Fluctuating hotend temperature and failed prints

[Alloy]

I am currently experiencing a problem where I set the extruder temperature to 215C but the temp will over shoot the set point up to 220C-222C and then fall to just under 215C and level off around 215C when the print starts. However, during the print, the temp will sit around 205C-210C and some times drop to ~180C but bounce back to 210C the next reading. I have an extra hotend so I took the thermistor out of the new one and tried in the used one and it did the same thing. I put the new thermistor back in the new hotend and tried that and it did the same thing. I had 4 prints fail on me while trying different parts and of course when I am sitting there waiting for the first line of code with an error it doesnt crash. When I find the stalled printer it is repeating this line of code "printer stopped deu to errors. Fix the printer and use M999 to restart! (temperature is reset. Reset it before restarting)." Also I checked the resistance on both thermistors and they both read ~100K ohm at room temperature and ~65K ohm when held in hand.

[Jules]

Maybe a wire breaking? I'd check the connections at the connectors and power supply first, then start checking for broken or partially broken wires. (Guys?)

[Alloy]

Thanks Jules. When it comes to stuff like this, everyone hopes it is something simple like just replacing the thermistor, and nobody likes having to tear the machine down to investigate. First thing I looked at was the thermistor wire from the print head to the board and I found a spot that has intermittent conductivity when flexing and bending the wire. After this post I am going to solder in a splice for a temporary fix and see if I can get the wire with the connectors or just the connectors from maker gear.

[Jules]

Look at the bright side....at least you know how to solder! (Not in my skill set!)

[Alloy]

Look at the bright side....at least you know how to solder! (Not in my skill set!)

I cheated. I used a crimp connector because thats what was readily available, but trust me I can solder . On the bright side, I am back up and running. During warm up the extruder still overshoots by 5C but it comes down and holds at set temp.

[Jules]

Yeah, that overshoot by a few degrees is the PID....it's supposed to do that. (You can fine tune it a bit, but it generally doesn't add much time so that's okay. Nothing to be concerned about.)

[Alloy]

Yeah, that overshoot by a few degrees is the PID....it's supposed to do that. (You can fine tune it a bit, but it generally doesn't add much time so that's okay. Nothing to be concerned about.)

This is bringing me back to my auto controls class days when we had to control a thermal chip and choose the best way to do it (Lead, lag, PID...)

[Jules]

i tend to forget that guys seem to instinctively know what the heck a PID is.