See my post in this thread for some useful parameters.
http://www.cnczone.com/forums/showthread.php?t=40774
Also, power machine down. Hold the following buttons P and CAN while powering maching back up. Hold them until machine is completely powered on. Move the axis to middle of travel then perform zero return. The P/Can tells the machine to ignore soft limits and allows it to home on limit switches.
Bob