Bed MINTEMP - Broken Thermistor Wire
Just wanted to report that my printer stopped last night after about maybe around 80 hours logged with a bed MINTEMP error. My understanding is that the usual cause for this is improper strain on the cable at one of the critical attachment points or zip-tie pinch points. Ironically this happened during Toms3d live MK3 unboxing video where his machine was not being 100% cooperative either. Maybe some kind of Christmas curse? 
Since I have some decent RF test equipment I was able to accurately locate the wire break using TDR to within approximately 2mm and was surprised that it was not broken at one of these points but within the segment of cable inside the cable wrap that was approximately 3cm away from the heatbed ziptie point. Since the wire is quite a bit longer than necessary I repaired it and secured the repair underneath the heatbed with kapton tape. The printer is back up and running fine with the repair.
I will file a support request on this issue, but I did want to mention in this forum to see if anyone has had a similar problem. I expect that doing failure analysis like I did is not typically done and its usually just assumed that the cable breaks at a pinch point, but that's obviously not always the case!
Re: Bed MINTEMP - Broken Thermistor Wire
I'll have to remember to keep my TDR handy, good idea to use it. Now that's thinking outside the box. For those that will be putting their MK3's together do you have any recommendations that would prevent it?
Re: Bed MINTEMP - Broken Thermistor Wire
I cannot imagine what caused it, let alone suppose what would prevent it. It is possible that strain under the bed still caused a break at the weak point further up the cable. I did leave slack under there, but perhaps not enough. The cable is now installed with a 1.5cm loop underneath the bed that should provide adequate strain relief in all circumstances. Mainly this was to get the repair out of the cable line, but it may be a good idea to emulate in new installs.
Another solution would be to move to a cable less prone to strain for the thermistor. The cable appered to be 24 or maybe even 26 AWG when I cut it. A larger gauge wire or a twisted cable would be less prone to breakage of any sort and I do not think Y axis movement would be compromised by a slight change here. The wire jacket appears to be some kind of hi-temp material too like silicone or teflon, so that may also be a factor of why it is the way it is.
I just completed a 21 hour print without issue, so we are holding for now. Also FWIW broken thermistor wires seem to be my bugaboo with 3d printers in general, and I am always exceedingly careful with them.
Re: Bed MINTEMP - Broken Thermistor Wire
I just had this same issue I think. I don't have a TDR(Or know what that is) to track down the break, but I had left slack very similar to the picture and have some sort of break. Not sure how to go about repairing it at this point. I will also file a support issue.
Re: Bed MINTEMP - Broken Thermistor Wire
I just ran into the same problem. I was getting the bed mintemp error and found that I can get readings if I wiggle the cable bundle just right. There's no strain on either end of the cable and it appears to be a broken line within the bundle. Kind of annoying. Maybe my Printrbot isn't getting retired just yet.
Re: Bed MINTEMP - Broken Thermistor Wire
I had to cut out the middle of the cable where the damage was. I am pretty sure it happened when I pinched the cable between the bed and the frame during the first wizard check. I didn't realize it could hang loose. It worked for about 100 hours of working, then would occasionally lead to mintemp bed error. I simply spliced a length of wire between the thermistor and the connector and now everything is cool.
Re: Bed MINTEMP - Broken Thermistor Wire
I've just had the same problem, again the break isn't at a pinch point. I have spliced in a heavier gauge wire, leaving a little more slack at both ends, and this seems to be doing the trick. I hope this is a long term solution and I don't have to spend more time repairing a new printer.
Re: Bed MINTEMP - Broken Thermistor Wire
I am having a similar problem. When I first assembled it seemed to worked fine. The next day I got a MINTEMP error. I opened up the controller box and pulled and reconnected all the connectors which solved the problem. I had the same problem today but was not able to get it to go away by reseating the connections.
When I run the self test I get Selftest error ! Please check : Heater/Thermistor Not connected
Does this mean it is definitely the temperature sensor on the bed?
Can just test it for continuity with a multimeter meter at the connector?
Re: Bed MINTEMP - Broken Thermistor Wire
Well, it was just the printer temperature causing my problems.
If I preheat and power-cycle teh unit everything is fine.
