I have a taz 4 and suddenly the z axis does not act right. It moves UP ok, but when it comes down it gets to a point where it stops and the steppers seem to hum. While this is happening I can turn the leadscrews by hand and if I manually trip the Z limit switch it stops. The perculiar thing is that when I try and home Z, no matter where Z is when the machine is when it is turned on, it will not go below 0.0, even when homing. Sounds like a limit switch issue, but the switch seems to be fine as it does this even with the z-limit switch unplugged.
Basically it seems like it is not allowing itself to go past 0.0, even when zeroing z-home.