OS x mountain lion dosen't detect external projector

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Open System Preferences > Display, press Option key and press Detect display

It hasn't disappeared. Open System Preferences > Display, hold Option key and press Detect display at the bottom

I've got a similar problem. Mine shows that it is connected to the monitor, but there is nothing displaying on the monitor. The monitor isn't detecting any output from the Macbook Air - and my co-worker's Macbook Air (2011 - 11" model - same as mine) does the same thing.

Running Lion, this wasn't a problem. This has come up since upgrading to Mountain Lion.

It seems it's a widespread bug with Mountain Lion. I have a Mac Pro with a 3 monitor setup, and I consistently have 1 monitor not turn on (connected into the middle display port, of the 3 that I have). Interestingly, the computer DOES detect 3 monitors and is reading it as if it is on, even the detect displays button doesn't work. I've found simply disconnecting the monitor and reconnecting fixes the issue, but still a hassle nonetheless.

I am having the same exact problem. Apple just recently released a fix to address audio output from the TB port, so I installed it in the hopes that it would fix the VGA problem. It still hasn't fixed the problem. How do we file a bug report with Apple regarding this? I was wary about upgrading...shoulda trusted my instincts. :-\

Same here. I am a teacher and connect to a projector on a daily basis. I have never had any issues with my 2011 MBP or 2012 MBA, both ran Lion. I just purchased this 2012 MBP with Mountain Lion and although it shows on my screen that it has detected the projector and that it's connected, nothing is showing up on the projector screen. School starts next week. Let's hope Apple provides us with a fix soon.

I also have the projector problem with a new MBA5 running Mountain Lion. I am a teacher (Univ. Illinois/Chicago) and I ended up giving a couple of lectures with chalk and slate last week. Apple support did not help, but a couple of days ago one of the IT people at UIC game me a decent workaround. Just reboot the machine _after_ plugging the projector (VGA + dongle) in the Mini Display port of the MBA. Then, the projector will show properly. Better yet, the projector will continue being detected even after the laptop is shut and reopened, or if the projector is unplugged and replugged. Of course, you cannot reboot without plugging the projector, or else the problem will return...

I've found that I can use an external monitor flawlessly using a DVI dongle, but still can't get it to display via VGA at full resolution. If I back the resolution down to 1024x768 on VGA, it will display, but will still disappear every once in a while.

Same problem here!

Hardware:

Macbook Pro 13" Early 2011

Problem:

After upgrade to Mountain Lion I can't connect ANY external display or projector.

Thins that I've already tryed:

1) PRAM NVRAM reset - DIDN'T WORK

2) Fresh install Mountain Lion - DIDN'T WORK

3) Try others HDMI or VGA adapters - DIDN'T WORK

4) Downgrade (reinstall) Lion - all my adapters and external display worked again

Here is the chronology of the problem:

1) With Lion - Everything worked fine

2) Upgrade to Mountain Lion (10.8.0) - My HDMI adapter stoped work, but my VGA adapter worked fine

3) Update to Mountain Lion (10.8.1) - My VGA adapter died too. Now none of them work. No HDMI and no VGA.

4) Update to Mountain Lion (10.8.2 12C53 - I have an mac developer account) - DESPERATE TRY TO SEE IF APPLE FIXED, BUT NOTHING. SAME PROBLEMS, ANYTHING (HDMI and VGA) WORK.

I have not tried the HDMI connection, but I use the VGA connection frequently. I am happy enough with the workaround I described above, given that I do not reboot my MBA5 running ML very frequently. In the rare occasions that the output does not go to the external projector, I just reboot with the projector plugged in.

Got the detect display button appearing following instructions then... The display that all previous OSXs detected doesnt work.

DOWNGRADING ....... back to Lion or even Snow Leopard it seems.

Same problem here. I use my 2012 MBA for lecture classes every M,W,F....and now it fails to recongize the lecture hall projector. 10.8.2 is my OS...and 10.7x had no problems. Exceedingly frustrating!!!

I was able to solve my problem.

My issues connecting to a projector were actually pretty severe. It started with just my VGA/DisplayPort connector not working, so I switched to HDMI; then HDMI would not work; then I started to get a black screen with the cursor only, and discovered that if I blindly clicked on a menu, stuff would appear, but only in a very disturbing way.

Eventually, any time I plugged in either connector, my Retina MBP's resolution would seem to switch, but then the screen would go blue, and nothing would work--the machine froze and had to be force-shut down and restarted.

My solution:

I installed Display Menu, a free utility on the App Store:

http://itunes.apple.com/us/app/id549083868

Before plugging in my HDMI, I set the display resolution to a non-HiDPI setting (e.g., 1680 x 1050). For some reason, doing that with this utility made it work for me. My resolution went to Full HD (1920 x 1080, IIRC) and everything worked OK, the projector was recognized and displayed my screen. I have not tried the VGA adaptor or tried to turn off mirroring, though. Didn't need to and didn't want to deal with any other hassles...

Thanks for the tip!

Yesterday I'd the same problems connecting to my new Dell monitor, and your answer just save me time.

I'm using "Display menu" app, as you recommend, with the mirror option on ( it seems that without it it doesn't work).