There is a ground wire and the center post is a resistance wire (resistance back to ground). Using your multimeter with the function set to resistance (ohms) and set to the lowest reading, connect one lead to the center post and the other to ground. With your tank full, the resistance should be 30 ohms. Make certain you have a good ground connection, too. If you get something like 14~18 ohms resistance with a full tank, your sender isn't doing the right things. Let us know, please.
