Bad Temp reading at Onyx

[Jimminy]

First post here, cheers.

I've just finished the build on a V2 Max machine and all seems well except the bed temperature reading never gets past 16.7, even though the bed is hot. The temp reading at room temp gives me about 12.6, so that might be right. The hot end seems fine.

If I manually set the bed temp to 80C the bed heats up and is hot enough to burn if touched more than a moment, so it's functioning in that sense, but the print doesn't start. My workaround is to set the min temp in the ABS definition to 16C instead of 80, heat the bed manually for a few minutes, start the print, then manually override the bed temp to 80 (it sets the target to 16C). I've gotten good prints this way so far, but want to get it fixed because I don't know if this hack is causing the thing to try to keep elevating the heat on an already hot bed.

Is it possible there's a calibration or mistyped line in the eeprom? If I try to PID the bed I get a failure message after a while, but no further explanation.

[Earthbound]

Something is wrong with the hot bed thermistor or its wiring. If RAMBo is reporting 12.6C room temperature, that's a low for a heated indoor space (~55F). If you have an appropriate meter, check the resistance of the thermistor and its wiring when unplugged. Should be ~100kohm.

I would not continue using your workaround. Without a relatively accurate way to determine and control the heated bed temperature, print results will be random at best and you risk damaging the bed. When you set the target temp to 80C and the thermistor says it hasn't gotten there, the bed is running at maximum power.

[Eaglezsoar]

First post here, cheers.

I've just finished the build on a V2 Max machine and all seems well except the bed temperature reading never gets past 16.7, even though the bed is hot. The temp reading at room temp gives me about 12.6, so that might be right. The hot end seems fine.

If I manually set the bed temp to 80C the bed heats up and is hot enough to burn if touched more than a moment, so it's functioning in that sense, but the print doesn't start. My workaround is to set the min temp in the ABS definition to 16C instead of 80, heat the bed manually for a few minutes, start the print, then manually override the bed temp to 80 (it sets the target to 16C). I've gotten good prints this way so far, but want to get it fixed because I don't know if this hack is causing the thing to try to keep elevating the heat on an already hot bed.

Is it possible there's a calibration or mistyped line in the eeprom? If I try to PID the bed I get a failure message after a while, but no further explanation.

I would like to welcome you to the Forum.
I agree with Earthbound's answer to your question.
Please feel free to use the forum for future questions, comments or helping another user.
Happy Printing!

[Jimminy]

Thanks for the welcome, glad to be here.

I don't really understand the readings I'm getting when I put a multimeter on the thermistor. I took the bed up and poked around the underside checking for a short and measuring resistance.

I checked the resistor and it gives me 4.7k as it should, but the thermistor is a different matter. Then again, I'm no electronics buff.

I get different readings depending on which side of the thermistor I put the red lead, different from what I understand of resistors in general, which is that it doesn't matter what direction the current flows. But on to the actual readings taken from the 2 solder points of the thermistor, not the points for the white wires.


red MM lead on thermistor + side (meaning the same side as the + for the 12VDC label on the bed, for reference)
5.12 on MM 20k setting
-4.3 on MM 200k setting

Then switched around so the black MM lead is on the 12VDC + side
9.9 @ 20k setting
21.9 @ 200k

These are just all different readings that make no sense to me. I repeated the tests and always got the same results, so at least it's consistently confusing.

I hope someone can tell me either what I'm doing wrong or if this is just a bad thermistor.

Also, should there be continuity between the 2 pads of either the thermistor or the white wires?

Thanks all.

[Earthbound]

I get different readings depending on which side of the thermistor I put the red lead, different from what I understand of resistors in general, which is that it doesn't matter what direction the current flows. But on to the actual readings taken from the 2 solder points of the thermistor, not the points for the white wires.

red MM lead on thermistor + side (meaning the same side as the + for the 12VDC label on the bed, for reference)
5.12 on MM 20k setting
-4.3 on MM 200k setting

Then switched around so the black MM lead is on the 12VDC + side
9.9 @ 20k setting
21.9 @ 200k

The reading across the thermistor should be the same regardless of which meter lead is on either side. The thermistor is not a polarized part. Take the measurement with the thermistor lead unplugged from the RAMBo!

[Jimminy]

Ah, that changes things. Switched to the 200k setting I get a reading of 98.3 which I think is right.

[Eaglezsoar]

That reading is fine.

[Jimminy]

Then I think it must be something in the Arduino settings. But I have no idea what to check. Maybe I'll try the PID autotune for the bed again, but I got a failure the last time so that told my nothing other than...what, there's a problem somewhere?

[Jimminy]

So tried the autotune again and as suspected it timed out because it never hits (or even comes close to) the target temp. The bed is pretty hot, but the temp reading never fluctuates beyond about 16-17.

[Polygonhell]

I would check you have the right thermistor plugged into the right connector on the board.
Also check the resistance at the connector both at room temperature and when the board is a little warm.
You can also get very strange results if the thermistor lead is touching the metal through plating on the bed.
The fact it's reading 17C which seems low for room temperature, implies something is touching something it shouldn't somewhere.
If you have a couple of resistors in the 50KOhm range you can connected it to the connector and verify it reads a higher temperature to validate the RAMBO board isn't borked.