Solved HBP Temp fluctuating

This is a new problem within the last 5 days of normal use. The temp on the heated bed is fluctuating a lot and sometimes reporting 0 degreess. I've had a couple of failed ABS prints recently due to this issue, and I'm watching one crash/burn right now. The RepRap Discount LCD is reporting temps between 0-70 with a target temp of 110. It generally stays at 0 longer and more frequently than the higher temps.

What is the problem? Is the thermistor failing? Other issue?

My printer:

• Robo3D R1 with bottom cover, hexagon hotend, etc
• No modifications that could affect the HBP
• Firmware: Auto_Level_6_10 with a few motion modifications (I've been running this for a while since before the problem
• Hotend heater running in PID mode and tuned
• HBP running in bang-bang mode

Temp settings:

• ABS: hotend: 240, HBP: 110
• Max temp in firmware is 245 and 120

If there is any other relevant info I can provide, please let me know.

 

OK, now that it's cool I checked the resistance across the thermistor. It's reading at 98.8 at room temp. Both leads to the RAMPS are showing 0 resistance as well. Can I try connecting it to T2?

 

Sorry, that was confusing: I tested across the leads on the connector (disconnected). I also tested across the thermistor alone and both legs of the leads independently. The thermistor reads 98.8 at room temp, then I put a incandescent lamp close to heat it up and it read 81 ohms, as it should.

I'm going to try it on T2. What do I need to change in pins.h to do this?

Code:

#if (TEMP_SENSOR_1==0)
#define TEMP_1_PIN         -1
#define HEATER_1_PIN       -1
#else
#define HEATER_1_PIN        3    // EXTRUDER 2
#if (TEMP_SENSOR_1==-1)
  #define TEMP_1_PIN         5    // ANALOG NUMBERING - connector *K2* on RUMBA thermocouple ADD ON is used
#else
  #define TEMP_1_PIN         14   // ANALOG NUMBERING - default connector for thermistor *T1* on rumba board is used
#endif
#endif

#if (TEMP_SENSOR_2==0)
#define TEMP_2_PIN         -1
#define HEATER_2_PIN       -1
#else
#define HEATER_2_PIN        6    // EXTRUDER 3
#if (TEMP_SENSOR_2==-1)
  #define TEMP_2_PIN         7    // ANALOG NUMBERING - connector *K3* on RUMBA thermocouple ADD ON is used <-- this can not be used when TEMP_SENSOR_BED is defined as thermocouple
#else
  #define TEMP_2_PIN         13   // ANALOG NUMBERING - default connector for thermistor *T2* on rumba board is used
#endif
#endif

 

I think the problem resolved itself. There must have been a loose connection that was not glaringly obvious. The HBP holds a steady temp...for now.