Very smart move šThe GPU die temperature sensor problem!You all know that none of the modifies BIOS versions for the Metal GPUs listed in the table on page #1 is able to provide the internal GPU temperature back correctly to by used by the iMacs internal Apple SMC software to control the (ODD) fan according to the GPU internal temperature.
To address this issue we recommended strongly to use a free software called Macs Fan Control.
A different solution has been described several times now and last week and was in the mood to reopen my iMacs again and did this simple mod.
I (carefully) separated the ODD temp sensor using my finger nails and placed it on the back side off the GPU sink (side by side with the sink temperature sensor). Then I used a simple and cheap two component epoxy alike glue and fixed the little sensor. During the first experimental phase I used just a 3M or Tesa strip to hold the sensor in place.
Just cut off the two cables in the near the sensor and exchange the cable ends with the plugs connecting the same colors simply together (grey to grey and black to black). So you can easily connect the original heat sink sensor to the ODD connector in the logic board and vice versa.
Doing this you have a working fan control again even when doing system upgrades or during the start phase. As you may have noticed the Macs Fan Control used before is a user land application and it is only working after primary login to your account.
I attached some pictures showing the Macs Fan Control in automatic (i.e. SMC) mode under full Valley load (WX4170) and the hardware mod.
Note:
Please use Macs Fan Control (or a similar software) in any case as a fall back if the replaced internal sensor fails of the glue does not hold it properly. It is a nice tool to monitor what is going on internally.
Tonight when I was rearranging all the cables inside my iMac, I saw the ODD sensor cable now attaching to the back of the case without much function and thus use heat tape to attach it to the X-brace of the card back. But your idea is much better