External monitor goes black!

Discussion in 'MacBook Pro' started by Thor774, Jul 24, 2017.

  1. Thor774 macrumors regular


    Sep 14, 2007

    I have a Retina MBP 2015 15" that is connected to an external HP Z27n monitor via DisplayPort. When my MBP is on my desktop I connect it to this monitor and close the lid (clamshell mode). Everything works ok when I am inside macOS, but if I have to restart the Mac while the external monitor is connected there is a point on the start process where the external screen is going black. I get a message on the monitor that there is no signal and it goes to sleep. If I open the lid of the Mac and turn the monitor off and on it starts to receive a video signal again and I can close the lid.

    The thing is that at the beginning of the starting process everything is working fine. I get the Apple logo and then the screen to input my credentials (I use FileVault). The OS then continues booting and I can even see the desktop loading with all the icons, but then suddenly the external monitor goes black and I have to do what I have posted before to get the picture back.

    I have changed ports in the Mac for connecting the DisplayPort cable but nothing changes. I have played with the settings on the HP monitor without success either.

    Anyone has any idea on what can be happening here? I do not restart my Mac everyday but when I do it this is very annoying. Any help will be greatly appreciated. Thanks.
  2. jerryk macrumors 68040

    Nov 3, 2011
    SF Bay Area
    Sounds like the graphics card cannot sync up with the monitor. My 2015 (w/dGPU) does that with certain brands/model external monitors, usually Asus. It is kind of a pain, but I don't use with an external monitor that often.

    I have to do the same monitor power dance you mention to get it to work again.
  3. Thor774 thread starter macrumors regular


    Sep 14, 2007
    I am pretty sure this problem started after the upgrade to Sierra. It was not like this on El Capitan. In that OSX version the system could boot normally without needing to restart the external monitor to get any picture.

    The most annoying part is having to open the lid before turning off the external monitor. If you turn off the monitor with the lid closed the Mac goes to sleep as it detects no external monitor anymore.

    I think I will contact Apple on this one, but have no big hopes because my machine is already out of warranty.

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