I’m not quite sure what is going on. I’ve tried between .300 and .525 volts on the z motor. If I lift on the z arm taking the weight off, it seems like it wants to spin, but basically just kicks left and right. If i let it rest and try again, not much happens. I took of the Z assembly from the motor and verified all connections from the 4 wire connector on the board to the motor, they check out. On lower power ~.2-.3v it seems to turn fine, but the direction it does so is random. Pretty much 50/50, but not alternatinating 100%. Any weight on it makes it jumpy. Higher voltages to .4,.5 result in less working and more jumpy behavior. I’m not sure what is going on and could use some assistance to fix this.
Try swapping stepper drivers and/or motors to locate cause of troubles.
Try 0.55 and you may want to check the wiring as well.
I wired another stepper ( not from the kit ) that has a 1x4 connector straight to the stepper drivers header. It worked as one would expect.
I wired other motors ( x and y ) to the z driver and they still functioned.
I questioned if it was possible that my ribbon cable was somehow miswired, etc. I cut the existing ribbon connections to the 4 wire connector, and soldered 4 pieces of a decent gauge hookup wire to each wire. The other end was directly soldered to one of the motor wires.
Motor => Driver
Blue => Yellow
Red => Orange
Green => Red
Black => Brown
The result was the motor fluttering/vibrating as before. I have taken the voltage up to .55 and although the behavior and intensity changes slightly with the variations in voltage, it all results in a stepper that isn’t spinning correctly.
If you are positive that the stepper is a the cause of troubles, please return it for inspection/exchange.
You can return it to your distributor or direct to:
Velleman Projects Tech. dept.
Legen Heirweg 33
9890 Gavere
Belgium
US residents: please call (817) 284 77 85 for RMA#
I don’t think that is the problem. Here is how one person dealt with it, from an article in Australia’s Silicon Chip magazine by Alan Ford, “I did have problems here with my Z motor stalling with a buzz when the print head was still some millimetres from the print bed — probably caused by the very slight eccentricity of my threaded rod/motor adaptor — despite resetting the Z motor potentiometer to 0.55V (the permitted upper limit explained in just before 002/031). I got around the problem by re-positioning the print head arm assembly down a few millimetres in the frame so that the print head was closer to the print bed while the Z nut was not quite so far down on the threaded rod.”
Have you solved this problem? I have the same issue.
Which problem do you mean? Stalling Z motor?
Did you test the motor itself?
You know the trick with the led on the motor wires?
Connect a led to 2 the motor cables that you know should pair and turn the motor by hand.
If the led lights, you have a working pair.
If not, you don’t have a pair, or the coil of the motor is bad.
I had my stepper motor behave as you describe after assembly and found out it was caused by a faulty line in the ribbon cable.
One coil of the motor wasn’t powered correctly.
I exchanged the wiring and then it worked.
ichbinsnur, thank you for that tip. I´ll try that.
My Z motor is behaving random, once it go up when i press the up button and the next time it will go down for example.
It seems like it was a bad soldering or the ribboncable. Now it works! 
Nice to hear!
New motor installed, it was working before put together. Took a video. Z is @ .51v in this photo.