My (factory) fuel gauge was very inaccurate (seemed to hover in the 1/4 to 1/3 range) all of last season. I ordered a new VDO sending unit per Formula's part #. When I installed the new unit it was identical to the one I pulled out - both have a single male spade that connects to a signal wire. As soon as I keyed on the ignition with the new sending unit installed, the needle on the gauge did not move. I then ground the signal wire and the gauge peaked at F. Thinking I may have gotten a dud out of the box, I reinstalled my original sending unit and now have the same result. Zero reading on the gauge when powered on.

Any ideas?

Did you check ground to sender was good

Unless VDO stuff is backwards every gauge i've dealt with pegs the gauge to "E" when grounded.........If there's only one wire going to the sending unit from the gauge and you don't have a ground wire attached to the sending unit....then the tank itself would need to be grounded somehow.... and the screws holding the sending unit should make the contact to ground for the sending unit.
So if you cant see a ground wire anywhere near the sending unit, i would take the sending unit wire off (coming from the gauge) and touch the tank....if it doesn't peg to "E" (or "F" it seems like in this case with the VDO) then the tank is not grounded....if it does, then either the sending unit is not grounded or its bad. And there could be a remote possibility that the spade connector has corrosion and is making intermittent contact too...

I guess they do run backwards....just found this on google:

The fuel sender should show between 10 and 180 Ohms, depending on how much fuel is in the tank. It should register 10 Ohms on a full tank and 180 Ohms on a empty tank. If your fuel level falls between full and empty, the Ohm output should register between 10 and 180 Ohms.

That's why one needs to be careful with the VDO's, as it sounds the OP was.

Wally. Good find, what is the fix for the issue? Do you just change the leads?

I grounded the sending unit wire to one of the sending unit mounting screws and that is what pegged it “F”.

Might just be the gauge
Remove the sender and reconnect the wires. Move the float thru its range to see if gauge works

I was working on a boat last night non vdo gauges . Because of this thread I i grounded sender wire and it pegged full. Boat had less than 1/4 tank

Did you hook up multimeter (set to ohms) to new sender and read ohms from empty to full moving the arm slowly ?