The fuel gauge was dead when I bought my 1979 300CD, and I dumped some diesel purge into the tank, and that fixed the problem. You might want to try this, and if it doesn't work after a few tanks it might be the sending unit. You might just need to pull it out and clean it up, and if that doesn't work then you might have a bad sending unit. You might also have a bad ground connection for the gauge, which is causing it to not work.