Need help with inaccurate extruder temp

Have inaccurate extruder temp on 3D printer. changed everything on extruder, no difference. Read out indicates extruder at 180, laser thermometer tells me 40 c. Problem likely with Ramps 1.4 or Mega 2560? Worked fine 2 days ago.

Readout is based on a thermistor. When thermistors fail, they go to an open circuit (infinite resitance, -40°) or short circuit (no resistance, +something stupid) either of which should throw Marlin into an error condition not to allow it to extrude. Being wrong like that isn’t a usual state, but can happen when the thermistor fails.

Often you’ll get a problem where the thermistor is no longer in good contact with the print head (or build surface) but that usually results in reading closer to room temperature than the heated element. In other words, reading 40° while something is actually 180°. This is where we can get into trouble, because the hot end can often get hot enough to cause some real trouble if unmonitored, so there are other safeguards to catch when this happens.

The laser thermometer might be a problem, they often have difficulty getting temperatures from shiny surfaces, as they also reflect IR (Which is what they’re measuring) from surrounding objects, which are (hopefully!) much cooler than 180°C. I would suggest that you try a candy thermometer or at least see if the 40 degrees is reasonable. You should be able to touch 40°C with your fingers, it should feel warm. If it’s reading 180° though, I’d advise extreme caution, that’s more than hot enough to burn you quite badly and pretty much instantly. That would be a terrible way to find out that the reading is correct.

In general:

Make sure that the wires going to the thermistor are okay. These are often sheathed in teflon, which the hotends can get more than hot enough to damage. If there is a retention screw, make sure that it’s not pinching through the insulation. Make sure that the wires aren’t being strained or damaged.

If you find any damage, or suspect it anyway, these thermistors are inexpensive, and not difficult to replace.

If you’re sure that it’s not the thermistor, (plug it into the bed temperature sensor port, for example) it could be the pull down resistor on the RAMPS board. Since what the Mega2560 board actually senses is voltage, it uses a known value resistor with the thermistor as a voltage divider network, and reads the resulting voltage. If the known value of the resistor is wrong, then the voltage will be wrong, and the resulting reading will be wrong. It is VERY unusual for these to fail in use, unless you’ve done somethign that can cause it to fail, ie: applied too much power somewhere, or shorted something out, and you’ll usually get an error that looks like the first example. -40°C or 300°C+

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Thanks, better understanding of how circuit works. Installed new 12V 40W 620 Ceramic Cartridge Heater and NTC Thermistor 100K 3950, no change. extruder head only hot enough to allow very little PLA through with manual feed. USB on 2560 toast about 5 months ago, been working off of SD card. Have ordered Ramps and 2560 replacement boards, will then play with existing on bench. Thanks again.

I think 180 is a little too low. The hotter the pla the more liquid it becomes (within reason) I personally set my extruder temp at 200 to 220, some filaments seem to flow better at the higher temperature. At 60 degrees the pla will still be solid, think heated bed temperature.

I would agree that 180° is a little low. For certain though, under 100°C PLA should still be solid, and should not flow through the nozzle at all. Your extruder should be either grinding the filament down or skipping steps.

It seems to me that the readout of 180°C sounds like it may be right, or at least close if it’s flowing at all.

Fixed, replaced Ramps 1.4, somehow misreading extruder temp.
Thanks to all for input.