Unplug tank sensor ,

There are 3 terminals........G/W/31

With key ON, jumper W to 31....Reserve lamp should come ON.
If NO, then check fuse #5 .. [ spin it in the holder ]

Jumper G to 31..gauge should show full.

Jumper G to 31 , using a 50 ohm resistor...should show low fuel level.

If yes on all test , sender is the problem
If NO, then check that 31 is a good ground..........simply repeat the test using a jumper to a known good ground for terminal 31

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A Dalton