No more movement or heat

I've been using the same RAMP V1.4 board for many years now, but yesterday it stopped in a middle of a print and decided to never move again, even after several power cycles on everything. My setup is this: Repetier Host -> Networked Raspberry Pi 3B+ (Repetier Server) -> USB UART -> Arduino Mega 2560 -> RAMPS V1.4.

Here is what is happening: When I click the print button in Repetier Host, it usually moves the nozzle up in the Z direction a little bit while heating up the hot end. Now, I hit print and it makes no movement and never heats up, never progressing through the print. I can see in Repetier host that it has set a target temperature for the hot end, but the temperature sensor stays at room temperature (it looks like a valid reading of the temperature sensor, moving a little bit due to noise, but staying near room temperature).

I initially suspected that it was something related to Repetier Host/Server, so I grabbed a new raspberry pi and loaded it up with OctoPi, and when I tried to print, it did exactly the same thing: No Z movement, no heating up, but temperature sensor looked good. So I think that eliminates the issue being the raspberry pi and up.

I then removed the Arduino/RAMPS stack and removed the RAMPS shield, and with the USB UART connected to the Arduino, I can see that the Marlin firmware is functional, and I can send and receive commands.

My guess is that the high power stuff isn't getting 12V for some reason, due to the fact that both things are not happening: movement nor heating. As far as I can tell, I see no visual signs of electrical issues on the board: No corrosion, no burnt parts, no popped ICs.

Other things I've checked: Power supply is producing 12V, and I see a blinking LED on the RAMPS board during power up, and the Arduino has blinking LEDs as well.

I'm wondering what I should do next to help isolate the problem. Any suggestions? I'm pretty handy with a multimeter and oscope.

Edited 2 time(s). Last edit at 10/16/2022 10:55AM by rdb9878.

The big yellow square things on the ramps are polyfuses the smaller of the two is hotend heaters and stepper the larger one is for the heated bed only.

Poly fuses are meant to self heel when they cool down, but they can just die.

Check for 12v on the 12v aux pins, no 12v, its a dead polyfuse

This plug the fan is on is 12v aux


Edited 1 time(s). Last edit at 10/16/2022 12:28PM by Dust.

I just measured the 12V-AUX pins as you show, and I did measure 12V. So that means the poly fuse is still good?

I measured with no loads (just 12V going in). Should I also repeat test with everything connected and trying to print?

Edited 1 time(s). Last edit at 10/16/2022 05:33PM by rdb9878.

Ended up buying a new RAMPS 1.4 and the problems went away. Even used the same stepper driver daughtercards, so it probably was a bad polyfuse.