Tank Unit Sender
Tank Sender Unit (1940) -- If by now you have concluded that your tank sender is intermittent, why not rebuild it yourself? I did, and it works just fine.
Siphon gas from tank or run it dry, rinse with water and drain again, especially if you have to use heat on the strap nuts in the trunk well. Use every precaution that common sense dictates when working around gasoline. The tank drain pipe plug is rotted in place and will tear the tank if it is torque too much. (Don't use heat on this item, please!)
When you see the problem, I'm sure a number of solutions will come to mind. Make sure that the strap will not short to the case and replace the cover, using a few small machine screws and nuts (#4 or #2, I forget which size I used). Use some RTV or Permatex to seal the cover. If you find the slide wire burned, it is because someone over the years jumped the battery wire to the tank wire on the gas gauge. Most of the time the strap will be broken. (Clean the slide wire with alcohol before reassemble.)
Buick Bugle, June, 1988, James Cox, #12125.