How to autotune PID controller for heated bed?

Greetings, I'm using the RepRap 2020 Prusa i3 offered by Folger Tech. After assembling the printer, I'm now trying to calibrate everything properly. My main problem right now is that my bed reaches the desired temperature, then continues heating to beyond that temperature. For example, I have it set to 120C for my ABS prints and it is currently at 135.5C and climbing shortly after starting my current test print. I have just discovered it tops out at 135.8C and stays level there. I can only think that it may need the PID controller tuned. I was able to tune my extruder PID controller, however I cannot test my heat bed PID tuner. I can't find the correct G-Code to autotune the PID controller for the heat bed. I've tried M303 P1 S , M303 C8 S175 , and M304 as indicated on [reprap.org], however none of these return any values in the window and act like nothing was typed. I see in the config file where to adjust the values, but I cannot figure out the G-Code needed to test it. On a related note, is there anything that needs modifying besides

#define DEFAULT_Kp 20.95
#define DEFAULT_Ki 1.15
#define DEFAULT_Kd 85

when autotuning? Thank you for any assistance you can provide on the matter, every bit is appreciated!

Edit: Ah... the temperature is still slowly going up but occasionally fluctuates around temperatures. Currently at 137.4. So maybe it's not PID related?

Found bed auto tune code M303 E-1 S60 C8

This did not resolve my heating issue. My bed hits 120C, then heats the extruder, then continues climbing in temperature. Repetier is recognizing that the bed temperature is 130C instead of 120C but my printer is not cutting/reducing power to the bed's heating element it appears. Looking for any suggestions on why my print bed likes the number 131 instead of 120. I've tuned the PID controllers, I can't find any issues in wiring but maybe there's something I'm not checking. Thank you for your time and assistance.

Edited 4 time(s). Last edit at 02/14/2016 07:54PM by Zannyth.

You have to replace the PID values in firmware, reflash and then use "M502" followed by "M500" to refresh the EEprom values.
-Olaf

Maybe running PIDautotune with a higher target temp ends up with better values.

"M303 E-1 C8 S90" to run autotune on the bed at 90 degreesC for 8 cycles.

Thank you for your suggestions! I am adjusting the values in the Arduino code and reuploading it to the board when I tune them if that's what you meant by replacing the values in the firmware and reflashing (sorry I'm still learning this jargon). I haven't used those codes before though so I'll try those before my next print.

Edit: Huh. And now it's heating properly. All I've done recently is unplugged the heating element cables to the board and plugged them back in.

Edited 1 time(s). Last edit at 02/15/2016 08:41PM by Zannyth.