# Prusa i3 Z axis only goes down (even on up command)

I have just finished building a Tronxy P802M Prusa i3.

When I try to move the Z-axis, using the hardware buttons in the LCD menu (without a computer connected), it only goes down, when I both increase, and decrease, the value of Z.

Any suggestions?

• See similar issue at 3dprinting.stackexchange.com/questions/4090/… – markshancock May 30 '17 at 14:23
• Increasing the value of Z makes it go down as well? Does it make any unusual noise? I had to loosen the nut on the right lead screw, otherwise the right motor wouldn't turn when I increased the value for Z – Davide May 30 '17 at 16:26
• @markshancock - I thought that was similar too, except in this case it moves down for both up and down commands, which is odd. – Greenonline May 30 '17 at 16:49

It could be a hardware fault.

First check, and compare, the control board connections to the motors of the three axes. You don't state it in your question but, presumably, the X and Y axes move correctly? If so, then compare the connections for the Z-axis motors with the connections for the motors of the axes that work as expected.

If they are correct then the problem is likely to be with the firmware.

Have you...

• homed the Z-axis yet?
• installed the endstops?

Using Marlin? Before you do a G28 homing the axes will only move towards the endstops. But also check your endstops with M119 to make sure they are triggered at the right time. On older Marlin, you may need to set DISABLE_MAX_ENDSTOPS (on a machine that has no max endstops). Newer Marlin uses USE_XMIN_PLUG, etc., to specifically set which endstops are connected. If the switches show the opposite state (off when triggered) then set the [XYZ]_(MIN|MAX)_ENDSTOP_INVERTING flags, as needed.

Likewise, from Building a Prusa I3 3D Printer:

You will probably also find the motor will turn only in one direction. This is normal for now as we don't have end-stops installed and haven't homed the axis - so the software doesn't know how far it can go in one direction or the other.

As Mark states in his comment, the P802M uses a Melzi board. From Github: Repetier-Firmware/boards/Zonestar P802M/:

There are some printers sold under different names like 'Zonestar P802M', 'Prusa i3 P802M DIY kit', 'Anet A8-B', etc, which have LCD 20x4 with 5 keys controller connected to Melzi V2.0 board via 10 wires cable. Keys are connected to a single analog input using resistive divider.

• I believe the P802M ships with a Melzi board running Repetier. – markshancock May 30 '17 at 20:22
• @markshancock - yes, thanks. Just updated my answer with a description of the controller. – Greenonline May 31 '17 at 1:35

Thank you guys, it took me a while but it was a faulty cable, One stepper motor worked but the other did not. But, apparently, both have to work for even one to turn.

The cable had one wire isolation still in place, so there was no connection from the board to the stepper.

I also had another cable with the wires connected in the wrong order, as well.

Bottom line - it is working. And printing well.

Now it seems like I have problem with the z-sensor (stopped working after a few hours of printing). But I'll figure it out tomorrow.