Best way to debug erratic hotend temperature V2 Max ?

[RobertPhoenix]

I have been having trouble with erratic readings from the hotend. Now today I tried to do a print and find that the temperature reading was mostly sticking to about 10 deg F but sometimes jumping up to 180 or so.

The hotend was definitely hot ! (even when temp was showing about 10)

Previously I was thinking it might be noise - and maybe it is actually much more than noise - like a bad connection somewhere. To start debugging, does anyone know what voltage I should see across the thermistor at the hotend, say at normal room temperature and at something like operating temperature ?

[duvdev]

It sound like the temp sensor it not sitting in its place. It happens to me too.
Make sure the the sensor is fix in the hole and there is enough rtv that will not let it move

[Eaglezsoar]

If you disconnect the thermistor from the Rambo and measure it's resistance at room temperature it should measure around 100K (anything near 100k is good)
You problem sounds like the thermistor is not making good contact with the heat block as duvdev has indicated. The thermistor could be coated with RTV at the
bottom and cannot read the temperature reliably.

[RobertPhoenix]

Thanks for all the help. I thought I would post an update about solving the problem. I had installed a connector at the hotend so I was able to disconnect the connector and put a 100K resistor in the place of the thermistor. I then got a reading of exactly 25.0 displayed for hotend temperature. All connections seemed to be OK and the thermistor itself measured about 91K.

Eventually I discovered that one of the wires soldered to the thermistor leads was broken inside the insulation at exactly the point where the wire entered the crimp connector. Simple to fix, but I do remember this as an age old problem with connectors that have some movement. You really need to have a transition from the rigid connector to the wire that is going to flex (hence all those extended grommets that are used on things like hair driers). So now I am thinking about how to make a better solution for the connection between the thermistor and the flexible wire that goes into the Bowden cable bunch so that this doesn't happen again after extended use. Using stranded wire between thermistor and connector might be a good start.