OK, I have looked at the Wilcox videos on troubleshooting C3 fuel gauges.
The one issue that is a little fuzzy, back at the sending unit., I should be able to confirm sender operation. If I I hook my ohm meter to the power ( center stud) and the ground spade connection, I would think as I run the float up n down, the resistance should vary from 0 to 90 ohms. Mine is zero through the entire float range telling me the sending unit has failed (rheostat has an open)
Any experience to help here!

Bob

 

Pull sending unit. Look for a broken wire on the inside. I've seen senders go bad but zero all the way? Sometimes those copper strip's just get corroded and you can clean them up and it works again.
pull that sucker out and look it over before you spend money.

 

It is possible to clean the contact surfaces if you use a #2 Pencil's Eraser and carefully erase the corrosion on the surfaces. Another easy way is to use DeOxIt contact cleaner as it removes all the corrosion very quickly and easily.

I was able to get my fuel level sensor working and simply bent the float mechanism to get it full at "Full". Now the gauge works great and lets me have a more accurate fuel level amount inside the fuel tank.

Getting a good solid ground is also very important on these older Corvettes. For this type of work I love my "Power Probe" electrical testing tool. I can apply either 12 volts or ground at the tip simply by using a roller switch and this allows testing of the motors and other components in the electrical system. A very handy tool when dealing with old Corvette electronics and weak grounds.

 

Using the ohm meter, it appears that the rheostat in the sending unit is "OPEN" (broken wire). Seeing as how, no matter what, I have to drain the tank and pull the sender, is the best path of remedy, to try n repair a 54 year old unit or purchase a replacement.
Are you saying that the repaired original (if repair is possible) unit would be better than a replacement one?

Bob

 

Absolutely, hands down a rebuilt original would be leaps and bounds better than a Chinese repro.
Your call, but a original, gen. GM part has long been discontinued.
I rarely agree with Mr. McCloskey, however the eraser method of cleaning the copper strip's of the rheostat and the use of deoxit are my go to methods.

 

Update: So, back at the sending unit (still in the tank) I took the center sensor lead and grounded it. Gauge goes to empty. OK, the wiring and the gauge are just fine.
Next, drain the tank and remove the sending unit. With the unit out on the bench, I connect my Ohm meter between the center sensor lead and ground - Dam, the meter goes 0 to 90 ohms. What the heck? Not leaving well enough alone (and not wanting to drain a tank to remove the sender again) let's clean everything up. I loosen the metal housing (three bent tabs) and look inside at the sliding contact and the rheostat windings. There is rust on the metal housing at the far end of the resistor windings. By the bent clip that holds it to the housing. Figure that is a possible ground to the rheostat. Also, the little insulator strip under the rheostat is not centered under the windings and looks like the rust is underneath. Time to be gentle and see if I can clean that up without breaking the rheostat windings.
Reassembled the rheostat housing and put the ohm meter back on the contacts, Viola', sender is now operational. Just need to reinstall and make sure everything is working at the gauge. It should.
Appreciate everyone's help. Being a caretaker on one of these old machines is an exercise in patience. I have been looking after this machine since 1973. Just want to get it roadworthy again and enjoy it for a few more years.

 

Cool! And a whole lot more satisfying repairing than replacing!

 

Bob
Glad you got it worked out...and it does require patience. As a reference this article describes the steps you encountered along the way.

Restoring your C3 fuel sending unit