first step is to install Morphos on HDD/SSD
If you run Morphos from LiveCD the fans runs on highspeed
second - go to the open firmware and put this line
setenv boot-device hd:,\\:tbxi

third - you can use tool
https://www.morphos-storage.net/?page=System%2FMonitoring&file=iMacFanCtrl_0.2.lha