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I just finish building a Graber i3 printer from mixed parts (a MDF clone of Prusa i3 MK2). For the hotend, I've bought the Greetech MK8 extruder. However, the temperature sensor is driving me up the wall.

I've already checked the wires and connections, but the darn thing keeps showing a steady 500 degrees Celsius. In Marlin, it shows it right up (I'm using the 1st option for a 100k thermistor), in Repetier Firmware (using the same one), it shows 0.00°C, until I tell the printer to heat up, when it shoots to 500°C just like before with Marlin. The documentation for this extruder only lists it as a "100K NTC Thermistor", so I tried to select one of the NTC options on the list for both Marlin and Repetier and the temp sensor reads steady 3.600°C!!!

Measuring it with the multimeter, it shows around 60k, it's 31°C outside.

Is it broken or am I selecting the wrong thermistor type?

Documentation for MK8

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A brand new thermistor shows about 95-105k ohm (depends on the multimeter quality), so it looks like that one is out of the range. To be sure that it is a thermistor, you could connect a 100k potentiometer, play with it and see readings on the lcd. If the readings are OK, then mainboard is good so replace the thermistor.

A 100k thermistor curve

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  • $\begingroup$ Thank you! I'll buy a new thermistor tomorrow and test it out, if that's the problem, I'll mark the question as answered. $\endgroup$
    – user3430483
    Mar 18 '18 at 23:51
  • $\begingroup$ The thermistor was fine, however there was a slight cut on the protective sleeve around its legs, so it was shorted against the metal block of the hotend. Used some thermal tape to unshort it. For those that use that type of hotend (MK8 from Greetech), the correct option in Marlin is 11 (Beta 3950). Thanks a lot! $\endgroup$
    – user3430483
    Mar 24 '18 at 22:31
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If your printer is a cheap one that uses a ribbon cable or other combined cables, verify your thermal sensor's polarity. I know, I know - it's a resistor (in most cases) and that means there's no such thing as polarity. Bear with me - I just helped a friend diagnose his printer (Geeetech, for the record) for showing 500C on one sensor and 'def' on the other, after replacing the mainboard with a different/more capable model. Turns out, they SHARE PINS, which suddenly makes polarity important. Swap the pins in the connector, and it might just work. First time I've run into this. We swapped only the Ext0 sensor and left the bed disconnected - Ext0 read normal, so we swapped both, and everything's been perfect since.

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