I am certain Jerry knows more about the 4100, but in the 3100 full is 30 ohm and 0 is empty. Or at least it is that way on the 1952 model. You can check the fuel level with an ohmmeter if the prior owner didn't burn out the sender with his 12v conversion. If he connected 12v to it, it could have been fried and that means you'll get no ohm reading (open circuit). If so, you might want to remove the sender and see what is happening there. I will tell you rebuilding the original sender isn't easy. I've tried it and the problem is the shaft is steel and the housing is some sort of potmetal which over time wears out to the point the wiper flops around and only makes occasional contact. If you need a replacement sender, there's a guy on eBay selling a decent 30 ohm sender. But as I posted a month or so ago, some of the senders sold by some of the suppliers are junk. The makers (whoever they are) use 90 ohm senders and drop blobs of solder on the windings to reduce the ohm rating. You can see this here:
http://i1154.photobucket.com/albums/p535/JonGoodman/IMG_8579_1.jpg This was bought from a well-known supplier of old truck parts and it never worked right. I'm sure you can see why.
Jon
