Taz 4 z axis issue

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.

Check for missing setscrews, broken z motor couplers (especially if you hear a clicking sound). If those are OK it may be mechanical binding, but that occurring suddenly Is very unusual. To check for that, loosen the x belt and loosen the x idler block on the x rods and try running it up and down and see if you get the same result. If that fixes it move it to yhe middle, re tighten the block and re tension the belt.

It could also be a failing motor

It moves up and down fine, no binding as I wheeled it down to the bed by hand. It moves evenly, never getting out of level. When I move it by the LCD readout knob it simply will not go below 0.0 it just stops there. When I try and home z, no matter if it is through pronterface or by the LCD readout knob, it gets to the before-mentioned 0.0 and that’s when the stopping/humming occurs.

0.0 should be z minimum. It should set that when you hit home. If it’s not, I have no idea why or how to fix that.

I’m thinking the board has gone goofy.

I swapped the board for my spare and it seems to be fixed.