new 20 gallon tank

[VileZambonie]

If there is no hole in the dash then it wasn't equipped with dual tanks so you would use the single tank wiring diagram.

[Rough75C20]

So, hooked up the wire to the #30 tan and tried the truck out. Fuel gauge still reading F with an empty tank. Made sure the ground at the tank is clean, and the ground of the dash cluster is clean right above the emergency brake release. am I missing something? bad resistor in the gauge? printed circuit is new but haven't tried checking power to the gauge. no blown fuses that I can see.

[bd]

Check the GAUGES fuse.  Ground the tan wire.  Switch the ignition ON.  The fuel gauge needle should drop to Empty.  Disconnect the tan wire.  With ignition still switched ON, the fuel gauge needle should swing past full to the ~3:00 o'clock position.  If the gauge needle doesn't move, the gauge is either dead or the wiring is hacked, and you'll need to start tracing to locate and verify I+, ground, and signal connections to the gauge.

For reference, see "Functional Tests of Factory GM Electric Gauges."

[Rough75C20]

So troubleshooting the wiring shows that the contacts on the printed circuit aren't touching on the back side of the gauge cluster. when pulling the gauge out slightly the Fuel gauge drops to past E. I would say between the 9 and 10 o'clock position. Is that normal for an empty reading? I went to go fill up the tank to test, but I have the filler neck upside down on the driver's side which wasn't allowing fuel to go in. also have a building pressure issue in the new tank I need to figure out. Shouldn't the vent in the top of the sending unit vent the tank from doing this? Maybe the line is pinched somewhere when installing the tank. I also thought i purchased a vented cap. It's the gates one from amazon that looks like the original tank cap. My plan was to open the tank cap and blow air through the vent to see if its plugged.

[bd]

With the signal wire grounded, the fuel gauge should indicate Empty.  With the signal wire disconnected, the fuel gauge should indicate well past Full with the needle pointing ~3:00 o'clock.  Correct the connection issues between the gauge pins and PCB.  The clips (image) may need to be gently retensioned using needle-nose pliers.  Don't overbend them or you'll introduce gauge fitment problems! 

Excerpt from the Application Notes in "Functional Tests of Factory GM Electric Gauges" previously linked:

"The instrument cluster gauges (excluding speedometers, clocks and tachometers) establish electrical connection with the flexible printed circuit board (PCB) through shiny metal spring clips (Fig. 8) that snap through the back of the instrument cluster housing.  The PCB and clips are notorious for accumulating dust and tarnish to the point of degrading electrical connections, while the spring clips tend to lose tension over time, degrading connections even further.  Hence, anytime the cluster is disassembled for gauge service or new gauges are installed, the clips should be burnished along with the PCB where the clips make contact.  But, be careful - the copper foil of the PCB can peel easily from its plastic backing.  A soft pencil eraser or crocus cloth works well for burnishing the copper foil.  Once the spring clips have been removed from the housing, they can be gently reformed to increase contact pressure with both the PCB and gauge pins.  Furthermore, inspect and burnish the connections between the wiring harness plug and PCB."

[Rough75C20]

Perfect. Thank you for the info BD!