[VetteVet]

Originally Posted by lilbluetruck

tried your link, but was not working.

Wondering anyone can help me troubleshoot. My sender was replaced 5 years ago. Was working, but my gauge needle was reading on the next set of lines (3oclock) position. Now stopped reading. my tank is completely full, gauge sitting correct now, but at 3/4 tank. I took off the ground and the gauge burries to about 5 oclock. When I put it back on, it goes to 3/4 of a tank. Should I be replacing the gauge or new sending unit. Thank you!

Your gauge is showing the symptoms of a lack of ground from the sender. You should run a temporary ground wire from a good ground in the cab to the fuel terminal on the fuse panel. It is usually the middle terminal on the end of the panel. Your gauge should go to completely empty with the key on. If it does then you need to find the open circuit on the tan wire that goes to the sending unit in the tank. It's possible that the sending unit isn't grounded to the tank or the tank isn't grounded to the cab.

If you measure the ohms between the sending unit terminal and the tank you should get 90 plus ohms with a full tank. That will tell you if the sender is good.

The fact that your needle is going to the five o'clock position when you remove the ground wire could mean that the gauge isn't grounded in the dash or the gauge resistor is bad.
the link posted by HO 455 will trouble shoot the complete system.