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  1. #1

    Oval MFD #3280252 voltage range of the AC RPM signal on the Yellow wire ?

    HI all.

    Does anyone know the voltage range of the AC RPM signal on the Yellow wire, idle and full speed?
    Fuji engine 780

    Jet ski at the lake, just trying to develop my own engine rpm tachometer gauge.

    Click image for larger version. 

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  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Interesting project. Welcome

    Will you be using that VFD display on the actual unit? Sunlight readable?

    If you don't like the voltage swings on the Yellow wire you can instead tap into the CDI trigger coil feed. There was a service bulletin for one model (Fuji engine?) where the stock MFD was erratic with RPM display so the MFD Yellow wire was re routed to tap into the CDI feed from the stator.

    I don't actually know the maximum possible voltage from the stator charging coil on yellow but I expect it will be dozens of volts.

    And of course the AC signal is distorted by the LR module during the actual charging half-wave cycles. Not to mention the battery voltage DC bias when the engine is stopped.

  3. #3
    Well its Graphic OLED display.
    NHD-3.12-25664UMB3 by Newhaven
    http://www.newhavendisplay.com/oled-c-119.html

  4. #4
    Here i got some more pictures.
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    The more you look at these photos, the more your brain will wrinkle.

    Now my approach below with Optocoupler.
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  5. #5
    BlueFishCrisis's Avatar
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    Quote Originally Posted by K447 View Post
    There was a service bulletin for one model (Fuji engine?) where the stock MFD was erratic with RPM display so the MFD Yellow wire was re routed to tap into the CDI feed from the stator.
    Yes, Fuji 780

  6. #6
    Thanks all of you.
    OK i need to look into that, just trying to get arduino development board rpm test circuit done before the long weekend.

  7. #7
    casey67's Avatar
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    Quote Originally Posted by Chris O. View Post
    HI all.

    Does anyone know the voltage range of the AC RPM signal on the Yellow wire, idle and full speed?
    Fuji engine 780

    Jet ski at the lake, just trying to develop my own engine rpm tachometer gauge.

    Click image for larger version. 

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ID:	329907
    I'm not an expert on the subject.
    But I don't think the MFD converts the voltage signal to a RPM reading.
    I'm pretty sure it converts the frequency to a Rpm reading.

  8. #8
    HI all.

    I Know this is a bit late, but for the curious this is my intention to count the frequency.
    I was on the lake only two times last year.
    Real life comes around and slows us all down all the time.
    I just tow the ski to my house the other weekend so hopefully i can run some tests.

  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Chris O. View Post
    ... my intention to count the frequency.
    ... hopefully i can run some tests.
    The Polaris MFD/MFI/NGI tachometers take the Hertz frequency on the Yellow wire, multiple that by ten, to display engine RPM.

    So 60Hz AC signal on Yellow wire becomes 600 RPM.

    120 Hz is 1200 RPM.

    1000 Hz would be 10,000 RPM

    The Yellow wire signal is a clipped waveform. Half sinusoidal and half not.

    The Yellow wire is also used to control battery charging (this is done by the LR voltage regulator module) and this is why it has a half-wave shape with a DC bias voltage.

    The LR module sits between the Yellow wire and electrical ground. The LR module varies the average current through the Yellow wire to control battery charging voltage.

    The Polaris MFD will display RPM with only a couple of volts of AC input but the input circuit must also be able to withstand something in the neighborhood of one hundred volts. This fairly high voltage can occur at high engine RPM when the battery is already fully charged and does not need much charging current.

    Your input circuit must be able to withstand both positive and negative voltage spikes from the Yellow wire.

    When the engine is not rotating the Yellow wire sits at full DC battery voltage, not at electrical ground voltage. Your input circuit must not impose a parasitic DC current draw on the Yellow wire when the engine is not running.

    If I am not mistaken the Polaris MFD shows RPM rounded to the nearest 10 RPM, it does not show RPM increments less than ten. The less useful NGI version only shows RPM in increments of 50 RPM.

    The MFD update rate is at least once per second, as I recall, and it may actually be faster, perhaps updating the displayed RPM every half second

    Tip: Have you seen the videos I posted some time ago showing the MFD testing every mode and warning?

  10. #10
    Tip: Have you seen the videos I posted some time ago showing the MFD testing every mode and warning?
    WOW thanks K447
    I haven't seen video of the MFD testing, can you post a video link please.

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