Hot end thermistor only reads when RAMPs 1.4 is powered off

My DiVinci 1.0 running RAMPs 1.4 started to behave weird lately. If the printer is off (Ramps not getting 12v) but the Mega is being powered via USB (5v only), the thermistor reads perfectly fine. But when the printer is powered on, it instantly stops reading.

It's a generic 100k Thermistor with option 5 in Marlin.

Put on another one, same reading. Even when it's dangling outside the hotend, no shorts.
I've also swapped it to T2/pin 15 (default is T0/13) on Ramps/Marlin, same thing.

Was working fine for the past 3 years till today. I did had a plug on the thermistor wires and experience loose connection before, could it somehow fried the analogue reading on Ramps? Are both pin 13 and 15 sharing hardware? Bed thermistor (T1/pin 14) is working perfectly fine by the way.



Will I have to replace the Arduino or Ramps?

Anyone experience this before?

Edited 1 time(s). Last edit at 06/24/2018 01:34PM by chilicoke.

Could be a few things

1) the voltage regulator on the mega is doing something strange, pulling 5v to a weird voltage (12v is converted to 5v on mega)
2) a short on your hotend, when 12v is applied its causing this (unplug hot end heater and E from the ramps and turn on with 12v, does it still go to 0 degrees ?)

I've fix the problem, and you're right that I did accidentally short 12v to hotend thermistor after pulling all the wiring. The Mega was still behaving weird after fixing the wiring so I replaced it with a new one, all is good now. Ramps was fine and didn't need replacing.

Thanks!