DESPERATE - Heated bed heats but the temperature readings are way off

Hey RepRap guys!
I am a very sad guy with a problem i cannot solve my self.. Right now my only solution seems to be: Buy a new ramps board and arduino board maybe even a bed to simply just "start over".. But as a student.. my $$$$ is not limitless...

My printer:
Modified Prusa i3 Hephestos. A printer i love and is capable of very nayz prints (except for a stupid heat bed!!!).
It's running the stock bq ramps board and the heated bed is a mk3 alu bed.

Backstory of problem:
I got my printer a while ago and everything was working fine. Also the heated bed. At some point i accidentally(i was all new and wasn't even aware the solder pads were so exposed) put the metal paperclips that hold the glass plate in place across the solder terminals on the heated bed. After this my heated bed has been acting wired. My guess is i either shortcircuited something, or gave the thermistor input 12V. maybe frying something on either the ramps board or the arduino board..

Problem - Bed behavior:
When i start my printer the bed temperature usually shows between 22C and 24C (room temperature).
If i start a print with a normal PLA bed temperature around 50-60C following happens:
* The bed starts heating up to somewhere above 40C - i can feel it is warm to the touch.
* The temperature reading though still says around the start point 22C. Maaaaaybe rising to max 27C.
* The printer goes in failsafe and shuts down due to temp error(heating without reading increasing temp.)
* If i instead set the temp to 27C the plate heats to way more than 27C but i am sometimes able to start printing.

What i tried
I have tried the following things:
* Replaced the thermistor
* Meassured the resistance of the heated bed - roughly 1.6 Ohm.
* Tried to use T2 as a temp input - which failede misserably, had to completely reload the software - Guess I don't fully understand the pin.h file or something.. A step by step guide to do this would be cool!!
* Double, trible, and more checked the solderings and wirering.


I am now throwing the white towel.. I have googled, i have measured i have replaced sensor i tried to use different input on the Ramps board.. Nothing seems to work!
Please help !! - THANKS FOR ALL INPUTS!

Turn ON the the bed and measure the voltage starting from PSU to RAMPS to heatbed pins in RAMPS and all the way to the heated bed while it is warming up.
(Is the red led lit on the heat bed?)

PSU to RAMPS should be 12V
From RAMPS pins to bed should be at least 10V

Possible failed parts on RAMPS, polyfuse, bed mosfet, maybe burnt wires?

I see your point, but as i said, the bed gets warm, no problem with the heating!
But since the arduino/RAMPS/sensor dosen't detect it the printer turns off after a minute or something.

Will do the meassurings when i get home though!

Oh forgot to update!

I replaced the ramps board and now everything works like a charm!

Conclusion: You have burnt traces on the RAMPS when you made the shortcut. Otherwise the other temp input should've worked (with corrected firmware )

You could isolate the heatbed with cork or something. I'm using thin Styrofoam covered with aluminum foil ( sold as insulation wall paper ) and a sheet of MDF.

The foam has a cutout where the solderpads are, so I can't short them ever again.

Yes my conclusion as well

And i already covered the pads so it does not happen again xD