Taz 5 Heating Bed - Temperature error

I recently noticed a temperature error of the heating bed of my Taz5, which prevents me from printing correctly. I usually print PLA, where I set the temperature of the heating bed to 60°C - the LCD screen shows that the heating bed is heating up, and once it stops (when the LCD screen shows 60°C on the bottom), the heating bed does not actually have the 60°C. It only has heated to about 35°C - I measure it with a pyrometer. (

My conclusion was that the temperature sensor of the heating bed does not measure correctly (anymore), so I bought a new heating bed, plugged it in, set the temperature to 60°C and I still have the same problem! The screen says that it has heated up to 60°C, but the heating bed is way to cold to print on. Thats 80$ wasted.

I can now only conclude that something on the printer motherboard has stopped working, but before I replace that, I wondered if maybe someone knows a solution for my problem. Thanks

THe thermistor the heated bed uses is the same kind the hotend uses. if you have a spare hotend thermistor you could attach it to the bed temporarily, run wires into the control box and see if that one is reporting correctly. It would be a $3.00 test rather than another $80 one. The thermistor readings are resistance based. It is possible that the taz 5 bed connector at the back of the control box has been damaged (its not rated for the full 15 amps the fuse can draw, so it sometimes uh, melts a bit. Normally that would affect the heating element feed, not the thermristor reporting, but if it melted in the right place maybe the thermristor wire has a very bad connection now? It could also be a mostly broken wire in the harness run. I’m thinking like a single copper strand of the cable still intact, which would still run current but at high enough resistance the thermristor is reading it wrong?

If its the board itself, they are expensive so you don’t want it to be the board. I’ve seen more of the panel mount connectors fail than boards on the heated bed side, so thats where I would check next. bypassing that connector and the wire run with a new wire harness to a hotend thermristor would also clear the board of any possible damage (or confirm it)

Alright I just removed the thermistor from a hotend, connected it to the temperature wires of the heatingbed, fixated the thermistor on the heating bed, heated up, and once again it reports a wrong temperature. I suppose something on the board has melted…

Any chance you have a weird firmware loaded? It could be set to the wrong thermistor type.

nope its not the firmware, i have the one installed for the dual extruder v2, and it worked fine until yesterday…

It’s possibly the mosfet on the board then. Best price for new ones is direct from ultimachine.com. Best price for used is going to be itworks3d.com if they have them.

alright so i bought a new rambo v1.3 board, replaced it with the one installed, and finally the temperature problem is gone. although i want to state that i dont see anything molten or broken on the old one. anyway, it works again now.