Yosemite does not recognize external displays...

I know I'm late to this thread but I reckon other people are still having this problem and I'd like to share how I resolved my issue. And very easily at that.

It was a simple case of poor electrical contact between the MiniDisplayPort adapter and the MBP Thunderbolt jack.

I took a dry, clean, new toothbrush and gently poked and swirled the bristles into the Thunderbolt jack on the MBP. Easy does it you don't need a jack hammer. ( Compressed air or electronic contact cleaner would be the more "professional" way but I had nothing to loose by trying the brush ).

Bingo - worked like a charm - all good now.

Since I updated my Macbook Pro retina late 2013 15" to Yosemite it hasn't been recognizing my screen when it is awoken from sleep modus. Also, when it awakes I don't have any sound on my external audio, I get the speaker with a stop sign.

My setup:

- I work with the lid closed and have one external display connected via HDMI

- I have an external stereo connected via 3.5 mm

- I work with an apple wireless keyboard and magic trackpad.

Problem:

When the MB is in sleep mode I can't get it to wake up using only the wireless keyboard or magic trackpad. I have to open the lid to get the external screen to work and than close the lid. After that I have to unplug the 3.5mm audio cable and plug it back in to have sound and be able to change my volume.

Not the biggest of problems, but it is quite annoying because it used to work fine in Mavericks!

Hope you guys can help, thanks in advance.

I've got the same issue with my Late 2008 MacBook Pro 15". There is no signal to my Acer monitor. When I plug it into the mini-display port, my MacBook screen goes black and comes back as expected but there is still no signal to the Acer. Resetting the PRAM/NVRAM has not had an effect. I'm not sure if the SMC is getting reset. Everything was working fine with Mavericks.

I've tried multiple PRAM/NVRAM resets. I'm not seeing any indication that the SMC is getting reset. I'll give it another shot now.

I did the procedure for SMC reset a couple times followed by holding down the PRAM/NVRAM reset keys for 3 or 4 resets. Still nothing.

Folks,

I did not know that others are seeing the same issue. I just posted a query on the OS X Yosemite group with similar description. I too have tried the SMC and PRAM reset, plugging in/out cables, detecting displays by pressing Option etc. No success so far.

Thanks!

So apparently the one thing I didn't try was actually unplugging my monitor from the wall. I hit the power button several times but unplugging and replugging did the trick.

Mac keyboard does not work with file vault password. What's up with Apple releasing Yosemite with this problem. I prefer apple products bc I avoid these headaches, at least that's how it used to be.
Sent from my Surface Pro

Anyone care to describe what connector you are using for your external monitor? I see that some have resolved without mentioning changing the connector, but my attempts to use the methods described have failed to resolve the issue. That leaves me wondering what other factors could be complicit and that boils down to the connector, the cable, and the monitor. The later two should be non-factors, but the connector could be an issue when you consider Apple's habit of making "non-compliant" connectors (3rd party Lightning anyone?) obsolete.

I'm using a Monoprice brand Mini DisplayPort to DVI connector on a MBP 17 Early 2009 to a Samsung SyncMaster 213T that has worked fine since Snow Leopard. I don't get any warnings that the connector is non-compliant. Oh, also, the Yosemite install sets the graphics card to the energy-saving option. I have switched to the high-performance card to no avail.

I'm having the same problem. 2013 MBA, 13" display. Upgraded to Yosemite a couple days ago, haven't been able to get the external monitor to work since. I've reset the PRAM and the SMC. The external display is a Samsung LN46A650A1F. I'm connecting via Thunderbolt->HDMI. The adapter is manufactured by Monoprice.

I appreciate any help. Thanks,

Philip

My problem is a bit different. My external monitor works just fine, but my MacBook Pro display does not turn off. The logo is lit and when I crack the lid the screen is on. I use this MBP as a desktop with a Thunderbolt display, it worked just fine before the update to .10, The beta on my other machine worked fine too, until it also was updated to .10...Any suggestions??

Hello z33k3r

This discussion thread is really funny..

What cable you are using...what size monitors...restrating in safe mode, is irrelevant !

(Unplugging the power to the monitor!)

Look, if everything worked well before downloading YOSEMITE...and then it stopped working well after you downloaded Y0Semite...then the problem must be the...CABLE!

You can not fix this... only APPLE can.

Pls prove me wrong. I would like to eat some humble pie.

Peace.

--Marconi