After narrowing down the possible culprits, I remembered that my GPW fuel sender had similar problems. Grounds were good, but the resistance of the sender never changed when manually actuating the float.
The fix is simple. Drill out the hollow rivets holding the bottom plate onto the top. You can try to save the gasket, or not, permatex makes a good gasket in a tube (or you can old school it and use a hammer and the bottom plate as a template.
Check the resistance of the slider wire... not sure what correct is (it was 0200 when I did this) but infinite OHMS would probably be bad. There was also a lot of calcium (or whatever that white stuff is that water leaves behind when it evaporates).. any tube and tile cleaner will clean that right up. Just use a brush on the resistor wires... I don't know how the phenolic will take chemicals.
The culprit seems to be that dirt/calcium gets in the shaft bushing and creates a 'dirt bearing' that breaks the ground path for the fuel gauge. Clean that out and you have a good ground remember the variable resistor is grounded through the wiper, which is grounded through the sender body, which is grounded through the tank, which is grounded to the frame by fuel lines. Since the top/bottom halves of the sender are separated with a gasket all you have is a are a few rivets and screws for ground... but that's only AFTER the wiper is grounded via the float shaft through a bushing.
Cleaned it all up, hit the bottom with some permatex and screwed it back onto the tank... got a 1/4 full reading (I dipped the tank with a stick and thats about right).
I don't know if this is an original sender unit... but once cleaned out internally it seemed to work.
