Not trying to refute this as it's been a couple seasons since I looked at it, but my recollection of how the PCM handles temperatures is that it uses a proprietary unit that allows the module to deal with a wide range of temperature readings with a single byte (0-255). I referred to in in my notes as 'engineering units'. I'm fairly sure the temperature based table lookups used this unit raw and the .xdf converted it to a human readable conversion. I only recall this because I spent a considerable amount of time figuring out how to determine which ECT voltage divider / range the PCM was using based on cranking spark advance for the diy-ltcc project. I was certain enough this was the case I fired off a question to Banish (who I believe did some calibration work for GM at some point) asking if they had an official unit name for this.
There should be comments in the diy-ltcc source pertaining to this. I certainly recall the (temp_units * 0.75) - 40 = degrees celcius equation. If I get motivated to look I'll dig into the source and try to refresh my memory.
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