the 57 chassis manual, which should theoretically be very similar, is here:
https://www.teambuick.com/reference/...ssis/index.php
57 is the first year of the 364ci though, so there may have been a major redesign from 56 when they were still using the 322ci.
unfortunately, the closest thing we have in the other direction is the 52 Service manual, which is prior to the introduction of the Nailhead and still used a straight 8. that manual is here:
https://www.teambuick.com/reference/...shop/index.php
according to the 57 diag page, a short across the fuel level sender should read empty and an open circuit should read full ... so it would appear that the problem is within the sending unit itself, not with the circuit to the dash.
this could be mechanical or electrical in origin.
the first thing i would check would be that the tank float actually moves up and down ( ie, isn't sunk at the bottom of the tank and never moves ) AND is properly coupled to the potentiometer.
once you've verified that the mechanicals are working properly, put a multimeter across the sending unit. obviously, minimum resistance ( close to zero ) should show when the float is at the bottom and resistance should rise fairly smoothly as the float moves to the top of the tank.
i would suggest getting a can of electrical contact cleaner if the pot gives jumpy readings as the tank fills ... but if it's showing shorted regardless of the position of the wiper you'll probably have to get a new rheostat on there. sketchy resistance values are typically due to corrosion or grime building up between the wiper and the carbon resistor element which means that R goes *up* the more the pot deteriorates.
https://en.wikipedia.org/wiki/Potentiometer
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