So, I noticed there is a 47k divider resistor on the R1003. I’d like to read a 100k thermistor with the R1003, so I desoldered the 47k resistor and tied a 100k resistor on there instead. This will work right?
Not without firmware modification, the pullup resistor value on R1003 should be 4.7k (not 47k)
Smoothieboard performs thermistor measurements with a 4.7k pullup to 3.3V, R1003 is following the same for firmware compatibility. I suggest you resolder the 4.7k (or just use another channel for testing first) and tie your 100k thermistor to ground and pick the proper type in configuration
also to know which GPIO connects to which channel, depending on the slot look at the table here
make sure that you use analog capable slots, slots #1 or #8. PS: if you have to use lost #8 you MUST disable UART0, I recommend using slot #1