Check the ground connection for rust or something foreign. On the half ton the wire which goes to the gauge has a connection in it under the cab. Sometimes corrosion gets in there. Emery cloth works wonders. You can also run a jumper wire from the power post on the sender to the gauge and see if it changes. If yes, there is a break or short in that line. You might also want to click the ignition switch on and off a few times. Been known to work - I do it all the time on my computer monitor!

1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice