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Open Roads Forum  >  Tech Issues

 > Fridge Thermistor Replacement Question UPDATE (Fixed)

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BFL13

Victoria, BC

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Posted: 02/11/20 01:06pm Link  |  Quote  |  Print  |  Notify Moderator

dougrainer wrote:

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


Not clear on what an AUTO temp refer is that is different.

Not clear if an adjustable tstat is the same thing as the Temp Settings 1-9.

My Norcold 641 runs with no thermistor at all, but just keeps running and getting colder. (But is not in power back -up according to the Temp setting lights which are not flashing.) It does not have that snowflake lamp that says the thermistor is broken or whatever.

I am running the fridge on AC now at home until it gets colder and then will aim my IR temp gun at the fins. Before starting I checked and did see the upper right and lower left fin temps are the same. Will report later on that test.


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dougrainer

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Posted: 02/11/20 02:52pm Link  |  Quote  |  Print  |  Notify Moderator

Norcold and Dometic make AUTU TEMP refers. That means there is NO adjustable tstat and the temp is a constant at about 38 degrees
MOVING a Thermister on a Adjustable (Norcold 1-9 and Dometic 1-5) refer will NOT yield colder or warmer temps if wanted. The Thermister senses the temp of the fins and turns the refer ON and OFF depending on the set of the tstat(1-9). That sense is the OHMs reading. FYI, a BAD Thermister will still test(OHM READINGS) TO SPEC. Moving a Thermister on an AUTO TEMP refer can be done, but only changes the final temp no more than 4 degrees.
The FIN temp will be 10 degrees colder than the refer temp---- Fins at 24 degrees-----refer will be 34 degrees.
For another posters question about moving the Thermister on an Adjustable tstat refer. Lets say you have the Thermister in the center of the fin top to bottom(Factory setting). At 5 setting lets say 35 degrees in refer. The thermistor is sensing at 25 degrees(10 degrees). You move the Thermister UP(Fin will be colder there). The Thermister will now read 25 degrees a little sooner, but still shut off the refer at that 25 degrees.
Moving the Thermister UP will yield colder temps in a AUTO tstat refer
Doug

BFL13

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Posted: 02/11/20 03:42pm Link  |  Quote  |  Print  |  Notify Moderator

dougrainer wrote:

Norcold and Dometic make AUTU TEMP refers. That means there is NO adjustable tstat and the temp is a constant at about 38 degrees
MOVING a Thermister on a Adjustable (Norcold 1-9 and Dometic 1-5) refer will NOT yield colder or warmer temps if wanted. The Thermister senses the temp of the fins and turns the refer ON and OFF depending on the set of the tstat(1-9). That sense is the OHMs reading. FYI, a BAD Thermister will still test(OHM READINGS) TO SPEC. Moving a Thermister on an AUTO TEMP refer can be done, but only changes the final temp no more than 4 degrees.
The FIN temp will be 10 degrees colder than the refer temp---- Fins at 24 degrees-----refer will be 34 degrees.
For another posters question about moving the Thermister on an Adjustable tstat refer. Lets say you have the Thermister in the center of the fin top to bottom(Factory setting). At 5 setting lets say 35 degrees in refer. The thermistor is sensing at 25 degrees(10 degrees). You move the Thermister UP(Fin will be colder there). The Thermister will now read 25 degrees a little sooner, but still shut off the refer at that 25 degrees.
Moving the Thermister UP will yield colder temps in a AUTO tstat refer
Doug


Doug thanks for taking the time to explain all that.

So we are all agreed that the fins are colder higher up. That is what my IR temp gun says too in my test just now. After that, Doug has managed to confuse me even worse! [emoticon]

So we have
1. AUTO Temp fridges,
2. We have Adjustable tstat fridges (using the Temp setting button)
3. AUTO tstat fridge (maybe a typo?)

With 1. you can move the thermistor and get maybe a 4 degree change to what the AUTO temp is. 38F nominal can be made to go from 36 to 40? Or is that 34 to 42?

Which way does Doug say to move it to make that 40 go to 38? Or 36 go to 38? ( The link posted earlier says move it down to make the fridge temp go from 42 to 38 so he does the 4 degrees with wider spread?)

With 2., Adjustable tstat moving the thermistor will NOT yield colder or warmer temps if wanted. BUT,

Moving the thermistor UP will get you colder temps in 3. an AUTO tstat fridge, but

If you move it UP in a 2. Adjustable tstat fridge, it will not change the fridge temp, but will make it get to the thermistor reading for shutting of the cooling (shutting off the heat source) "sooner"

To me, that "sooner" means if the cooling stops sooner, the fridge will not be as cold as if it had run on longer. That is what happens if it does not shut off--it keeps getting colder and colder in the fridge section (and in the freezer too)

So that would mean it does yield warmer fridge temp. What part of this don't I get? [emoticon]

Then Doug says with 3. (typo? ) moving the thermistor UP yields colder temps in that type of fridge.

