Fridge Thermistor Replacement Question UPDATE (Fixed)

Moving a Thermister on the fins up or down only changes the result by about 4 degrees. Moving a Thermister is for AUTO Temp refers that have no adjustable tstat. Doug

BFL13:
You never said whether this is a Dometic circuit board or after-markey. Dinosaur electronics boards have an adjustment to enhance/change thermistor input to the circuit board.
Raising the thermistor lowers the temperature in the refer.

In the link near the bottom, he is challenged by somebody with the same idea as wolfe10. His reply is there.

What does not help is saying that the fridge is warmer or colder as a result of moving the thermistor. Freezing is still the same temperature. What they should say is that the fridge temperature gets too cold before it shuts off or that it is still too warm when it shuts off.

BFL13 wrote:
wnji says it is the other way around on the fins, see above. It seems it is colder high on the fins ( being closer to the freezer above ). Colder is more R. When it gets up to 25ish for R the control stops heating.

However it is true the cold air in the actual fridge goes to the bottom, just to make it all more confusing.

I suppose the 1-9 temp settings are adjustments to R.

In all honesty I don't know for a fact which way you move it for cooling but just trying to explain that the picture was illustrating the fridge target temperature and not the actual fin temperature. If it's the other way, that website you found has it wrong.

wolfe10 wrote:

BFL13 wrote:
.... and the thermistor to high on the right and it still went just under freezing. :)

High and to a side are WARMER areas in the refrigerator and will cause it to run more/operate at a cooler temperature.

Likely not enough to cause your symptoms, but they can affect fine-tuning of the refrigerator temperature.

wnji says it is the other way around on the fins, see above. It seems it is colder high on the fins ( being closer to the freezer above ). Colder is more R. When it gets up to 25ish for R the control stops heating.

However it is true the cold air in the actual fridge goes to the bottom, just to make it all more confusing.

I suppose the 1-9 temp settings are adjustments to R.

BFL13 wrote:
.... and the thermistor to high on the right and it still went just under freezing. :)

High and to a side are WARMER areas in the refrigerator and will cause it to run more/operate at a cooler temperature.

Likely not enough to cause your symptoms, but they can affect fine-tuning of the refrigerator temperature.

Thanks for the explanations. Makes sense now.

Yes, my old Dometic fridge had to be set at 1 and the thermistor to high on the right and it still went just under freezing. At least it did cycle from there. Had to replace that fridge for other things wrong with it.

I will get the replacement part for the Norcold as in the OP. If that makes no difference, I will check out those other gadgets mentioned. Good tips on that, thanks.

I did have some calibration problems with fridge thermometers, even though my new multimeter checks out as in the other thread. I have two fridge thermometers and they disagree by more than moving the thermistor among the fins can do. Cure is to have only one thermometer! :)

BFL13 wrote:
According to this

http://bryantrv.com/docs2/docs/norcoldnxseries.pdf

At 70F it should be 11.1-12.0 kOhms. Mine is 8.2 MOhms
At 51F it should be 15.5-16.5 kOhms. Mine is 12.7 MOhms

At 70F the little Dometic one is 3.4 MOhms.

So assuming mine is out by having lower R on the whole range, then instead of shutting off the heat at 25, mine would still have been way lower so it never shut off. The little Dometic one would be even lower in R so it didn't do any good.

Perhaps my jury rig should have used both thermistors in parallel. ISTR they would add their Rs doing that. ??

Anyway, unless somebody says no, I plan to order the replacement thermistor part. I see in the manual above that it goes on the 10th fin from the right, at least to start with.

Purely based upon resistance, your MOhm failed thermistor should have told the fridge it was super cooled, however I suspect that any reading too far out of range will cause the fridge to simply cool full time. That would be preferred to having spoiled food.

The old Dometic one looks bad too as it should be around 8k Ohms @70F from what I found. At any rate, MOhms is way too high unless there were some models with a vastly different scheme.

For what it's worth, parallel resistor don't add or subtract. They follow this formula:

Rtotal = 1 / (1/R1 + 1/R2)

BFL13 wrote:
Yes it didn't work when I swapped to the old Dometic thermistor. I have not yet checked either one for R. Based on above posts, first I need to confirm the Norcold one is not right before getting a new one or one of those other gizmos.

Camping this weekend I just turned off the fridge when the fridge thermometer said it was getting too cold, and back on again later.

I am really mixed up on how the thermistor works. AFAIK they have more R when cold (at least those in-rush ones do), so that would mean the signal to turn off the fridge is when R gets that high.

But then, they say to move the thermistor up where it is warmer on the fins to make it shut off sooner. Makes no sense to me. Oh well.

http://rvrefrigeratorrepair.com/uncategorized/the-thermistor-shuffle/

The thermistors do get higher resistance when colder. As for moving it around on the fin, I think your confusion comes from interpreting their picture. The red area in the upper right is showing where you move the thermistor to make the fridge temperature follow the color (red=warmer). The upper right is also where the fin temperature is colder which in turn causes the fridge to cycle off sooner.

According to this

http://bryantrv.com/docs2/docs/norcoldnxseries.pdf

At 70F it should be 11.1-12.0 kOhms. Mine is 8.2 kOhms
At 51F it should be 15.5-16.5 kOhms. Mine is 12.7 kOhms

At 70F the little Dometic one is 3.4 kOhms.

So assuming mine is out by having lower R on the whole range, then instead of shutting off the heat at 25, mine would still have been way lower so it never shut off. The little Dometic one would be even lower in R so it didn't do any good.

Perhaps my jury rig should have used both thermistors in parallel. ISTR they would add their Rs doing that. ??

Anyway, unless somebody says no, I plan to order the replacement thermistor part. I see in the manual above that it goes on the 10th fin from the right, at least to start with.