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I think it's because I'm using a Mini DisplayPort cable, not a Thunderbolt cable. The MDP cable and Target Display Mode works great with a mid-2014 MacBook Pro. Can someone confirm that a Thunderbolt cable will fix the problem? Have you connected a 2018 MBP to a late 2009 27" iMac?
I've already read the support article: https://support.apple.com/en-us/HT204592
MacBook Pro with Touch Bar
The late 2009 27" iMac is supported, but not with a Thunderbolt cable or adapter. You need a USB-C to Mini DisplayPort adapter to connect your Mini DisplayPort cable to a Thunderbolt-3 port on the MacBook Pro
https://www.uptab.com/uptab-usb-c-type-c-to-mini-displayport-adapter-4k-60hz-silver/
or you could use a USB-C to Mini DisplayPort cable.
The late 2009 27" iMac is supported, but not with a Thunderbolt cable or adapter. You need a USB-C to Mini DisplayPort adapter to connect your Mini DisplayPort cable to a Thunderbolt-3 port on the MacBook Pro
https://www.uptab.com/uptab-usb-c-type-c-to-mini-displayport-adapter-4k-60hz-silver/
or you could use a USB-C to Mini DisplayPort cable.
Apple does not make a "Thunderbolt 3 (USB-C) to Mini DisplayPort adapter". You probably mean the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple
"Note: This adapter does not support DisplayPort displays like the Apple LED Cinema Display or third-party DisplayPort and Mini DisplayPort displays. "
Also see:
About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Support
"This adapter is not compatible with displays that use DisplayPort or Mini DisplayPort, such as the Apple LED Cinema Display. Thunderbolt and Thunderbolt 2 are not the same as Mini DisplayPort , though the ports have the same shape."
Thanks for your reply, macjack. According to the article, Late 2009 27" iMacs do support Target Display Mode.
Plus, as I mentioned, I've been using Target Display Mode successfully with a mid-2014 MacBook Pro.
The question is whether my iMac and Target Display Mode is compatible with a 2018 MacBook Pro. I'm hoping someone in the Apple Community can confirm whether they use that combination or have first-hand experience with it.
Thanks for your reply, Malcolm. I forgot to include the detail that I'm using an Apple Thunderbolt 3 (USB-C) to Mini DisplayPort adapter plugged into the 2018 MacBook, then using the Mini DisplayPort cable to my iMac. All connections look good. However, command+f2 does nothing.
A 2009 iMac does not support Target Display Mode, from the Support Topic you linked to.
iMac (27-inch, Late 2009) Mini DisplayPort to Mini DisplayPort Mini DisplayPort or Thunderbolt
Yes, my bad eyes! Not sure how I misread that.
2018 MacBook Pro connecting to late 2009 27" iMac Target Display Mode: Command+F2 does nothing
