Z limit fails in the down position

I have Taz 6 that I have been using pretty hard for at least three years. I have replaced one stepper motor (Y) and swapped the X and Y steppers due to some print failures but otherwise the unit has been pretty solid.

Today the lower Z limit switch function failed, it just keeps driving down until I switch the power off. I did trace the wires back to the printed circuit board on the unit and checked the switch function…the switch is definitely closing when depressed.

I did try running the unit all the way to the top and the top Z limit switch does work.

What is my next step?

I assume Z only drives down and you can’t make it go up. I had a similar issue but in the opposite direction. The Z axis driver chip on the circuit board failed. Ended up replacing the Rambo board. I think they have a design problem with the driver chip they are using. Looking at the datasheet it appears to me that chip was intended to drive only one stepper motor. They are driving two on Z axis. I think the chip overheated and shorted. On top of that, earlier versions of the board had a little heat sink glued to the top of the chip. The chip overheated and the glue let loose. The heat sink dropped down and shorted out on one of the 24 volt traces. Damaged the board so I had no option other than replace it.

This is a picture of the diver chip that failed and the heat sink that came off.

The Z axis drive works in both directions and the upper limit does stop the drive as it should.


I did connect the printer to my computer and ran the diagnostics as suggested by the Lulzbot customer service team; all of the limit switches seem to be working (state changes from “Open” to “Triggered” when the switches are activated.

I uploaded the latest Cura updates and now it is working normally again…I am happy that it is working but I am confused as to why it had failed previously.