fuel gauge erratic
 

i have a 97 blazer.fuel gauge will go from 1/3 to 2/3 when you fill it up.it will go up and down a little,but it never goes to full or empty.if you unplug the sender,it doesn't move until you cycle the key,then it goes empty.i think there is a module in the system,but i am not sure.i would like a wiring diagram to see if it is there and where it is.any ideas?