The fuel gauge in my pickup pegs at 3 o'clock too--I checked all the wires,grounds,etc,but it turns out the sending unit rehostat must be junk..
I had swapped another tank into my truck and I failed to test the original sending unit (Truck had dual tanks and the drivers side leaked,so I just plumbed up the passenger side and ran it off that one,till I came across a good used drivers side tank)..I swapped the original sending unit to the "new" tank,and was bummed out to find after I had it all bolted in,the gauge would just peg out..
My truck is a diesel so the sending unit is a bit different than a gas truck,having the "water in fuel" sensor in the sending unit,plus another hose nipple for the factory water drain siphon hose feature..
I happened to have several gas sending units hanging around from other tanks I knew were good,plus one brand new one ,and I was able to get the plug and wires off the tank's sending unit and plug in one of the known good ones ,and grounded it--when I moved the float up and down with the key on,the gauge responded normally..
So it figures I'll have to drop the tank again someday..it weeps at the seam a bit anyways,so it will need replacing sooner or later..
There is a "right and left" sending unit for dual tanks,though I dont see why one just couldn't be spun 180 degrees and used on either side..I was told I can swap just the rheostat from one of the gas sending units onto the diesel one ,so I can retain the water in fuel sensor and siphon hose feature,though I doubt I'd miss either one of those things..
I found this troubleshooting guide for GM gauges online,its confusing,but might help diagnose the issues with them..
Functional Tests of Factory GM Electric Gauges
