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Thread: What are the two ignition signal donuts really doing in V12?

  1. #1
    Join Date
    Nov 2015
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    Turku - Finland
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    1991 850i 6 Speed

    What are the two ignition signal donuts really doing in V12?

    I have been wondering about the engineering on these things.

    Earlier in the week I had a total failure of the 7-12 side pulse generator aka. donut sensor which was fortunate as it was now easy to diagnose* and cure but it also left me wondering what is the function on these buggers.
    *Hooked a separate spare ignition wire with a spare donut to the #12 and all was well again in less than 10 minutes. This "diagnostics" kit should be in every (V12) E31 garage btw.

    Earlier I though those "camshaft sensors" were indicating the #6 & #12 workcycle timing but how can that be. Spark is already fired for the cylinder (that is the signal that donut is detecting and transmitting back to DME), timing is atleast roughly there, is there additional finetuning going or what?
    Ignition is mechanical, that is the job of the distributor and engine will start and run for a second or so before shutting down if a donut sensor is dead...mine did. DME will need to trigger coil to ground to make the spark so I'm assuming cranksensor will do this.
    In my E36 (M52B28) the camsensor was broken (to be specific it was a non OEM sensor that was not working right) for years and only difference was in midband power. Othervice engine was pretty ok. E36 does have more modern electronics and individual coils and fuel injection so I'm assuming crank sensor was used as a backup in this case and DME ran with some error management mode.
    Not so in our M7x engines.

    Another theory was that E31 (V12) uses donuts as catalytic converter safety check, no spark -> no fuel & no catalyst break. Would any properly formatted and fed signal satisfy this function or does it need to be also timed to correct cylinder and with cranksensor to function? Could we increase the reliability of the engine by simulating the signal with a simple electronics if the timing is not critical...
    Seems also a bit lacking to just monitor one of 6 cylinders if this is the case, what about the rest 5?

    In some materials I have seen a mention of semisequential fuel injection being dependent on the sensor but a failure of the sensor will shut the ½engine down so there must be some other needs for the sensor. Fuel injection might benefit from the signal still.

    Does someone have better insight on this?

  2. #2
    Join Date
    Aug 2013
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    Hesperia, CA
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    30
    My Cars
    91 850i 165,000mi

    Ignition Sensor for Cat Converter Protection

    From Training Manual
    Attached Files Attached Files

  3. #3
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    1991 850i 6 Speed
    So it is indeed for the catalytic converter protection (only monitoring 1/6 cylinders to do this) but also the models without cats have this to my understanding.

  4. #4
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    Guys it's very simple, CPS lets the ecu know which cylinder is on it's induction stroke.

  5. #5
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    99 840ci Sport Indv
    Its primary function is (as Dragon says) is to let the DME know the camshaft position - the only other rotary sensor (the crankshaft sensor) cannot supply this information as it rotates twice for each revolution of the camshaft.

    As a secondary function, the DME disables the injectors on that bank if it fails to receive a signal from the CIS after a few of seconds of engine running. This is handy for diagnostics as, if everything else is functioning correctly, the affected bank will actually run quite well for a few seconds then switch off.
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  6. #6
    Join Date
    Nov 2015
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    Ger
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    E31 850iA 1992
    i think this sensor is only for catalysator protection.
    Without this monitoring and a broken rotor in one distributer, the M70 engine is still alive. The other bank runs the engine.

    I think there is no other reason for this sensor. No knock detektion in this engine and the injektors are in groupe wired - no sequentiel fuel injektion. Knock detection is possible without camshaft position. For SFI camshaft position is needed.

  7. #7
    Join Date
    Sep 2018
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    Belgium
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    1990 850i and 1970 2800C
    interesting thread.... i can understand that the crankshaft sensor does not give all info as it rotates twice for one full engine cycle with 12 ignitions.... so the donuts give info on what bank ... but how does the engine starts ? As mentioned , that info comes only after firing 6 or 12 .... ??? Once "good" running i can imagine crankshaft sensor and donut info give exact location ... but what about the first startup ?

  8. #8
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    E31 850iA 1992
    There are 2 crankshaft sensors. One for each bank.

    A crankshaft sensor is enough to run a engine with distributor or wasted spark igniton.
    Camshaft sensor is needed for sequential fuel injection and cylinder SELECTIVE knock control.
    M70 doesn't have one of these.
    Last edited by E31 850i; 05-20-2019 at 04:24 PM.

  9. #9
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    11/88 E32 750iL+98 E36M3
    Here copied from M1.1 and M1.3, same for the M1.2 and 1.7:
    Cylinder Identification (CID)
    A cylinder identification sensor is used to identify cylinder firing sequence. The sensor is connected around the HT lead of cylinder number 4 (4 cylinder) or cylinder number 6 (6 cylinder) adjacent to the distributor. As the HT pulses travel along the HT lead, a small AC signal is induced in the sensor and returned to the ECU. The ECU utilises an ADC to transform the signal into a digital pulse.
    Fuel injection
    The Motronic ECU contains a fuel map with an injector opening time for basic conditions of speed and load. Information is then gathered from engine sensors such as the AFS, CAS, CTS, and TS. As a result of this information, the ECU will look-up the correct injector pulse duration right across the engine rpm, load and temperature range.
    The injectors are arranged in two banks with injectors 1 and 3 (4 cylinder) or 1, 3 and 5 (6 cylinder comprising one bank, and injectors 2 and 4 (4 cylinder) or 2, 4 and 6 (6 cylinder) making up the other bank. Each bank is connected to the ECU via an independent ECU pin. The Motronic 1.1 & 1.3 multi-point injection system pulses the injectors semi-sequentially and once every two engine revolutions. During engine start-up below 600 rpm the ECU pulses all injectors simultaneously. Once 600 rpm has been attained and if the ECU has received a signal from the CID sensor, each injector bank will be pulsed alternatively according to which pair of cylinders are approaching TDC. If a signal is not received from the CID sensor the injectors will remain on simultaneous operation. However, if the CID sensor subsequently sends a signal to the ECU after the engine has commenced running, the ECU will pulse the injectors semi-sequentially after the next deceleration phase - even if the CID sensor then ceases to send a signal. During start-up from cold, injector pulse duration is increased to provide a richer air/fuel mixture and pulse frequency is also increased. In addition, the ignition timing is also retarded. Injector frequency & pulse duration and degree of timing retard depend upon the engine temperature both during start-up and immediately afterwards. If the engine is restarted within one minute of the first start occurance, less overall fuel is injected to reduce the risk of fuel flooding into the engine.

    https://forum.bmwland.ru/index.php?a...e=post&id=6521

    If the inductive sensor on the plug wire (CID) fails then it goes into gang firing, since the m70 is a semi sequential design meaning that it triggers the injectors of each side in 2 banks of 3 every other revolution, when it goes into the gang fire, then it triggers all six at the same time every revolution but decreases the volume of fuel by 50% .
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