The link guy says:
"....cooling fins to another you are effectively readjusting the resistance and “tricking” the control board. For instance, if your refrigerator is running around the 42 degree mark consistently you can move the thermistor to a location that gets cold more slowly so that the food cabinet actually drops to 38 before the control board shuts of the heat source."

He then says moving it down to the left makes the fridge colder.

I think he means by, "gets cold more slowly", that it doesn't do that, but what he is trying to say is that it will keep cooling longer so it takes more time ("slower") to reach the lower temp shut off point.

* This post was edited 02/11/20 03:55pm by BFL13 *

wnjj

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Posted: 02/11/20 04:04pm Link  |  Quote  |  Print  |  Notify Moderator

Doug is confused about adjustable fridges not being affected by the thermistor location. Lets say for example that the fridge is satisfied with the top of the fin at 23F, the middle where the thermistor at 25F, the bottom of the fin at 27F and the fridge compartment at 35F. Now when you move the thermistor to the top the control senses "too cold" and the fridge will stay off until the fin temperature is 25F at the top, 27F at the middle and 29F at the bottom, with the fridge at 37F.

This is what's known as closed loop feedback in engineering. If you add an offset in the feedback, you affect the output.

The only thing an adjustable fridge does is allow the user to change the target temperature by changing the thermistor sensed resistance value where the fridge cycles. You still have minor control over where that temperature is achieved by the location of the thermistor. You're not really tricking the control board but rather expecting its set temperature to be achieved at a different physical location within the fridge.

This is just like installing your home heating thermostat near an open window. If you let cold air from outside hit the thermostat, your house will get warmer everywhere else, however you can also still decide what temperature "warm" is by adjusting the set point.

(Added F to the temps)

* This post was edited 02/11/20 04:59pm by wnjj *

BFL13

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Posted: 02/11/20 04:29pm Link  |  Quote  |  Print  |  Notify Moderator

We better use units for the "25" !

I used 25 to mean 25 MOhms (which my meter uses. I am hoping that is the same as a kOhm), which is the approx R the Norcold thermistor reaches at 35F. At 32F it should be 30-32 kOhms

Now we have the fin temp at 25F when the fridge is at 35F. So my 25 for R is too low for when the fridge is near freezing, which is when the fin is 25F... Are we going crazy yet? [emoticon]

I think wnjj and I are on the same page in all this. Anybody not with us on that? If not, please explain.

* This post was edited 02/11/20 04:44pm by BFL13 *

enblethen

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Posted: 02/11/20 04:34pm Link  |  Quote  |  Print  |  Notify Moderator

You are correct saying it is Norcold then you say you are using Dometic part. Confusing!


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BFL13

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Posted: 02/11/20 04:42pm Link  |  Quote  |  Print  |  Notify Moderator

enblethen wrote:

You are correct saying it is Norcold then you say you are using Dometic part. Confusing!


Sorry, I tried to say I stole the thermistor out of my old Dometic that was replaced by this Norcold to see if it would make the Norcold work properly. Didn't work as noted earlier.

wnjj

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Posted: 02/11/20 05:01pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

We better use units for the "25" !

I used 25 to mean 25 MOhms (which my meter uses. I am hoping that is the same as a kOhm), which is the approx R the Norcold thermistor reaches at 35F. At 32F it should be 30-32 kOhms

Now we have the fin temp at 25F when the fridge is at 35F. So my 25 for R is too low for when the fridge is near freezing, which is when the fin is 25F... Are we going crazy yet? [emoticon]

I think wnjj and I are on the same page in all this. Anybody not with us on that? If not, please explain.


Definitely not the same. 1 MOhm = 1000 kOhms. (Mega vs kilo) Now your meter may have a MOhm setting that can still read something like 0.022 which would be 22k.

Otherwise yes, we agree. [emoticon]

BFL13

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Posted: 02/11/20 05:38pm Link  |  Quote  |  Print  |  Notify Moderator

I was not reading the meter correctly. It starts off with MOhms symbol at the top with OL meaning "open leads", then I put it on the thermistor and it reads 9.19 kOhms.

I did not check the units before, just the number and ASSumed it was still in MOhms. Seems it will read in either depending on how much R there is. So that solves that. [emoticon]

dougrainer

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Posted: 02/12/20 05:36am Link  |  Quote  |  Print  |  Notify Moderator

Only 2 refer temp control systems
1. AUTO TEMP refer----NO tstat settings
2. Tstat Refer--- adjustable tstat settings

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