Incorrect buildplate temperature

[eh_bee_see 0]

I am having an issue with my Ultimaker 2+ where the buildplate temperature registered by the Ultimaker doesn't match the actual temperature but I don't get any errors, just bad buildplate adhesion. When I heat up my buildplate it increases to the expected temperature and works for a little while, but after a few layers, the temperature reported on the printer keeps going up (it got up to 105C when I had it set at 65C). I stopped the print and checked the actual temperature by touching the buildplate and it was pretty much cold (no warmer than 30C at most). I heated up the buildplate manually to see what is going on and it does heat up, but once it hits its set point is seems to cool down while the temperature displayed on the screen keeps increasing. I checked all the connections for the buildplate temperature sensor and they are fine. Has anyone else had this problem or have any ideas for a fix?

[gr5 1,399]

Pretty much the only way to get this is if the temp sensor has a bad connection.  As the connection slowly opens/disconnects, the resistance at the breakage doesn't go infinite but instead may add 100 ohms and this makes the reported temperature sky rocket in an instant.

 

There are 3 very common locations where the bad connection can be.  2 are trivial to fix.  One is at the PCB under the printer.  Everything is labelled.  The heater sensor has thinner wires than the heater power.  Actually this may be a crimped connector and may be the less likely place for a problem to occur.  Or it may be a terminal block where bare wires are held in by push connector in which case it's a common fail point.

 

The second spot is where the wire connects to the print bed.  This is easy to tighten back up.  There is a screw connector (well there are 4 - two for heater, two for sensor) and this can come loose after a time.  It's really easy to take apart the print bed (completely remove the 3 leveling screws and remove the cable clamp).  Then remove and re-tighten the 2 sensor wires.

 

The third spot that commonly fails is the solder under this screw block where the block is soldered to the bed.  If you have a friend who is good with the soldering iron this is a trivial fix.

 

Don't do any of this if your printer is less than a year old in which case you should contact your reseller first as I assume you don't want to violate your warranty.  Tell them you have an intermittent sensor connection on the bed and it reports 105C when it's clearly much cooler.

 

In the meanwhile you can print on a cold bed with blue tape but make sure you wash the blue tape with isopropyl alcohol (to remove the waxy surface from the blue tape).