iMac runs slow and fan running fast after replacing display

Sorry not really, you see the design of the system requires the display as its part of the heat management system. Between the physical placement to prevent air from entering and its own thermal sensor you need it.

Besides, it looks funky without it ;-}

Really? =( I am kind of a DIY guy. I was thinking to cover it with an acrylic sheet,

remove the stand and mount it somewhere. I think it is cool. LOL

I like your style ;-)

BTW, I ran into the same problem, just that besides the fan-speed and performance, not even my new screen would work.

However, I forgot to transfer the temperature-sensor from the broken to the new screen, as I only stumbled about this post after I put on my old cracked screen - again, which isn't working, so I am using a HD-TV via Thunderbolt/HDMI.

Now the system is at light-speed again and the fan is resting peacefully. But I am wondering if the new screen did not work because of the missing input from the missing temperature sensor??? Is that a possible cause for the whole new screen not to work?

Facetime showed a message (on the TV with the new LCD "installed & not working") that it will only switch on the camera once the temperature is back to normal. Now I am wondering whether the same applies to the LCD?

It's possible to run the Mac with the faulty screen in place and an external monitor connected to the auxilliary monitor socket.

I did this for a while but the fans ran annoyingly at max speed.

I used the free online software programs that control the speed of the fans and all was hunkey dorey UNTIL I turned off and restarted the computer.

It was a great hassle to get the free fan speed program to work properly again requiring re-installs of it.

If you never turn off the computer it's OK.

I contemplated screwing the whole carcase to a board, screen side touching the board and securing this "computer-board" to a wall.

A new wall-mounted computer box with all of the connections facing you. I procrastinated and stored it away instead because of the fan-speed hassles.

Later I heard you could buy a monitor screen temperature-sensor cable from "OW" or even cheap online Chinese that could fix this fan-overspeed problem. DIY' ers can work this out.

Sorry no more communication from me for personal reasons.

It IS possible to trick iMac thinking it is using internal monitor, just get the broken screen out, cut the logic board from the screen panel (the board that have 30pin connection and contain thermal sensor), keep this board connecting to iMac motherboard, iMac gonna run fine, no safe mode slowness, no max fan speed at all.