Chosen Solution
I recently replaced the original hard drive with a solid state unit. Since then, the cooling fan begins at low speed as usual, but very quickly reaches maximum speed and remains at that speed until I restart the computer. Prior to the new hard drive, the fan never went past low speed.
I don’t recommend software solutions as they’ve been shown to not be as good and people tend to misuse them killing their system. The proper way is to install OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade
Did you replace the temperature sensor when you upgraded to the SSD? If you didn’t that would cause the fans to max out. You can also use this program that another iFixit user used to control the fan speed of your Mac.
