Because most automotive temp. sender units are NON lineair we have 2 options to connect sender to a micro (arduino, etc... )
- First you have to map the values of the sender you wish to use.(see GM values as a nice example)
a) look for them up on the net.
b) measure the values of your sender at different temperatures.
c) Use your temperature gauge at 13.8V and pot resistor ( 5K ) to ground. - Depending on how accurate you wish to work ( most older cars normally have a 3-5% variation from correct value )
P.S. Nobody really cares about values below 40 degr. Celcius
a) record ADC value per degrees Celcius ( or F) with high and low bounderies
Create a lookup table for these values , it's a lot of work, but the most accurate, it uses a lot of valuable memory space ( these days not so important anymore )
b) dot the mapping values on a piece of Graph paper . from 40 to 120 degrees C . cut the graph up in say, 4 straight lines.
You now have for lineair lines to work from ---> 4 formulas , Guess you can work it out from here.
Hope this helps 