iMac always auto detects for external monitors after waking from sleep or from display off

Question

iMac always auto detects for external monitors after waking from sleep or from display off

I've got a Dell 24" monitor (2408WFP) plugged into my mid 2011 iMac via DVI cord and thunderbolt attachment. Things work fine.

When I need to leave my computer, I usually turn the displays off using the shortcut keys: CTRL + SHIFT + EJECT

This works perfectly and both displays go black. However, when I return to my computer and reactivate the iMac, the iMac display will turn blue as if I have just plugged in a new monitor even the external monitor is still plugged in. It usually flashes blue twice before displaying the password prompt. I can usually just log back in as normal. I've never had this happen before using Snow Leopard. It's annoying but strange.

However, even more strange is that sometimes when returning to the desktop, the iMac will do the auto detect display thing and then when I log back in, the external display just shows static as if there is no signal. I've checked the cable a million times and everything is as tight as tight can be and both ends are plugged in properly. Unplugging the displayport adapter from the iMac and then reattaching it restores the desktop on the secondary display although sometimes, it results in the secondary display showing a gray background picture instead of my normal picture. Replugging the secondary display seems to fix that too although sometimes it doesn't.

Why is this happening?

- Problem with the adapter?

- Problem with the DVI cable I'm using?

- Compatibility issues with my external monitor?

I need to stress the point that this has never happened before with Snow Leopard.

If I am the only one experiencing this then I suppose it may be an issue with the iMac or adapter or even the cable. But if anyone else can shed light on this problem, I'd really appreciate it.

iMac, Mac OS X (10.7)

Posted on Sep 5, 2011 8:41 AM

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Answer 1

Best reply

ToothpickProductions

Level 1

135 points

Sep 7, 2011 5:25 PM in response to Michael Cheung

It might help isolate the problem if you try a few things:

1) Does the problem occur if you Do NOT have the external display connected into the configuration?

2) In original config: does the problem still occur if you have the Password Log-In Disabled?

3) Are you running the (2) displays in Mirrored Desktop mode or as Extended Desktop? Try both.

4) Does the same problem occur if put the whole computer is put to Sleep (Opt+CMMD+and then waken up... instead of just 'Display Sleep'?

Just some things to try...

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Answer 2

Sep 8, 2011 2:05 PM in response to ToothpickProductions

Thanks for your advice.

I've tried out all the things you mentioned and the problem still remains.

1. The computer doesn't do the blue screen auto display detection thing when nothing is plugged in.

2. The computer still displays the blue screen twice even with no password protection

3. Tried running the displays in both modes and the problem still occurred. However, when waking the display back up, the main iMac display would appear in it's normal resolution before resizing to match the 24" external display.

4. Sleep mode results in the same thing.

I'm not really sure what's going on here. I'm assuming it's a problem due to Lion but I'll have to do more research before coming to that conclusion. I don't really want to bring the iMac into the Apple store either since it's kinda heavy.

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Answer 3

Sep 8, 2011 4:01 PM in response to Michael Cheung

yeah... as far as the blue screen flashing, that seems to be the normal occurance when the computer re-sync's (like when the computer detects the change of adding or removing a display). As long as it does finally recover and show the proper Finder / Log-In window.

The bad part is your issue of after logging in and the external shows static (or no signal). But this is the problem you are saying still occurs (in your above #3 and #4 steps)?

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Answer 4

Oct 4, 2011 5:41 PM in response to ToothpickProductions

The static doesn't happen often and I can't seem to reproduce it. It may be due to a faulty adapter. I'll wait until 10.7.2 comes out and see if this happens again. I just find the re-syncing thing to be pretty annoying, that's all.

Regarding the gray background colour on the second monitor, I've learned that a way to "reset" that is to go into the terminal and type in killall Dock

This restores the previously chosen wallpaper. So apparently, the dock also controls the desktop background. Not really a fix, but it'll do for now.

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Answer 5

Oct 21, 2011 5:02 PM in response to Michael Cheung

10.7.2 seemed to have removed the auto screen detect thing since it now instantly wakes the secondary monitor as well with no blue screens.

No problems with the wallpaper either so far.

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Answer 6

Oct 23, 2011 5:45 PM in response to Michael Cheung

This has happened to me, also, since one of the last updates of Snow Leopard. It's a driver problem, as it used to work just fine. I've got the latest Lion version and it still happens to me more often than not, but not EVERY time.

I wouldn't be surprised if the appearance of it working for you now is just temporary...

/mike

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Answer 7

Nov 22, 2011 1:59 PM in response to Mike Mitchell4

You're right in that it temporarily went away. It's happening again-- every single time I use the computer. It's ridiculous.

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Answer 8

Mar 14, 2012 6:36 PM in response to Michael Cheung

Hi Micheal,

I've been experiencing the exact same issue with my new iMac I just bought. I bought it refurb so I wasn't sure if it was a defect with the hardware or not. Since I'm not the only one I will asume it's not for now.

Setup:

- iMac 27-inch 3.4GHz (Release May 2011)

- Dell 2709W external monitor

- Hooked in with mini display port to DVI adapter

- Lion 10.7.3 all up to date to this day

From what I've been able to gather the issue presents itself only when my Dell 2709W enters "Power Save Mode" after I do CTRL + SHIFT + EJECT or when the diplays simply goes to sleep by themselves.

Although if I hit that combination and the displays goes to sleep but I don't wait long enough for my Dell monitor to actually "Power Save Mode" (wait like 2 sec) then touch the keyboard then the 2 displays (iMac and Dell) resumes from sleep without the issue presenting itself.

I'm tempted to buy the mini-display port to HDMI just to see if the issue is still present with that adapter. If I do I'll update my post with the outcome.

If anyone else has any info to help with this issue it would be greatly appreciated. It's very annoying when you resume from sleep and you find all of your windows that where opened on the external display before sleep squeezed into your main display (iMac).

Thanks!

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Answer 9

Jul 26, 2012 8:51 AM in response to mlavigne016

Just upgraded to Mountain Lion and still have the same problem. Secondary display takes too long to wake up, and all the windows get moved to the primary display. Ugh, is this ever annoying--makes having a secondary display almost more trouble that it's worth.

Come on, Apple, this has been an issue for a LONG time now...

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Answer 10

Nov 3, 2012 10:40 AM in response to Mike Mitchell4

Hi exact same thing hapens on my new mini-mac running lion, seems random and makes working on the Mac a very frustrating experience, does anyone from Apple read these threads? It would be good if you would at least acknoledge this is a problem.

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Answer 11

Jun 10, 2015 2:17 PM in response to Michael Cheung

This happens to me as well brand new 5K iMac with two Asus 21" hooked up mini display to hdmi

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