MG Car Repair: fuel gauge, gauge needle, fuel gauge

Question
Filled up the car yesterday and fuel gauge shows the tank only half full. Is the gauge or the sending unit faulty? After driving approx. 75 miles the fuel gauge dropped slightly. This is on a 73 mgbgt. What do I do to repair the problem.
Thanks, Bill

Answer
Hi Bill,
If all the other gauges operate correctly, I would either drain the tank or run the car until it is almost empty and jack the right side of the car up high and remove the sending unit from the side of the tank and check to see if the arm is sticking. Then plug the hole in the tank with a rag or something so as to not ignite the fumes and connect the wires back on to the sending unit and manually operate the arm to see if it works the gauge. Also remove the power wire from the sending unit and see that the gauge needle deflects all the way in one direction and then contact the power wire to the ground wire to see if the needle deflects the other direction all the way. This is a rough test of the gauge. If all that don't net you an answer check the instrument stabilizer to see that it's output is about 10v.
Howard