Heated bed reporting 0 degrees, no errors, won't heat

[Whom 5]

I really hope this isn't simple as I did my best searching and came up with nothing other than the usual heatbed P100/ solder connector issues.

 

So here goes (actually I hope it IS simple)

 

I needed a new motherboard so I bought one from Aliexpress and updated it with the latest firmware. It looks pretty convincing if it is just a knockoff? I was hoping it was from the same production line...  

 

The motherboard "temp3" input pins are showing 109 ohms, the same as the sensor being unplugged, which is spot on for the temperature in the room. 

 

When I go to "set buildplate temp" it reports 0/0 and it never climbs when I set it higher (e.g. 0/60), and the build plate is not heating.

 

When I go to print, it sits there preparing the print waiting for a buildplate to heat up that has no intention of heating up, and remains at 0 degrees. 

 

So what could cause the motherboard to have the correct input resistance, but report 0 degrees? An outright broken board or something salvageable? 

 

Thanks for any insight 

 

 

[Whom 5]

It got weirder!

 

So I plugged in a spare nozzle temp sensor into temp3 and it then read a temp correctly and all was good, it would even heat up ok.

 

But if the sensor was unplugged an error was flagged right away. So it must be something to do with the sensor, even though I can read the correct resistance from it. Maybe the machine reads the resistance in a different way than my multimeter? 

[Whom 5]

I fixed it! 

 

I tried measuring voltage across the sensor and found that it was 1.3V when the nozzle was in the mV. So I disconnected the power to my (also NEW) heated bed, and the temp worked fine!

 

So I then saw lots of surface solder splatter around the connector on the bed which was shorting the two middle pins. After cleaning that off, it worked.

 

So the sensor had been confused by the short, but not enough to throw any errors.