problem fixed
I followed Peggie's advice and removedthe sender. The sender is a perforated cylinder, about 2" in diameter that hasan internal, rod running up and downits center. There is a slideron this rod that must connectto a transducer. Somehow the slidergot stuck. I freed it by sloshing thesender in a bucket of clean water.Of course, before I did any of thisI had the tank pumped and then Iflushed it about three timeswith clean water. Nevertheless,this job ranks near the bottomof things I want to do to maintainmy boat. The bad news is thatI expect the slider will getstuck again and I will have to repeatthis task.