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Unfortunately, to replace the sending unit on a 77 the tank will need to be dropped.
Before dropping the tank here is a good troubleshooting document from Willcox Corvette that covers 77-82.
LINK: http://repairs.willcoxcorvette.com/w...it-testing.pdf
I have a fuel level dip stick made up for my 77 so it will give you an idea where your ohms reading should be in relationship with YOUR fuel level.
I will post a photo of that later this morning.
If you are having fuel gauge reading problems, test the sending unit by itself to prove that replacement is required, before going to the trouble of dropping the fuel tank. Use the approach mentioned above by bmotojoe to manipulate the sending unit float while watching an ohmmeter connected directly to the sending unit output lug (with sender wire removed) and chassis ground. If it shows reasonably smooth resistance change as you raise/lower the float and reading ranges from 0 to about 90 ohms, the sending unit is fine.
P.S. If you find that the the connections for sender wire and ground wire were in questionable condition, that might just be your problem.
I completely forgot...
I did these calculations when I repaired my LOW Fuel warning light module.
You will have to calculate your level in ohms. 90 being full
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