Z-axis motor fail?

[ruppelgl]

I recently finished building a Prusa I3 Pro B (acrylic) and had everything working properly. Now one of the motors on the Z-axis is not turning in the correct direction. The other motor responds correctly to up and down commands from Repetier Host. However the second motor sometimes turns clockwise, and other times counter clockwise, often in the opposite direction of the motor that is functioning correctly. Sometimes it turns one direction and then reverses during the same move. This happens whether the threaded rods are connected or not, with or without the connectors. I tried switching the motors on the board (to see if it might be a firmware problem) but the same motor continues to malfunction. The Z endstop is correctly attached and functioning correctly. I have only had the printer operational for about 1 week so there is very little time on the Z-axis motors. Is this a defective motor?

[Mark]

Seems like a defective motor, if you plug this motor to the X axis and it still behaves like this, then it might be dead. And have you checked the motor cable? Sometimes a dead cable will cause this problem too. you can swap it with the other cables to check it.

[ruppelgl]

I tried the tests you suggested. When the motor is plugged into the X-axis cable it seems to perform correctly, but when the cable is plugged into the X-axis motor, the X-carriage does not move. This suggests that it is the cable. I removed the cable to check for continuity; the red, green, and black wires show continuity but the blue wire does not. There are no obvious breaks at either connector.

[Mark]

the cable is dead, you need to contact the sales to get a new one, or just use some dupont cables to replace it, it will work too.

[ruppelgl]

I tried connecting the motor using Dupont cable which I had and the motor vibrates but the shaft does not turn (the other Z-axis motor responds normally). It seems that the motor may be dead as well.