The computer knows there's a fan because it sees tacho output. If it doesn't see tacho, shrug. You can get an external temperature-controlled PWM controller for a few units of your local currency on AliExpress, steal 12V from somewhere (Molex header or whatever) and run the fans off that. Figure out where to put the temp sensor to get the desired effect.
There are far better ways to do this, but they require software engineering, not €3 and 15 minutes.
The computer knows there is a fan because it knows when there isn't a fan. By subtracting where there is a fan from where there isn't a fan, or where there isn't from where there is (whichever is greater) it obtains a difference, or deviation...
How does the computer knows that? You mean the parts that can meassure temperature will meassure where it gets warmer, or where it doesn't get warmer, altough it should?
How does the system knows, it is not a local heat pipe, transferring heat away?
This meme makes perfect sense in almost all contexts - at least continuous ranges are involved. I salute GP for fitting it for use with a discrete case.
The problem is not the fan, it’s the fan controller on the motherboard. I doubt a nonfancy fan controller will bother to drop off the bus/whatever if it doesn’t have fans connected, and the comment by 'patrakov upthread seems to confirm this.
