What is the "vertical sync ribbon cable" for in iMac 27" late 2009?

Discussion in 'iMac' started by macpokerstars, Jul 18, 2014.

  1. macpokerstars macrumors member

    Sep 29, 2010

    I accidentally broke the "vertical sync ribbon cable".
    For those who don't know which one it is, it's the first one you have to pull out when you lift the LCD up: https://d3nevzfk7ii3be.cloudfront.net/igi/ljO1oq2TyeXgaQLO.medium

    Even though I broke it, the iMac is still working fine.
    If it's still working, what is the purpose of that cable anyway?

    Note: for some reason, my CPU's fan keeps spinning at 2500 rpm all the time. Is it linked to the cable I broke?

    Thanks in advance
  2. DeltaMac, Jul 18, 2014
    Last edited: Jul 18, 2014

    DeltaMac macrumors G3


    Jul 30, 2003
    (This was answered the FIRST time you asked this same question, nearly a year ago)

    Here's another answer, similar to the first.

    The display, and your video will, more or less, work OK with the vertical sync cable disconnected.
    That vertical sync function is basically your screen refresh (keeping the screen stable when objects on the screen move, for example). Without it, you will see odd visual effects, depending on what you do. Games that require a lot from your vid card, will just not look right, and you will see "tearing" on the screen (vertical edges may not appear uniform, particularly when moving.) It will always be noticeable, but animations in games probably will be very blurry, etc. Or, move a large window left to right across the screen, and watch the vertical edges. You will see a lot of distortion while the window is in motion.
    If you are OK with that distortion, then don't fix the cable.
    (And, now you likely can't un-see that distortion … :( )

    It's likely that you left a sensor disconnected what you opened up your iMac before.
    You can get the fan under control with a utility that gives that function, such as SMCFanControl...
  3. macpokerstars thread starter macrumors member

    Sep 29, 2010
    Hi there,

    Thanks for your response. I never noticed someone ended up answering my initial post!

    I tried to repair it and it might work again.
    Is there a way of knowing via OS X whether that cable is correctly plugged?

    Thanks a lot
  4. DeltaMac macrumors G3


    Jul 30, 2003
    No, probably no way to test and get the answer "yes, that's working OK!"

    But, you should try the "drag a window with a running video" trick - before you try a repair.
    Then do the same trick again after the repair. The difference at the vertical sides of the window should be obvious.
  5. h9826790 macrumors G4


    Apr 3, 2014
    Hong Kong
    For some reason, if you perform the Oceanwave benchmark on a vsync enabled Mac, the result may lock at 60fps. If vsync disabled, you can easily get few hundred frame per seconds. This test may not be reliable, but easy to perform.

    On the other hand, you can register as a developer (free of charge), and then download x-code. After x-code installation, you can use Quartz Debug to enable/disable vsync. I don't know will the system accept to enable vsync if the cable is broken (accept but not perform). However, at least you can use it to disable vsync, check if you iMac perform differently. The result should be very clear, without vsync, you can easily see the graphic distortion in something like safari, or just moving a finder windows on the screen.

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