Re: Bed MINTEMP - Broken Thermistor Wire
Although the wire broke a few cm away from the attachment point in my case it could still be that pinching at the attachment point caused strain that caused the cable to fail at a weak point.
Prusa had committed a new STL of the bracket which secures cables to the heatbed on the mk3 github. I will probably go ahead and print it in case I have to dissassemble again. When I repaired the thermistor cable the wire is very ridiculously small; maybe 28 gauge. I believe this is the more serious issue. If it breaks again I will be looking to replace the entire length with heavier gauge wire.
Re: Bed MINTEMP - Broken Thermistor Wire
I am having this issue now too. I will get intermittent MinTemp Bed errors. I have written Prusa Support to see how it goes. I printed out the new bed cable connector and still got the error so I guess once you break the wire internally your screwed. I have already repaired a broken fan wire that was intermittently stopping during printing but since I was going by dead reckoning, DVM and wiggling the wire, I have to cut out a big chunk to make sure I got the break which I did because I could pull the with apart after pulling it out. Thanks for posting. I may just try to fix this too or wait for Prusa to get back to me. I declined Prusa sending me a new fan since I fixed it. I would like them to keep their money if possible to develop even better 3D printers.
Re: Bed MINTEMP - Broken Thermistor Wire
28ga really? thats insanely small and stupid to use in a situation which will see stress and bending constantly. What a dumb way to cut costs... Luckily i have a bunch of silicon wire on hand in different gauges. When my printer arrives im going to swap out anything under 20ga up to 20ga and hopefully not ever have to deal with any of these issues.
Re: Bed MINTEMP - Broken Thermistor Wire
I am also having this issue. I'd like to do some fixes like what was mentioned in the thread, but my printer is less than 30 days old, and I don't want to void any warranties. I've had several successful prints, but now, it appears my thermistor cable has separated in the wire bundle. If it's happening to so many people, it can't be user error, right? I took a video of myself debugging the problem and pinpointing exactly where the separation in the wire has occurred. Sent an email to Prusa Support and still waiting for a reply.
Re: Bed MINTEMP - Broken Thermistor Wire
I repaired the wires with a soldering iron. I removed the about 70% of the wire and replaced with a heavier duty wire. I added shrink tubing and about 1 inch of wire. It seems to have heald up.
Prusa sent me a repalcement and it is my back up.
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Chuck H
3D Printer Review Blog
Re: Bed MINTEMP - Broken Thermistor Wire
I am also having this issue. I'd like to do some fixes like what was mentioned in the thread, but my printer is less than 30 days old, and I don't want to void any warranties. I've had several successful prints, but now, it appears my thermistor cable has separated in the wire bundle. If it's happening to so many people, it can't be user error, right? I took a video of myself debugging the problem and pinpointing exactly where the separation in the wire has occurred. Sent an email to Prusa Support and still waiting for a reply.
Prusa has less than stellar response times on its email system I waited a week until folks here clued me in to using the excellent online chat system
contact prusa online chat system 24/7 support
log into your account on one of the prusa store pages and in the bottom right corner you will see the Chat system box click in it enter your Invoice number for you Prusa and start the chat system they do an excellent job and will take good care of you.
RE: Bed MINTEMP - Broken Thermistor Wire
Had this happen today with an intermittent on the bed thermistor. Finally found it right at the clamp on the bed. Does not look like they are using flex rated cable for this and seem to be routing the wires run right beside the screws which is an additional abrasion risk. Using continuous flex rated cables on these bending harnesses would be good and a small grommet to wrap around the cables when they are in the plastic clamps could also help reduce some of the strain and vibrations the cables are getting in this area. This is on an MK3S. This is the first real problem I've had with the printer in the 6 months I've been running it.
RE: Bed MINTEMP - Broken Thermistor Wire
@robot-stefan
Common problem. I make sure my printer is not hitting the glass in my enclose. No repeat problems in 12 months. I own 4 other Mk3s printers.
I have had similar fails with both bed and hotend thermistors so I keep 1-2 in stock at all times.
Parts to keep in stock:
I put a small piece of Kapton tape to keep the thermistor on the bottom of the bed.
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Chuck H
3D Printer Review Blog