I suggest you use
it enables you to set a fan speed slope profile min speed max speed and min and max temps etc for all three imac fans, CPU, optical hard drive, it also set the cpu and hard fans to 1200rpm minimum which now means my GPU die is operating at 68deg then playing 720p youtube video on a second screen.
donate to the guy if you like and use it
It works for me on an imac 27″ mid 2011 running mountain lion