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Open Roads Forum  >  Tech Issues

 > Fridge Thermistor Replacement Question UPDATE (Fixed)

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wnjj

Cornelius, Oregon

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Posted: 02/10/20 07:39pm Link  |  Quote  |  Print  |  Notify Moderator

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

Victoria, BC

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Posted: 02/10/20 08:03pm Link  |  Quote  |  Print  |  Notify Moderator

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! [emoticon]


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wolfe10

Texas

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Posted: 02/10/20 08:11pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

.... and the thermistor to high on the right and it still went just under freezing. [emoticon]


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.


Brett Wolfe
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BFL13

Victoria, BC

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Posted: 02/10/20 08:32pm Link  |  Quote  |  Print  |  Notify Moderator

wolfe10 wrote:

BFL13 wrote:

.... and the thermistor to high on the right and it still went just under freezing. [emoticon]


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.

wnjj

Cornelius, Oregon

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Posted: 02/10/20 09:42pm Link  |  Quote  |  Print  |  Notify Moderator

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.

BFL13

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Posted: 02/10/20 10:22pm Link  |  Quote  |  Print  |  Notify Moderator

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.

enblethen

Moses Lake, WA

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Posted: 02/11/20 10:25am Link  |  Quote  |  Print  |  Notify Moderator

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.


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dougrainer

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Posted: 02/11/20 10:33am Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 02/11/20 11:00am Link  |  Quote  |  Print  |  Notify Moderator

enblethen wrote:

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.


I said in the OP it is a Norcold. Not sure how some got confused.

Having an adjustable like that would be great assuming you can read the adjustment you are doing somehow.

I am still confused how this all works. Please correct if this is wrong.

The fridge cools from some higher temperature until it gets down to where the thermistor reaches say 25. Then the fridge warms up until cooling comes back on at say 20 (whatever, just an example here) so the fridge stays in the proper temperature zone just above freezing.

So if you move the thermistor higher on the fins, that makes its R go up, being colder there. So cooling starts from room temperature and stops when R is 25 same as before, but the fridge has not been running as long, so it is not as cold when cooling stops.

Now the fin warms up until R is 20 and cooling restarts. I am stuck on why that makes the restart temperature in the fridge also higher than before. I need more coffee I guess! [emoticon]

wnjj

Cornelius, Oregon

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Posted: 02/11/20 12:00pm Link  |  Quote  |  Print  |  Notify Moderator

The control board only has one input that represents the temperature at the exact location of the thermistor. That's it and it doesn't matter if it's an auto or adjustable model. The control board still has feedback from the thermistor at that exact spot. The only difference with an adjustable model is user-selectable choices of what the goal temperature is.

On any model, moving the thermistor to a colder location will result in a warmer fridge and freezer (they work together on the same cooling cycle). Moving it to a warmer location will make them colder (i.e. the fridge runs more).

The only question is which location on the fins is the coldest and which is the warmest.

If anyone want to tell me I missed something here, please provide a specific technical explanation of how so we can all understand better.

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