iMac Late 2015 - External Monitor Recognized But Black

Aye. Get help with video issues on external displays connected to your Mac

Otherwise just curious what is the make, model, and model # of

• the external monitor
• the Thunderbolt 2/HDMI adapter
• the HDMI jumper cable

Also what is the resolution and frame rate of the external monitor as shown in Display Preferences?

There's something about the late 2015 iMac Retina 5k. I posted a couple days ago regarding a similar wake from sleep/black screen problem. My machine is late 2015, 4.0 GHz, 32gm ram, Radeon R9. I've been running El Capitan with a Thunderbolt display as the second monitor for a long time with no issues. I have the 1tb fusion drive with a Bootcamp partition which works fine.

After upgrading to Sierra, then High Sierra, then Mojave, when I attach the second monitor (I've tried several, it's not the monitor) and it goes to deep sleep, the screen won't wake up and I have to do a hard reset by holding the power button.

I did a clean Mojave install to 10.14.2, same thing. I did the PRAM, SMC, etc. as many have. If I don't have the second monitor attached, it wakes as expected.

Do a search for iMac late 2015 black screen and you'll find a few variations, but basically when it goes into deep sleep it has a problem waking in a few flavors.

As I said, it works fine (second monitor connected to TB on back of iMac) if I start up in El Capitan on a different partition. So me thinks it's definitely a software-type issue. But I've not seen a definitive solution. This is very disappointing and frustrating as I need to go past El Cap, but I guess I'll be limited to one display. Heck, my Quadra 700 could do two monitors without issue.

This post describe it well

https://discussions.apple.com/thread/7778784

Thanks

As a followup, there's folks that try all kinds of things for this problem, such as this:

https://www.youtube.com/watch?v=q2cy1rMfJ50