After replacing a few 2010 iMac Seagate hard drives, I came across a few 2011 iMacs that did not have and external temperature sensor, or a wire pair going directly to the hard drives. I figured that they simply did not require a sensor or were getting the temperature from some other source, so I replaced the hard drives in a few. The fans ran at full blast. I could not figure out where the sensor was until reading a few pages into an Anandtech article on the 2011 iMacs. The sensor is once again provided by special firmware on Apple drives but these are wired through the SATA power connector.
The article explains that one the SATA power connector, pins 2 and 7 are carrying the temperature data. The problem is that the pins 2 and 7 referenced are being read on the motherboard side of the connector not the SATA power connector. I ended up having to dismantle an iMac to find those pins on the motherboard and trace them to the wires going into the SATA power connector. As it turns out, the two pins are the first wire and the fourth wire reading from the SATA power connector key side. Clipping those and soldering them together makes the iMac happy.