Well, it has been a long, long time figuring this out, but I believe I have got it. I'm referring to a thread I started last year about configuring the instrument cluster for adding the trans temp gauge. https://www.tahoeyukonforum.com/threads/reconfiguring-instrument-cluster-with-tech2-and-tis.99740/ Turns out, the Tech2 isn't needed, but an eeprom programmer is. Good news is, they're a lot cheaper than the Tech2. You can pick up a cheap one for $30. Anyway, after a lot on research, it became clear that there are a number of parameters that are stored in an eeprom (U15) in the cluster. Odometer, engine hours, VIN trip meters, some other identifying information, some vehicle settings, and still quite a bit more that I haven't figured out yet. There are 256 bytes on this eeprom, and I've deciphered about half of it. I originally theorized that the cluster model (there are a lot of cluster models for the GMT800) information stored there was what was used to determine if a trans temp gauge was present, but as it turns out, it is just a byte in the vehicle settings block that is set to 0x01 for no gauge and 0x00 if the gauge is there. I have managed to find a handfull of memory dumps from different clusters, and bought a Denali cluster off ebay to play with. And from these I have wasted many hours, well at least my wife thinks they're wasted, figuring out what and how these parameters are stored. I still need to pull the cluster from my Tahoe and reprogram it, but I have successfully turned the trans temp gauge on and off on the Denali cluster (I used my Tech2 to sweep the gauges to see that the gauge operated or not). So, HOORAY and success!