As mentioned above, installing a replacement hard drive does not allow the HD temperature sensor to work correctly. The original Apple drive has a unique firmware that allows the factory sensor connector to work as the logic board expects. Any drive without Apple firmware will not feed the temperature info to the motherboard and the internal fan speed will eventually crank up to its max RPM. Even installing a new drive that matches the brand of the original factory drive will not stop this from happening. I tried putting a Seagate 7200.xx drive in place of the original 7200.xx drive and the fan still went full tilt boogie.

So, I decided to buy an optical drive sensor (Apple part #922-9141) and use that in place of the original HD sensor. It plugs into the logic board where the old HD sensor did and sticks to the hard drive with an adhesive that is already on the sensor. After installing this and reassembling the computer, the system's fan behavior was normal.

I overcame the thermal monitoring problem by using the equipment already provided. The hard drive in the mac had failed but the board still had a functional thermistor on it, allowing me to unscrew the PCB from the old hard drive and keep the 4 pin cable attached. Then i tucked this PCB under the new drive and only plugged in the SATA connections into the new drive.

Apple used the Tx pin of the UART in some HDD brands/models to report the HDA temperature in real-time. I suspect that the firmware required to support this function would be at ROM level. I say this because one Apple user reports using the PCB of a failed HDD to report temperature information after replacing the HDD with one whose firmware did not support the sensor.

The following post explains the problem:

https://discussions.apple.com/thread/2712496?start=0&tstart=0



https://www.ifixit.com/Answers/View/85991/replace+the+hard+drive%2C+how+do+I+connect+Hard+Drive+thermal+sensor