1989 ford f250 7.3 diesel: fender mounted solenoid clicks but truck won't turn over?

hi, i have a 89 f250 with a 7.3 idi, when i turn the key,the fender mounted solenoid will click but nothing will happen.i tapped on the starter with a hammer, and still clicks! i have a starter/battery tester, and both batteries were ok and the starter was ok,