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Hello, does the gauge bounce up and down from full to empty or vice versa? Or i is mostly full and it drops down to empty?
Ok most common thing we see on these older style fuel senders is the orignal sender is a metal barrel that hangs on the sender arm in the fuel tank. Over time the hanger wears into the hollow metal float barrel and fuel leaks into the sender float and will make it sink. You can unplug the fuel pump connector and ground the sender wire and it should read full. and disconnected empty.
If it doesnt read full when you ground the sender wire you have an open circuit between the fuel sender connector and the instrument cluster or gauge issue. If it reads full the sender should be working properly, if it pulsed up and down it would be an issue with the instrument cluster voltage regulator.
Ok the wire from the fuel pump should be yellow for the gauge. If its always reading full the wire or sender is shorted. disconnect the fuel pump 4 pin connector it should drop to empty. if it does the sender cricuit has a short to ground possibly the sender.
With the pump removed you can move the float up and down and it should sweep. the gauge
Napa offers a good line of fuel pumps at a decent price. Just need to make sure they come complete with a new sender, some do not