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Thread: M52 engine runs with CPS disconnected WTF

  1. #1
    Join Date
    Sep 2010
    My Cars
    BMW 320i M52 1996

    M52 engine runs with CPS disconnected WTF

    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,


    Last edited by mamij; 05-31-2013 at 07:20 PM.

  2. #2
    Join Date
    May 2013
    My Cars
    325is, 328i, 318is
    Yea, i was gonna say you need to trace the wiring and make sure everything is plugged in to the proper plugs.

  3. #3
    Join Date
    Aug 2014
    My Cars
    1998 BMW 728i

    M52 Engine running problem / Crankshaft sensor related

    Quote Originally Posted by mamij View Post
    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.


    Hi everyone -

    I've Just stumbled onto this thread after keyword searching for 'CPS' 'Disconnected' & 'Engine running'...

    I've gotta tell you - I've had a complete nightmare with the electrics of my 1998 728i, but now, after all my woes I want to share my findings with anyone who's in need of a bit of help.

    The first post above is correct concerning the Cam & Crank sensor disconnection.
    The thing is...these clever boffins who design the micro-controllers (read ECU / Engine management / engine brain) aren't dull & they build-in some pretty nifty stuff. But.....they can't possibly envisage & cover every possible fault condition within the limits of the design set to them.

    You may or may not know that most modern day ECU's have what's called a "limp home mode" designed into them.
    This feature allows the ecu to continue running the engine even with a majority of sensors going down on the system.
    A look-up table is employed, so as to substitute for lost sensor signals which then keeps the motor running (albeit - not as fuel efficient & as responsive as when in normal operation). It's there to simply to 'Get-you-Home' !!

    I've read a lot of posts on the 'net about problems on BMWs, with individuals taking readings of possible offending sensor pins etc, but a lot of the stuff out there is just misdirected guesswork. Not a lot of it is grounded in fact, other than the fact that the person 'musing' over the problem in reality doesn't know what they're talking about . This, as I'm sure you know is very misleading & frustrating, especially when you're trying your hardest & spending precious time to get to the bottom of where the fault is located & getting your ride sorted out

    With all logical fault finding, you must take a constructive approach to the problem.
    Unplugging sensors and the like won't tell you if you have a faulty fuel pump / blocked fuel line filter / damaged fuel pressure regulator

    But, if you've checked the pressure of your fuel line, and are confident that these items are ok, then its time to move onto the other electrical components within the system.

    But whooah - don't go diving into issues of spark leads / coils / injectors just yet. All these things need to be timed & controlled by the engine control unit, and it ain't gonna play ball if it isn't getting the correct timing information necessary for ignition.

    The extra little pointers I would like to put out there are this :

    I've proven & observed that the ecu can get itself in a 'tizz when it believes it's receiving a valid sensor signal, when in fact the signal is corrupt. The sensor isn't completely dead - its still giving a signal. The system doesn't have time to decide if the sensor is dead or faulty after which, it would reference to the LUT (look up table) for typical values. This condition will invariably give a hesitant & misfirie turn-over of the engine. If you get this condition then I'd advise disconnection of the Crankshaft sensor.
    It would appear that the BMW system can roughly 'guess' the timing information from the Camshaft sensor, BUT, it must be able to commit to the LUT and not be distracted by a corrupt signal from the Crank sensor.

    There are a lot of so-called replacement sensors for the BMW system but they are completely cheap knock-offs.
    Again, I have proven this to my loss of time & money. There's a difference between the OEM part and copied parts.

    In my situation, the crank sensor is located in the front of the engine, just below the coolant hose connection and above the timing flywheel. What I recorded was a loss of signal amplitude due to an increase in sensor temperature. Basically, the copy replacement part couldn't withstand the rising engine temperature which in turn would affect the sensor's internal resistance, thereby causing signal corruption and periodic loss of timing to the ECU. From cold, the engine would start perfectly, but after between 15 / 20mins (with bonnet closed) the trouble would set in. The copy sensors 'soak up' the heat as opposed to resisting thermal build up.

    How does an OEM part cope with this? Well, having carefully stripped the original part previously, I noted that the main body of the sensor encapsulated the inner sensor component in what can be described as a type of 'ceramic' material. This is what is the difference between the expensive OEM part and cheap knock off part.
    So then, by disconnecting a faulty sensor, we give the ECU the 'nod to fully commit to using it's default internal reference values. The engine does labour somewhat in starting, but it does start eventually. Keep the heat-absorbed copy part in situ and you'll not get that engine to full running ignition.

    "But what about the faulty original sensor?" I hear you ask (to be continued.....)

  4. #4
    Join Date
    Oct 2010
    My Cars
    1996 Z3
    Ok, continue!
    I have a no start, no injection no spark but fuel pressure. I replaced 2 cam & 1 crank sensor

  5. #5
    Join Date
    Dec 2014
    Northland, N.Z.
    My Cars
    HMW M52 SC
    Funny, Mine wouldn't, But what mine does is even strange, unplug the fuel pump relay and the bugger keeps running, not really any change..
    BMW just has parts laying around they decide to throw on cars for no reason.

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  6. #6
    Join Date
    Jan 2005
    My Cars
    11/88 E32 750iL+98 E36M3
    Quote Originally Posted by Apogeelover View Post
    .....sensor is dead or faulty after which, it would reference to the LUT (look up table) for typical values.....
    That is not LUT, that is Limited Operating Strategy (LOS)

    Limited Operating Strategy (LOS)
    In the event of a serious fault in one or more of the sensors or their wiring circuits, Motronic will substitute a fixed default value in place of the defective sensor. This procedure is often termed limp home. A serious fault occurs when the signal from the sensor is outside of its normal operating parameters. When operating in LOS the engine may actually run quite well with failure of one or more minor sensors. Since the substituted values are those of a hot engine, cold starting and running during the warm-up period may be less than satisfactory. Also, failure of a major sensor, ie the AFS, will tend to make driving conditions less easy. Once the fault has cleared, Motronic will once more accept the live signal from the sensor. The following LOS measures are taken in the event of a failure......see link below

    That is from the M1.1 and M1.3 but the basics are all same for all DME's, here all the details I posted back in 2010 already

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