High Sierra, Target Display Mode, and fans

[pizzabox]

I updated a 2012 MBP from Sierra to High Sierra a few weeks ago. As an external monitor I always use a 2011 iMac in Target Display Mode (Thunderbolt). The iMac runs Sierra.

Weird thing is, ever since the MBP HS update, the iMac seems like it's been running extra hot and the fans are definitely extra loud. Surely the OS of the computer sending the video signal shouldn't have any effect like this, right? And yet I swear the difference was immediate and has been stark.

The iMac was opened up and purged of dust not long ago, so I know that's not it. I haven't made any changes to the iMac. I don't keep apps open, am not running any new software, nothing. Doesn't matter if there's a user even logged in yet or not; usually, there isn't.

I could try manually decelerating the fans but I don't want the thing to cook itself. Could it be that HS sends out video of such a sophistication that this iMac is at its limit with it? Would that not be strange?

Am I just being crazy?

 

[Minghold]

This might not be the response you're looking for, but my advice is to also put High Sierra on the 2011 iMac (especially if it's a 27") and sell it for ~$250, then use the money to buy a big 4k monitor and whatever adapter you need.

 

[pizzabox]

Sure—the ultimate solution is to just replace the whole setup. I'm slowly coming around to the wisdom of it. What I have done in the interim is installed Macs Fan Control, which told me that the offending fan on the iMac is the HDD fan (I replaced the HDD years ago and I recall there being implications about the sensor at the time; dunno how/why something would have cropped up now). So I just capped the speed on that fan at 2k RPM. So far the iMac hasn't melted down.