Print bed thermistor constant 170°C reading ... worked before ;(

Hello,

I have a problem with my print bed thermistor, or better with the temperature display in Marlin.
Everything worked in the beginning
HotEnd Temp was displayed, bed temp was displayed I could heat up both and did the PID tuning. Then days later I got a MaxTempError Bed and the display shows constat 170°C

I check the resistance on both thermistors (HotEnd and Bed) both are aroun104kOhms

If I switch the plug and the heatbed thermistor goes into T0, the bed temperature is display as hotend on the LCD and is "heating" up if i press my thumb on it.
So I guess the thermistors are not shorted and working fine.

The temperature for the bed stays the same even if there is no thermistor pluged in at all
Hotend drops to 0 but the bed stays at 170

I am completely lost at the moment because it worked in the beginning and now I have this issue

config settings:
#define TEMP_SENSOR_0 5
#define TEMP_SENSOR_1 0
#define TEMP_SENSOR_2 0
#define TEMP_SENSOR_BED 1

E3D hotend and a reprapworld thermistor B57550G104J

Any hint or tip or procedure to fix this is greatly appreciated

Cheers
Axel

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Cheers
Axel

Prusa i3 Advanced Laser Cut / Marlin 1.1.0-RC8 / BLTouch / Pertinax Print Bed /
E3D V6 3mm - 0.4mm Nozzle / Titan Extruder with NEMA 17 Pancake Stepper

Shorting the thermistor input to 12V destroys the input and gives that symptom. It is usually the extruder where they can get shorted easily. Less common on the bed.

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[www.hydraraptor.blogspot.com]

uha bad news ;(

but how could I shorten the thermistor input to12V?

I have a the E3D fan connected to the 12V power supply. The additional cooling fan for the PLA was added after the symptoms occured.
How can I "repair" this? Replace the arduino board or the ramps? can i replace any part on the boards?
Sorry for the many questions, I just want my bed heating up again

ah and probably the most important one, how can I prevent this from happening again?

Cheers
Axel

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Cheers
Axel

Prusa i3 Advanced Laser Cut / Marlin 1.1.0-RC8 / BLTouch / Pertinax Print Bed /
E3D V6 3mm - 0.4mm Nozzle / Titan Extruder with NEMA 17 Pancake Stepper

Quote
LeChuck71
uha bad news ;(

but how could I shorten the thermistor input to12V?

I don't know without seeing your bed and its wiring, but it is the only reason I know of that causes analogue inputs to get stuck at a mid range value.

Quote
LeChuck71
I have a the E3D fan connected to the 12V power supply. The additional cooling fan for the PLA was added after the symptoms occured.
How can I "repair" this? Replace the arduino board or the ramps? can i replace any part on the boards?

If you have the skills to replace surface mount chips then you need to replace the ATMega on the Arduino board, otherwise the whole Arduino.

Alternatively if you have a spare thermistor input pin you can reassign that in the firmware.

Quote
LeChuck71
Sorry for the many questions, I just want my bed heating up again

ah and probably the most important one, how can I prevent this from happening again?

Hard to say without knowing what happened. Connecting 12V to any of the 5V I/O pins will cause damage. Normally the problem happens at the hot end due to poor insulation or broken wires due to motion.

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[www.hydraraptor.blogspot.com]

Hey nophead,

thanks for your help.

Well I'll check the wiring, actually I was more worried on my HotEnd isolation than the one from the bed but sh... happens
Sending some pics here to check if I did something wrong - which I doubt I mean all the 12V cables are isolated I am still wondering how they can shorten to the 5v IO line and the thermistors. I was worried that those tiny thermistor cables shorten and I get a false reading.

There is still one input left on the board so I will go with a pin reassign after its clear what caused the problem. Same as with a multicopter or the shuttle ... it stays in hangar until you know whats wrong
Replacing the atmega is the last option but I think getting a new one for 14€ is the best choice here - my soldering skills are ok but thats too much of a headache

More infos tomorrow

Cheers
Axel

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Cheers
Axel

Prusa i3 Advanced Laser Cut / Marlin 1.1.0-RC8 / BLTouch / Pertinax Print Bed /
E3D V6 3mm - 0.4mm Nozzle / Titan Extruder with NEMA 17 Pancake Stepper

UPDATE

I did a complete rewiring of my printer. The 12V cables run now separately from the 5V IO hope that helps to prevent any shorts
Reconfigured my pins.h with help of the Ramps 1.4 layout so that the bed thermistor is now plugged in the Hotend2 port
Works fine

Thanks for the hints and tips on rerouting

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Cheers
Axel

Prusa i3 Advanced Laser Cut / Marlin 1.1.0-RC8 / BLTouch / Pertinax Print Bed /
E3D V6 3mm - 0.4mm Nozzle / Titan Extruder with NEMA 17 Pancake Stepper