Thermistor trouble

So while I was heating up the nozzle the printer suddenly showed me a "Maxtemp - printer halted" Error. I thought it was a short between the thermistor leads so I checked the resistance which was about 118 kOhm at room temperature which seems plausible. And here starts the mystery.

- If I swap the bed and hot end sensors both seem to show normal values
- If I disconnect the hot end thermistor I get a reading of 0° (which is also normal)
- If I connect just the one end of the thermistor to the RAMPS board it shows normal values. The moment I connect the second wire the MAXTEMP error appears. This is true for one particular thermistor wire (doesn't work with the other one) and one specific pin on the RAMPS - the one closer to the display connector

Please help...

---

Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A, [email protected], [email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6

it's curious.
Have you solved it yet?

---

Ingeniería 3D Digital - Servicio de impresión 3D - Diseño 3D - Escaneado 3D e ingeniería inversa

Quote
icefire
- If I connect just the one end of the thermistor to the RAMPS board it shows normal values.

Sounds to me like the thermistor wires are short circuited to the hotend... Otherwise, you would not get a reading from only one wire connected. Although it might be a problem with the RAMPS/Arduino controller, as you say it only happens on one particular pin. So the short circuit might be on the RAMPs board or the wires connecting the thermistor or the connectors that plug into RAMPs.

... Dutch guy once advised me to fold a piece of Kapton tape over each wire to make sure they were insulated from everything. There are other (expensive) alternatives like Sifer.

Edited 1 time(s). Last edit at 12/14/2016 09:10AM by DragonFire.

What are "normal values"?
If you disconnect the thermistor and get 0degrees that's not normal. You should get an error message.
Have you got an additional resistor in parallel to the thermistor. (Short on the controller board). That would create a temperature reading (that may be 0). Then when you add the thermocouple it will pull the resistance down and can trigger the max temp error.
A change in reading from having just having one wire attached is a mystery but you say its only one pin on the RAMPS so have a good look at that. Could be that just putting anything on the pin is making it deflect and causing a change - may simply be a dry joint.