printhead not moving down

Hello everyone, our Mini suddenly stopped moving the printhead down towards the printhead. Inspection of the terminal commands revealed that the software thinks the printhead reached an endstop even though the printhead was clearly not close to either end of the rod. It is possible to manually move up the printhead but not down; horizontal movement is fine as well. I checked the vertical endstop switches and they seem to be okay. I would be very thankful for any hints where to look next.

I am not home or by my Mini, but has this all worked of be fore? If
the machine has not been successfully Homed It might not respond to commands to move from control software. I hope that is what you mean by manually moving the head. Not moving with your hands…

Try homing XY then Z.

Yes, it has been working beautifully for several months. Also, I was referring to moving the printhead by software only. Unfortunately, homing XY and then Z did not solve the problem. But thanks for the suggestion.

Am I correct in assuming it moves on the X axis ok? The Z axis Homing switch is at the top left side of the gantry, when it Homes it goes to the top and hits the frame… If anything is in the way of that movement to depress that switch then maybe that is the problem? Or that switch has a loose connector someplace or even a bad switch. Simple things to check before you call Support.

You may have the thermistor on the extruder shorting to the heat block. You would need to measure from the hot end block to both wires of the thermistor at the connector under the top cover. But I would suggest you contact support and see if they can help you.

If the thermistor wires are shorted or open you will not get a temperature reading that makes sense. The thermistor is just a (simplified) resistor that is more sensitive to temperature shorting the leads together or even an open would not any problems in the movement of the stepper motors or the drivers.

Actually one of them can be shorted to the heat block which connects to the Z axis inputs for the Z min sense. So it causes a false Z Minimum detection. Thus it thinks the hot end is hitting a bed washer. But this condition will not give a temp error but a Z movement sense error.

But the printer is not in the pre-print wiping and touch the washer g code mode so how would that stop the head from moving down in the manual control? Having one of the thermistor leads grounded I believe would cause a error in the read out. I think the end stop switch is the problem, perhaps the plunger is stuck in? The OP said he believes the machine thinks it has hit the switch.

It will cause the controller to think it has hit/touched one of the plate washers. Horizontal motion will work, but only upward vertical motion is allowed. It thinks it is already hitting the plate, and the control board will not let it drive the nozzle (down) into the plate. The Z-Min on the nozzle is a input so if one side of the thermistor touches the block, it thinks the nozzle is touching one of the washers on the bed. It does not cause the temp. reading to fail as the Z-Min is a input to the control board. It is not shorting the thermistor to anything but another input line, it would probably only cause a temp failure if the nozzle actually was touching a washer. Which would short the thermistor then to whatever potential state the bed is(Which I expect is ground). The (Z-Min switch) is the contact of the nozzle tip to the washer, which is a switch closed condition.

I have seen several hot ends with the no downward Z motion due to this short.

I can’t imagine that hitting a washer is required for a Z Min limit? Regardless I just installed a new print bed on the Mini I am selling and used my Fluke to measure the resistance between the print head and washers on the bed. I got about 5 Meg ohms between the two points. The machine works fine and auto levels as it should.
But you must work for Lulzbot in some capacity and troubleshoot everyday. I hope the OP contacts support and gets a experienced tech like you.

The wire you see attached to the side opposite of the heater cartridge is the wire to the other input on the control board.

But no I do not work over at Lulzbot, but I do visit them sometimes. :sunglasses:

I have refurbished a number of printers and hot ends though. Keeps me entertained, and helps me buy fancy printer parts and filament to play with. :ugeek: