Display sleep problem with external monitor

I have a very similar problem, dell ultrasharp 30" connected via the mini display port. Sleep works fine, waking up only my MacBook (2010 15" i5) wakes up, the dell monitor does not wake up. I have to disconnect the monitor and then plug it back in via the mini display port.

I am working on an iMac - since upgrading to 10.8.2 my display sleeps every few minutes while I'm working.... Every few times I can't get it to wake up....I have to reboot (at least 30 reboots since this morning)

After a full day of reinstalling Mountain Lion in recovery mode with the problem immediatly recurring, I noticed that my iMac was extraodinarily hot when I touched the area where you can feel the slight vibrations from the disk drive running -[ I was checking it to see whether the display was merely blacking out and wouldn't wake, or if the hard drive was actually lshutting down....]

Because of the extraordinary heat, I powered down the iMac for a few hours, until it was totally cool to the touch. When I turned it back on, the problem had disappeared..... I've been running the computer for a few hours now with no recurrance.

I believe the shut-downs/unwarranted "sleeps" were in response to overheating (Apple tech support told me that if it was overheating that I should get some beeps when it shut down spontaneously, but I did not get any indication of this fromthe computer -- perhaps because I have an older model iMac?

In any case, problem has now been resolved....

Just recently upgraded my iMac mid 2010 to Mountain Lion. I did a clean install on a brand new Sandisk Extreme SSD. The only issue I have is my secondary monitor via display port does not wake from sleep. The only way to correct this issue is to avoid having the system sleep or reboot when I need the external monitor. The external/secondary monitor bug has been an issue since Lion (wakeup with no background) but now my secondary monitor is useless unless I reboot.

This has been an ongoing issue with Apple and it looks like no one is working on a fix.

So dissapointed.....

I and a friend have similair issue.

Macbook Pro late 2009 and 2011. Issue has started since upgrading to Mountain Lion.

When connected to external monitor (in my case a Apple 23") and macbook is left on after a few hours it goes to sleep, till now all fine.

However when I try to restart:

1. Macbook does not react to mouse or Keyboard

2. light is out (not flashing)

3. machine is on, you can here it when you get real close

4. Macbook is very hot

Nothing helps but hard restart!

It seems an issue with Mountain Lion and energy saving. I have reset the PMU etc..

Similar issue here, but in my case neither display comes on at all. This occurs for me not when the mac had gone to sleep, actually, but when the display turned off due to energy saving setting or a hot corner.

When connected to an external display and going into sleep (either via energy saver or alt+cmd+eject), it cannot be woken up on EITHER display… I have to force a hard reset, always. I have reset the SMC and NVRAM numerous times, turned off "wake for network access" etc… no dice. Very frustrating considering I rarely use my computer away from a desk. This occurs on a 15.4" Late 2008 Unibody MBP with both a 23" aluminum Cinema Display, as well as a 24" Dell UltraSharp 2408WFP. Extremely annoying.

Update to the issue: the only (unfortunate) solution I've encountered was a full clean install of Mountain Lion. What a pain! This also solved the huge slowdowns I was encountering and the icon drag problem. It would be interesting to know how many of those with these problems merely upgraded to ML without a clean install.

I upgraded to mountain lion.

I've since upgraded to a new Macbook Pro Retina and did a fresh install of Moutain Lion as well and I'm still having the wake issues about 50% of the time. Disconnecting the display adapter and plugging it back in is a workaround for me.

I have a similar problem, when i put my mac pro in sleep i cańt detect the display, i have to unplug and replug the displayport,

Dd you find a solution ?

Yes and no. My fresh install of Mountain Lion mostly solved it. I still need to hard reset, but much less often.

If you disconnect the external monitor, does the MacBook Pro act normally when it wakes from sleep?

Hook up the external monitor and try booting into the Safe Mode. Shut down the computer and then power it back up. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart. For each disable/delete, you will need to restart if you don't do them all at once.

Safe Mode

Safe Mode - About

Apple support article:

Video Troubleshooting Internal and External Monitors

I already did it, and aslo reset the pram.

Even in safe mode, when i put my mac pro in sleep, the monitor don't work when awake.

One more information, i have an adaptor displayport / vga generic ( not from apple) do you think that this is the problem ?

I was having this issue for months as well. For what it's worth, a normal update (read: not a clean install) to 10.8.3 fixed this and sleep is back to normal working condition on my late 08 c2d MBP. Huge sigh of relief.