iMac 2012 Black Screen when in Target Display Mode

Problem solved. After reinstalling Mohave on the iMac, TDM mode is working when you enter TDM when the login window on the iMac is displayed (regardless of whether or not someone is logged in to any particular user account on the iMac). Apple support had me go through a specific set of steps ... turn off both Macs, turn on the MBP, connect the cable to the MBP. Turn on the iMac, connect the cable to the iMac. Then, the iMac should go into TDM automatically - if not, hit CMD F2. I had to to hit CMD F2.

If you try to enter target display mode while logged into a user account on the iMac, you get a black screen with no other indication that anything else is wrong. The connected Macs think the iMac is a display, but the iMac screen is black.

Thanks for the reply. That article says my mac is supported. I am using apple thunderbolt cables, fresh off a $75 purchase for the USB-C adapter and cable. The 2018 MBP needs the adapter, the 2015 MacBook air just needs the cable. When connecting either computer to the iMac, both the MBP and MacBook behave like the iMac is there. Choosing display system preferences and clicking "gather windows" brings up the iMac display preferences pane. And, I can rearrange the displays, using the "arrange" display option. I can drag windows from the MBP or MacBook Air screen to the iMac. But, because the iMac screen is blacked out, I can't see anything there.

So, the iMac is being recognized by the computer it is connected to ... as a big screen. The internet is filled with people with this issue. One guy found that an audio cable was messing it up. Another guy found his brightness was too low. Some lady said she had the wrong cables. Yet another dude said the short cable worked, the long did not (I'm using a brand new short cable, apple branded) Another dude said it was his keyboard (he had the fn box checked in the settings). Another reset the PRAM and SMC (I tried that as mentioned above)

Before lugging this off to an apple store ... trying one last chance to see if someone had another workaround that makes it work. I guess it's possible that the brand new cable is bad. OS10.14.4 is the latest version (in case someone sees this post in 2025).

I tried safe mode - because safe mode disables the port, the iMac won't go into TDM. The MBP, when in safe mode, does not send a single out the USB-C/thunderbolt port to the iMac. So, the iMac won't respond to the TDM key combination. Same happens if I put the iMac in safe mode, it does not send a signal out of the thunderbolt port so the it does not respond to the key combination.

I also unplugged everything from both computers.

I'm going to try setting up a new user on the iMac

Creating a new user did not provide a different outcome.

Yeah. The cable is brand new and apple branded. I tried cheaper cables and the iMac would not switch into TDM. I sprung for the apple versions and at least the thing now switches, but of course I have a black screen. It's possible that the USB-C to Thunderbolt adapter and the thunderbolt cable are bad.

I was on with apple care and ... no luck. When was done with them, I was left with two options. 1) buy new cables, 2) re-install everything on the iMac.

So, this morning I wiped the iMac drive clean and did a full re-install. As I thought, the MBP still would not work. But, the MacBook Air with 10.14.3 did work!!!!! So, I updated the MacBook Air to10.14.4 and now it's not working. :-(

So, something is up with 10.14.4 and my cable is good ... more searching.