Fridge Thermistor Replacement Question UPDATE (Fixed)

[BFL13]

Updated 10 Feb. and 14 Feb

My 641 Norcold cools down too cold and won't shut off. It is not in back up power--the temp light does not flash.

I have not yet fiddled with the light/thermistor to see if it is just a bad connection.

There is a replacement part available. As a temporary fix if fiddling doesn't help, can I swap out the sensor (snip the wires) and replace with an old Dometic 6 cu ft fridge's sensor (splicing in its wires to the Norcold's)

AFAIK it is all about the resistance of the sensor, but no idea what the circuit board would think.

http://rvrefrigeratorrepair.com/wp-content/uploads/2016/08/Norcold-Service-N61x-N81x.pdf

Thanks

[BFL13]

So the new thermistor arrived and checked out for proper R vs ambient per the table in the manual. Installed and ran fridge on AC for some hours with the thermistor on the 10th fin from the right per manual and Temp Setting at 4.

So far so good. Able to check fin temp with diagnostic screen 3. Have seen 24-30F spot checking and fridge and freezer temps are in the proper zones and holding apparently. Now I know about that 10 degree F diff, it helps understand that!

I want to confirm by using LP and actually seeing the flame go out and come back on cycling with the right temps holding, but it is looking good so far on AC.

Thanks to the info I got on here, it looks like I can fine-tune it if needed, by changing the Temp Setting and moving the thermistor and watching the fin temps and the fridge temps. I don't think the food cares that much though! Good enough is good enough.

So that's that, and on to whatever will be the next crisis in the RV. Always something! 🙂

[BFL13]

"What you posted is when the Refer is in a FAULT Mode. Norcold calls it a BOS(Back up/Operating/System). This allows the refer to still function when you have a fault with the Thermister or controls. A correct functioning refer will run until the Tstat is satisfied, then shut off. Doug"

I got that from the Norcold NX service manual

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

EDIT-- I read that wrong. Page 13 indicates that BOS (Table 1) can be adjusted by using the Temp Sets in Table 2.

ISTR that Table 2 was elsewhere and it said it uses the Thermistor, so that wouldn't work if the thermistor is bad so it has to be in BOS.

Can't find that, so I must have been mixed up.

[dougrainer]

BFL13 wrote:
I had the wrong service manual linked in the OP, now corrected. My fridge is an N641.3R, not an NX type. Same thermistor specs though.

I just discovered the diagnostic checks in the manual so will check that out. The thermistor has been snipped off, but I get no flashing temp setting, which seems wrong. There is a fault history screen that might say if it saw a fault back when it was running with the suspect thermistor. Will check all that out later.

EDIT--there was a thermistor fault sometime in the past. I erased fault history to start over with the new to us fridge on that.

I can see where Doug has to know all this stuff for so many different models with so many features that have different terminologies in the various manuals. Yipes.

Interesting description on the manual of how the Temp settings work, using the thermistor readings and timing protocols. I see the timing is to have longer time with cooling on and shorter time for cooling off as you go up from 1 to 9. They use "duty cycle" for that in percentage of time on.

But also each setting has its own thermistor reading for when to start and stop the cooling. I can't figure that out. How can the program for duty cycling times in minutes know how long it actually takes to warm back up after it gets to the low temp and shuts down?

From the NX manual:
Setting 2- 18 min on, 42 min off, 30%
Setting 5- 36 min on, 24 min off, 60%
Setting 8- 54 min on, 6 mim off, 90%

So let's say the thermistor reading for turning off is higher for 8 than for 2, so the fin has to be colder before shut off in 8.

Then it is off and the fin warms up. How can the fridge know how long it takes to warm back up? You can have different ambients, effect of venting at the back of the fridge, door sealing, what is in the freezer maybe? It can see when the fin is back up to the restart temp by the thermistor R getting back down to that point.

It does work, so there must be a way it can tell. ?

What you posted is when the Refer is in a FAULT Mode. Norcold calls it a BOS(Back up/Operating/System). This allows the refer to still function when you have a fault with the Thermister or controls. A correct functioning refer will run until the Tstat is satisfied, then shut off. Doug

[BFL13]

wnjj wrote:

BFL13 wrote:
Interesting description on the manual of how the Temp settings work, using the thermistor readings and timing protocols. I see the timing is to have longer time with cooling on and shorter time for cooling off as you go up from 1 to 9. They use "duty cycle" for that in percentage of time on.

I’m not sure if I found the same place but what I saw in the manual discussing timing based cooling was only for Backup Operating System (BOS) mode. That is only in use if the thermistor fails.

Yes, the timing info I quoted was from the manual for the NX models I had linked earlier, but it must be the same for the Ns like mine and others with Temp Settings.

I don't know how they can co-ordinate the warm up "off" times with when the fin actually gets that warm to trigger the thermistor R values that restart the cooling for each setting. Perhaps they just use whichever comes first?

[wnjj]

BFL13 wrote:
Interesting description on the manual of how the Temp settings work, using the thermistor readings and timing protocols. I see the timing is to have longer time with cooling on and shorter time for cooling off as you go up from 1 to 9. They use "duty cycle" for that in percentage of time on.

I’m not sure if I found the same place but what I saw in the manual discussing timing based cooling was only for Backup Operating System (BOS) mode. That is only in use if the thermistor fails.

[kellem]

gkainz wrote:

cavie wrote:
thermister is probably bad. google "Snip-the-the tip". Should fix your problem.

I googled "snip the tip" and got stories about Lorena Bobbit... argh!!!!!

j/k

Oh wow, that one got me....vivid memories of that story surfacing.

[BFL13]

I had the wrong service manual linked in the OP, now corrected. My fridge is an N641.3R, not an NX type. Same thermistor specs though.

I just discovered the diagnostic checks in the manual so will check that out. The thermistor has been snipped off, but I get no flashing temp setting, which seems wrong. There is a fault history screen that might say if it saw a fault back when it was running with the suspect thermistor. Will check all that out later.

EDIT--there was a thermistor fault sometime in the past. I erased fault history to start over with the new to us fridge on that.

I can see where Doug has to know all this stuff for so many different models with so many features that have different terminologies in the various manuals. Yipes.

Interesting description on the manual of how the Temp settings work, using the thermistor readings and timing protocols. I see the timing is to have longer time with cooling on and shorter time for cooling off as you go up from 1 to 9. They use "duty cycle" for that in percentage of time on.

But also each setting has its own thermistor reading for when to start and stop the cooling. I can't figure that out. How can the program for duty cycling times in minutes know how long it actually takes to warm back up after it gets to the low temp and shuts down?

From the NX manual:
Setting 2- 18 min on, 42 min off, 30%
Setting 5- 36 min on, 24 min off, 60%
Setting 8- 54 min on, 6 mim off, 90%

So let's say the thermistor reading for turning off is higher for 8 than for 2, so the fin has to be colder before shut off in 8.

Then it is off and the fin warms up. How can the fridge know how long it takes to warm back up? You can have different ambients, effect of venting at the back of the fridge, door sealing, what is in the freezer maybe? It can see when the fin is back up to the restart temp by the thermistor R getting back down to that point.

It does work, so there must be a way it can tell. ?

[gkainz]

cavie wrote:
thermister is probably bad. google "Snip-the-the tip". Should fix your problem.

I googled "snip the tip" and got stories about Lorena Bobbit... argh!!!!!

j/k

[dougrainer]

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

[BFL13]

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. 😞

[wnjj]

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? 🙂

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. 😉

[BFL13]

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.

[enblethen]

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

[BFL13]

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? 🙂

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