I'm confused here Shane - Did you still have your HD still installed in the system? I thought you swapped it out with a SSD. If you did the sensor is internal to the HD [http://www.ifixit.com/Guide/Image/meta/uo5O5NJPCoqZxEh1|Internal sensor].
You could get an external HD sensor or short the cable out (telling your system the HD is always HOT). I prefer getting a replacement sensor than using software to override the fan's.
But this is not the sensor I was speaking about in my comment.
Apple has a lot of sensors in their systems. In my '11 iMac I have four sensors just in my power supply two on heat sinks! We don't have any more 21.5" models so I can't tell you how many you have and your description on what you were looking for was a little vague.