It should be easy to access the sensor once you remove the masonite / hardboard tank covering that forms the floor of the trunk. As I recall the resistance range on the sender on my 1971 140 is 70 ohms (empty) to 10 ohms full. As noted by grumpydad, pulling the wire off the sender should cause the gauge to read empty. If it does, then the sender is likely faulty. You can confirm that the gauge works by connecting a 68 ohm resistor beteen the sender wire and ground (should read empty) and then a 10 ohm resistor between the sender wire and ground (should read full). If you need a new sender be sure to get he one used for you model year. Volvo used at least 3 different senders on the 140 and they do not interchange.
If the gauge appears to be faulty, do as burdekin advises and check the temperature gauge. A flakey fuel gauge and temperature gauge points towards a bad voltage stabilizer. However, it would be unusual / difficult for a stabilizer to fail in a mode where it gives a continuous 1/2 tank reading.
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