I have a 2014 15" retina MBP that I've hooked up to my old 2009 27" iMac using it iMac as a monitor via Target Display Mode while the MBP is in clamshell mode.

My problem is that the iMac turns really hot very fast. A lot hotter than it turned when I was using the iMac as my primary machine.

The iMac is running a fresh install of OS X Mavericks so there shouldn't be an issue with a lot of background apps running.

Any ideas on why this could be?

  • 1
    did you look in the Activity monitor ?
    – Ruskes
    Aug 2, 2015 at 19:55
  • Yeah, nothing that seems out of the ordinary.
    – inFever
    Aug 2, 2015 at 20:16
  • since Retina resolution wont work on the iMac, what resolution are you using?
    – Ruskes
    Aug 2, 2015 at 20:33
  • 2560 x 1440, the iMacs highest resolution.
    – inFever
    Aug 3, 2015 at 15:11

1 Answer 1


I'm experiencing the same problem. I have a current generation 5K iMac which I use as my main computer, and a 2009 iMac which I use as a second monitor using TDM. In TDM mode, the older iMac runs painfully hot. Switch back to regular mode, and that same iMac runs cool.

Typically, I don't even bother to login to the older iMac - I just turn it on and off with the power switch when I want to use it in TDM. Hence, there can be no question about background apps running hot.

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