Bridgeport with crusader i control

Hello im new here and just got my first cnc machine but i am having a problem with the z axis, when i power up the control and press the reset button the quill drive turns on and moves the quill down about 25-30mm and turns off the drives if i reset the drives again the same thing happens, but if i raise the quill until it touches the upper limit switch i can control both x and y axis manually and through a program without using the z axis. I have been told by a servo repair tech that it is probably got something to do with the scales or the wiring as i have also tried switching the drives with the same result. Any help would be appreciated.
Dave