So...a weird thing happened to me the other day, I was printing a new z axis endstop support (my endstops are the ones mounted in a pcb board, the ones with three wires.), everything is plugged in normally, then, when homing the z axis, and as soon as the endstop clicked, everything shuts down, and the printer disconnected from repetier. My first thought was that I probably plugged the wrong z endstop wires in the wrong pins, but everything was connected correctly.
Tried restarting the printer... when I plug in the ramps to the pc trough the usb port, everything lights up and works as normal, but as soon as I turn on the 12v power supply, the lcd screen kinda powers down, as if it doesn´t get enough juice, and the nema 17 motors don´t work. The funny thing is that when disconnecting the lcd screen from the ramps, everything works as usual, the motors move, the printer communicates with repetier, etc.
Reading around the forums I saw that a probable culprit could be the D1 diode, but it looks normal and it has continuity checked with a multimeter (At least I think I did correctly), other than that, I´m very limited in finding the problem due to my poor understanding of electronics.
BTW, I have a ramps 1.4 with an arduino mega 2560, an atx power supply with 18 amps on 12v.

Any help is more than welcome!!
Thanks a lot!!

Are you sure that you have not plugged in the endstop connector the wrong way? Since this would cause the end switch to make a short between GND and 5V, shutting down immediately the electronics and often also destroying the 5V regulator on the Mega board. The latter could explain why it now only works with the USB connected (which brings the 5V directly).

Yes! Absolutely sure. As a matter of fact I recall leaving the z endstop wires connected to the ramps board, and only unplugging the molex connector that goes to the endstop pcb board. I´ll attach a couple of images of the endstop. However, since my symptoms match what you are describing, is there anyway to test the 5v regulator on the arduino and finding out if indeed it was a miswired endstop?

Thanks a lot for your help!!

Unplug the ramps from the mega

Upload flash sketch, check its working

remove usb and plug in a plug pack, this goes threw the voltage regulator, check flash is still running.

If you have a multimeter also actually check the voltage like they do here https://www.youtube.com/watch?v=gMCcKfRmm8A

You can also remove the diode and do as this video suggests if you have a 5v source, or replace the voltage regulator or get a new mega