I want to add that sender problems are sometimes due to bad contacts within the sender even when NEW. You can fix them by modifying the sender.
I had a series of brand new sender units that didn't work. There are two problems that can occur together. The connection from the resistor assembly (specifically the ribbon-shaped connector) to the outside pin was poor/nonexistent. The push-on connector is supposed to maintain contact (continuity) between the internal metal ribbon, and the "inside" end of the pin (which connects to the gauge wire on the outside). Neither the ribbon, push-on clip, or pin should touch the tank metal, or the sender body.
So the problems are:
1. The ribbon-connector may be rotated such that it touches the top of the tank causing grounding AFTER installation.
2. The push-on connection is open or opens up due to problem 1.
The photos below show how to modify the inside-to-outside pin, making a more stable junction to the ribbon connector. Pictures A & B show the push-on clip connection, and how to rotate the ribbon connector away from the edge of the sender, to avoid hitting the tank after installation. In picture C, I removed the push-on clip by grinding off a few "tangs" with the rotary tool. Then pull the pin out, and clamp the "outside" end in a vice. Put 6NB-32 threads on the "inside" (gasoline-side) portion of the pin. The last picture (D) shows the pin re-installed using a brass nut in place of the push-on clip. The resistance after modification was 0.9 Ohms at empty, and 89.1 Ohms at full.