Sounds like the power wire to the top of the gage (pink wire I think?) is having some issues. I believe if it was always full, the ground wire would be the problem. With the key on, check for voltage at the terminal going to the float/sender. I know it's a PITA. Also, check your 20 amp fuse for the gages. Is the fuel gage the ONLY gage to go wonky? Just thinking out loud.
I think there's a rubber connector somewhere around the tank on the pink wire you can disconnect to check for power back there. And then you can check the sender side for continuity on the ohm setting.
If you're getting power to the fuel not getting power to the sending unit from the pink wire, follow it back and check for breaks, damaged insulation, etc.
I'm away from my CSM/ESMs so I can't verify. Someone correct me if I'm fubar'd. Wires aren't my strong suit.
I can't speak for everyone here, but when I do a sender replacement, I always test the new sender by plugging it up to the hot wire, then grounding it, turning the key on and moving the float manually. You can also do a resistance check on the sender to see if it's correct.
My gauge in my Regal was corroded from mouse pee and stuck on half a tank, damn near drove myself crazy before replacing the gauge. Lol. Yours probably isn't that.
The first thing I always do is unplug the pink wire and see if the gauge sweeps up above full. If it does it could be the sender or a ground problem, if it doesn't change the wire might be grounding or no power to the gauge.
Check the ground wire attached to the tank, and try putting it where the bolt and area isn’t as rusted.
If that doesn’t work, it’s likely a wiring harness issue, get a multi meter and check continuity from the gauge connector.
A personal tip I learned from rewiring my 1984 Pontiac Bonneville was you can cut the sender unit wire from the black plastic gauge connector (in my case it was a tan colour), wrap a longer piece of wire around, then hold that jumper to the back of the gauge while the car is running to confirm whether or not it moves.
If it works, you know it’s not a sender or wiring harness issue. If it doesn’t, then you know it is.
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