Z-axis problem: stepper reverse for 1 step [solved]

I have a rather strange problem has arisen with my z-axis: One of the NEMA17 steppers sometimes reverse its direction.
When I singlestep from pronterface on the Z-axis, one steppermotor suddenly steps 1 step back, and then resumes stepping forward.
I have single microswitch endstops at Xmax, Ymin and Zmin. I can home X & Y axis, but Z axis is a problem, as the stepper cannot decide whitch way it wants to step :-(

I will try to switch the connectors to the stepperengines to see if the error moves to the other stepper.

Any other ideas ?

(never buy a chinese kit - plexi is too thin: 4.8 mm)

/Klaus
Denmark

Update: I found the error. A bad connetion to a coil. I re-crimped the connector. Problem solved.

Edited 1 time(s). Last edit at 07/21/2015 02:20AM by klausbfrederiksen.

You wired the z-steppers in parallel or series?
I don´t know if it helps against reverse stepping, but wiring in series would connect the coils to act as one.
Maybe both would eventually run reverse then?
-Olaf