Gas Gauge Issues, '76 Eldorado

[joeinbcs]

Howdy,
I was about to replace the fuel tank sending unit on my car....'76 Eldorado convertible, 36K miles.
It sat a lot in the last 20 years, and from reading the posts here, I concluded that the sending unit must be bad.
Then, while working on an unrelated issue, I noticed that the fuel gauge reads correctly....but only after I've had the ignition on, then turned it off, in that brief time before the Twilight Sentinel turns the lights off.
Now I'm wondering if the fuel tank sending unit isn't the problem, but some other electrical issue.
Anybody else experienced something similar?
Thanks, Joe

[TJ Hopland]

Did you try disconnecting and grounding the electrical connection for the sender that I believe is back behind the license plate?   That will tell you if the gauge is working.

[joeinbcs]

Yes, I cleaned the connection and made sure it was a good ground, but had no effect.
The usual symptom is the gauge goes to beyond full when I turn the ignition on, and beyone empty when I turn it off...except when the Twilight Sentinel has kept the lights on when the ignition is in the off position.  Then it reads accurately..I believe...

[TJ Hopland]

If you ground the wire does the gauge reading change?

[joeinbcs]

If you ground the wire does the gauge reading change?

I just removed the rear ground wire, and there was no difference.  When the ignition is in the off position, the gauge is all the way past empty.  When I turn the ignition to the on position, it goes all the way to beyond full and stays there.  When I turn the ignition off, the gauge goes to half full (which I believe is the true indication of how much gas is in the tank), but then, after the Twilight Sentinel turns the lights off, it goes to beyond empty.

[robert G. smits]

Do you have any information from the PO if the gas gauge worked during his ownership. 

[joeinbcs]

Do you have any information from the PO if the gas gauge worked during his ownership.

Yes, I'm friendly with the PO, and he actually came to see the car and have lunch recently.
When I got the car, the gauge didn't seem to work.  When I asked him about it he told me that he recalled it working sporadically.  It has only seemed to indicate the correct amount of gas in the tank when I've had the ignition on (and the lights), turned the ignition off (then it reads correctly for a few seconds before the Twilight Sentinel turns the lights off, then the gauge needle goes back beyond empty.