If the 90 sender is like older ones there is a bobber (float that works like a fishing bobber) that slides up and down while contacting two thin wires. That tells the gauge were the level is. Near the bottom of travel the low fuel lite is activated by the float touching another contact. These contacts need use to stay clean. They get a film or coating or maybe its an oxide reaction with the gas, don't know. Maybe even a coating wears through on the lower areas of the wires and the lite contact. My wife never lets her gas get below 1/4. I've got a 67 250SL that isn't used much and when I get the level low it gets irratic response on the gauge. The sender is easily accessible on many benz's throught the trunk floor and a removeable access hole. The sender comes out simply. If you have some time you can take it out and measure the change as the bobber goes up and down. getting the bobber to slide up and down enough should clean the contacts and it should take a few months or years if you are lucky, to get coated again. It took 15 years to get this way.