iMac Target Display Mode black screen

I posted this on another question, not sure how to link it so adding again here.. I use 2009 iMac as an external monitor to my 2015 MBP.. all running latest supported OS as of today.

COMMAND-F2 on the iMac keyboard will toggle between the running iMac local display, and the Target Display Mode (TDM) when connected to my laptop via DisplayPort. TDM always starts out at 100% brightness when you switch to it.

COMMAND-F1 will adjust brightness lower until it goes completely dark. If that happens you need to switch back to iMac local display then return again to TDM which resets to 100%

Now the important part: COMMAND-F1 sometimes doesn't respond to lower the brightness. I found this seems to happen if local iMac screen is locked.. if I switch back to local iMac display and log in to the account to unlock it, then when I switch back to TDM, the brightness control will now respond.

So for those of you who still have this problem and nothing above helps.

It only worked for me only after logging the user out on the iMac.

1. You log off
2. Press CMD + F2 (sometimes you need try it two times)

But logging off the user every time I want to use the imac in a target display mode is quite annoying. I found another option which works for me:

1. Lock iMac (CMD + CTRL + Q)
2. On the login screen of the imac click: "Switch users"
3. The list of users should appear on the screen of the iMac
4. Now press CMD + F2
5. It usually does not work from the first try. So if the screen of the iMac is just black, press CMD + F2 to exit "target screen". And then press CMD + F2 once again. Always works for me from the second try and does not require a user to be logged off from the imac.

Hope this helps

Resolved. Had to uncheck 'Automatically adjust brightness' on the iMac's display control.

On the Macbook, change the rotation on the iMac display to 90°, then revert it back (or let it revert back after the timeout) to Standard. That seems to snap it out of its zero-brightness funk more or less every time much more frequently than other display changes.[1]

(Of course, you can't do this in clamshell mode, you need some functioning display to use System Preferences→Displays on.)

[1]: (I've seen it fail once or twice, but it is the most reliable workaround I've found yet.)

Update 2021-07-12: Apple discontinued support for Target Display Mode in all operating systems since High Sierra, and so it was likely that this was officially unsupported / broken even at the time the question was written. I also definitely used TDM on Mojave for a while, and I think maybe even on a Catalina beta for a while; it still almost worked but it was incredibly janky. So if you've upgraded your iMac … some of this fiddling might help you, but it probably just won't work.

On closer inspection, it looks like the display is not actually uniformly black. This led me to conclude that perhaps the backlight was just not turned on properly, and led me to discover the following workaround:

1. Log into the iMac
2. Change the brightness to a different level (e.g. up or down by one). The level seems to need to be different from the one previously used.
3. Enable Target Display Mode again, by pressing Command+F2