Dear Tom and Ray:

I have a 1988 Lincoln Town Car. When I have about five to seven gallons of gas left in the tank, my gauge goes past "Full." When I have about three gallons left, it returns to a normal reading. One shop said it was the gauge. Another said it was the float in the tank. Which one do you think is right?
Harry

TOM: I'd go with the guys who said it's the float in the tank, Harry.

RAY: Me, too. The float is what sends the signal to the gauge. When the float is at the top (i.e the tank is full) the circuit is fully "open." As the gasoline gets used up, the float moves down, and the resistance in the circuit is increased. As the resistance increases, the needle on the gauge moves towards the "Empty" mark.

TOM: But in your case, there's a part of the resister (when you have between seven and three gallons of gas) that's bad. And it's not creating any resistance there. That leaves the circuit open, which is why the gas gauge reads "Full."

RAY: If the gauge itself were faulty, it probably wouldn't happen so predictably. The fact that it's tied to a precise amount of fuel in the tank makes the float a heavy favorite in my book.

TOM: So you can either replace the resister, or just make the most of your unique situation. If it were my car, I wouldn't miss the opportunity to lend it to my brother everytime it had about four gallons of fuel left, and then tell him to "fill it back up" before he returned it to me.