How do you access and replace the fuel sensor in a Volvo 1990 740 GL?

The fuel sensor (which I'm assuming is the sender for your fuel gauge???) is an in-tank sensor on the float valve unit. To access the unit, you'll need to locate the large nut mounted to the top of your fuel tank (6" dia approx.). On our wagon/estate 740GL, this is located under the carpet in the cargo area, driver's side. Fold rear seats forward, remove carpeted panels. You will find a black metal, square cover plate held on w/ 4 screws. Remove it and you'll find the Fuel tank access. The fuel float, pump, sensor are all on the same assembly, but all pieces are replaceable. The float valve is quite expensive, but if removed and work carefully, will still function after pump or sensor are replaced. We found that our float was bad upon reinstallation and wouldn't register below 1/4 tank. It was not a problem with the sensor, but a stuck float valve. Hope this helps.