I have a 2014 iMac 5K. I find that the fans seem to have two speeds - slow, or top speed. Using SSD Fan Control, the optical drive seems to sit around 1200 rpm on startup at 43 degrees, and hits 100% max rpm when I launch anything taxing on the CPU. The other fan is the Hard Disk fan, which sits at the same temp of 43, but permanently reads 65,535 rpm, no matter what. It hasn't changed once! What is the reason for this?
There's quite a few possible reasons why fan speeds vary, but as a starting point you're probably best to see whether a reset of the System Management Controller (SMS) changes things for you.
To reset the SMC on your iMac
- Shut down your computer
- Unplug it from power
- Wait 15 seconds
- Plug the power cord back in
- Wait for an additional 5 seconds
- Press the power button to turn it back on