fuel gauge

I,ve just had 1 bad day, bought a 90 couple of days ago (no warranty) Gauge is reading full and I run out of fuel. Replaced tank unit and gauge (no change). Any idea,s. Truck is 1991

wiring fault at a guess

iv got the same problem. haven't tried to fix it tho just carry a couple extra gallons of fuel in the landy and check the tank before i go out.

Likley to be an earth fault or chaffed wires unless repalcement sender also faulty

Sender in tank varies voltage to gauge as the fuel level drops

Full tank = 12v supplied to gauge which reads full. Voltage drops as fuel level decreases.

Can check sender by removing and seeing if voltage fluctuates with a multimeter.

Effectively your gauge is getting 12v all the time

Mine does it now and again and it is the float in the tank sticking up.
Usually a swift bang on the floor or thump on the tank releases the float to show actual fuel.
Also sometimes if there is enough fuel going over bumps makes the fuel slosh about and reset the float.