Maybe can i do something via software?
I am not familar with the 2017 iMacs, but if they use the same HDD internal temp sensors as the mid-2011 iMacs, then they are probably the same.
You can use a software solution for the fan speed, but personally, I would just put in a cable with a external sensor while I had my iMac opened.
Will my Imac be damage without this cable?!
Are you asking if you don't use any solution, HW or SW, and just let the fans stay fast? If so, I don't think this would damage anything, except shorten the life of the fans.
If you are asking using a software solution to control fan speeds, could this cause damage? This is a guess, but maybe. If you are regulating the fan speed lower than what it needs to be, then this could damage stuff.
Just get the sensor, and you won't have to worry about it.
[doublepost=1561059356][/doublepost]Wait a sec, I am pretty sure the 2017 doesn't have the temp sensor.
I haven't opened it yet, but I have a Late 2012 iMac, and when I looked into replacing the Fusion Drive's HDD with a SSD, I don't recall having to get a temp sensor.
I would assume the 2017 is the same. I would still look into it, but I am pretty sure that there would be no software or hardware solution needed, because there is no HDD temp sensor.