.

    Their function is to send voltage signals produced by changing magnetic fields developed by the rotating ring gear to the ECU, for calculating the crankshaft speed and position. These are important signals for the fuel injection and ignition timing.

    Effects of failure
    A faulty crankshaft sensor can cause the following:
    Engine will not start
    The engine is misfiring
    Engine stalls
    Storing a fault/trouble code

    Causes of failure:
    Internal short circuit
    Wire short circuit/open circuit
    Mechanical damage of the ring gear
    Soiling through metal abrasion
    Short circuit to vehicle ground

    For fault recognition consider the following system tests:
    Read out fault/trouble codes
    Check electrical leads and plugs for correct fitting and contact
    Check for soiling and damage

    The checking of the crankshaft sensor can be difficult when the sensor type is unknown. Before checking it must be differentiate between a hall sensor or an inductive pick up. It is impossible to differentiate between them by sight every time. When they have three pins in the plug it is not sure which type it is, for that you need specific manufacturer’s data and details from the parts catalogue. If the type is not clearly identified don’t use an ohmmeter because it can destroy the sensor. When the sensor has a two pin plug it is usually an inductive pick up. With the inductive pick up you can check the internal resistor, a short circuit to earth and the signal. For this remove the sensor plug and check the internal resistance, if it is between 200 and 1000 ohm (according the given value) the sensor is okay, if it is 0 ohm there is a short circuit and by Mohm there is a open circuit. For checking a short circuit to earth measure each pin of the sensor to vehicle ground, measured value > 30 Mohm. Measurement with an oscilloscope must have a strong enough sinus signal. AC voltage can be checked by measuring across the pins and spinning the wheel. With hall sensor you can only check the signal voltage (rectangular signal) and the operating voltage.

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    From the Bentley Repair manual instructions, E32 crankshaft position sensor - resistance and specifications

    As for
    E36 - 323i and 328i with M52 engine produced 6/95 - 12/98
    E36 - M3 with S52 engine produced 1/96 - 12/98
    E39 - 528i with M52 engine produced 3/96 - 8/98
    Z3 - with M52 engine produced 7/96 - 9/98
    Z3 - with S52 engine produced 1/98 - 1/99
    the story is different
    new version 12141709616 Crankshaft position sensor, that is a hall sensor, 12V
    like for example
    HELLA: 6PU 009 110-421
    VDO: S107230001
    VEMO: V20-72-0403

  9. #34
    Join Date
    Oct 2016
    Tenerife, Spain
    My Cars
    1997 BMW E36 323i
    I think I've found the answer on why the M50 and M52 actually have an IAT sensor.

    I did some research on what the IAT actually does in the M52.. The DME doesn't need it at all to get a proper reading of the mass of air being sucked into the engine, as the MAF gives the complete picture, independent of the temperature (mass already reflects the air mass and density which changes with temperature).

    So, what I think is, it is used to fine-tune the ignition timing. So, if the IAT is giving wrong values, it will affect the torque of the engine a lot.
    See this topic: https://webcache.googleusercontent.c...&client=safari .

    Quote (after installing proper IAT sensor): "I'm happy to say it worked brilliantly! Sure enough, once everything was assembled it fired up perfectly and sat there idling quietly. I took it out for a drive and the engine was so smooth with none of the spluttering at low revs it previously had."

    This is from Wikipedia -
    Also it is dependent on the temperature of the engine with lower temperature allowing for more advance."

    So, if your IAT is giving too high temperature readings, or is severely heat soaked, ignition will be retarded too much.
    And, if your IAT is giving too low temperature readings, ignition will be advanced too much.
    Both having impact on your performance, and also potentially causing knock in the 2nd case.

    Also learned (in the RomRaider forum, lots of DME expertise) that mounting the IAT in the intake manifold causes a lot of heat soak at idle, because of the high vacuum in the manifold, which hinders the IAT from releasing its heat.. So, it would have been better if BMW engineers had mounted it in the throttle body (on the side where there is no vacuum) or, better, in the rubber boot between MAF and throttle body (metal of throttle body also causes more heat soak).. It's a big improvement that the M52 IAT has a plastic housing, compared to the M50 metal housing (more heat soak). Also the M52 IAT is a different type of sensor which reacts faster to changes than the older M50 model.

    So, I think the summary on page 1 (first post of this FAQ) needs some updating. I'm quite certain the DME doesn't take into account the IAT to calculate injection time, as the MAF sensor + coolant sensor + oxygen sensors (in closed loop mode) + rpm (from the crank or camshaft sensor) already give enough information to calculate it. Perhaps the DME does use it, but it shouldn't have much influence. I think the DME only uses it to calculate ignition timing, along with using some other sensor values like coolant temp, rpm, engine load (mostly MAF).

    I'll probably post a question in the RomRaider forum to verify this, but I think my assumption it's correct: IAT is not used for fuel calculations, as it's superfluous above all other more relevant sensor data, but it IS used for ignition timing.

    Update: Have been looking at some RomRaider MS41 screenshots.
    The base map for fuel injection time is based on engine load and rpm.
    The base map for ignition timing is also based on engine load and rpm.
    Then there is, at least for some Subarus, a map that retards timing with higher IAT temperatures. See screenshot 3 of this post:
    1 . (also see image 4 for an ignition base map, comparable to how it works with MS41).

    Update 2: I'll ask in the RomRaider forum what the IAT actually does in the MS41 DMEs. No need to change the summary just yet ;-) .
    Update 3: Just asked: Here is the link: . Here is the link to the misfiring 323i I refer to in the RomRaider post: #92 .

    P.S. This only applies to engines/DMEs that use a MAF sensor. If a flappy style AFM (or a MAP sensor, which is used with Megasquirt and other brands) is used then the IAT is definitely needed to calculate the air mass, because the air flow meter only measures air volume and doesn't take into account air density, which is mostly dependent on the air temperature.
    MAF started to be used in M50, M52, S50, S52, M44, M43TU, M60, M70, S38B36 (and virtually all newer engine revisions).
    AFM was used in the M40, M42, M43, and all older BMW engines like M10,M20,M30,S14,S38B35,M88, etc.

    Update 4: My assumption seems correct. This is what someone writes in the RomRaider forum:
    "Maf should not need iat to calculate air mass. Like with map in speed density mode it's needed. It [the MAF sensor] measures it correctly no matter what the air temp is. IAT sensor is mainly needed for ignition timing adjustment. If you check the iat/ect multiplicative map, there is whole range of iat temps that affect timing.".
    The IAT is quite certainly used when the DME goes into Alpha-N mode though, when you disconnect the MAF sensor. It then uses the TPS, the RPM and most probably the IAT to estimate air density (and of course coolant temp also to enrich mixture during warm-up phase).
    Last edited by ed323i; 01-19-2019 at 01:40 PM.

    1997 E36 BMW 323i
    (European) 275k km (171k miles), with following small mods:

  10. #35
    Join Date
    Oct 2016
    Tenerife, Spain
    My Cars
    1997 BMW E36 323i
    Here's another interesting addition:
    I discovered something very interesting today: At least the M52 MS41 DME can run with only one of the two position sensors connected. So it can run with ONLY the crankshaft sensor, and also with ONLY the camshaft sensor. And, it runs better with a bad sensor disconnected than with the failing sensor connected. I'm 95% sure this also applies to the Motronics 3.1 M50 DME.
    It's also very important to only buy the original OEM sensor (Bosch for Bosch DME and Siemens for MS41 Siemens DME), also explained in the details-link below:

    Here are all the details: #50

    1997 E36 BMW 323i
    (European) 275k km (171k miles), with following small mods:

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