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Open Roads Forum  >  Tech Issues

 > Fridge Thermistor Replacement Question UPDATE (Fixed)

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gkainz

Arvada, CO, USA

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Posted: 02/12/20 08:02am Link  |  Quote  |  Print  |  Notify Moderator

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


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BFL13

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Posted: 02/12/20 08:09am Link  |  Quote  |  Print  |  Notify Moderator

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. ?

* This post was edited 02/12/20 02:05pm by BFL13 *


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kellem

Shenandoah valley,VA

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Posted: 02/12/20 03:11pm Link  |  Quote  |  Print  |  Notify Moderator

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.

wnjj

Cornelius, Oregon

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Posted: 02/12/20 08:55pm Link  |  Quote  |  Print  |  Notify Moderator

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.

BFL13

Victoria, BC

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Posted: 02/12/20 09:31pm Link  |  Quote  |  Print  |  Notify Moderator

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?

dougrainer

Carrolton, Texas

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Posted: 02/14/20 06:54am Link  |  Quote  |  Print  |  Notify Moderator

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

Victoria, BC

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Posted: 02/14/20 08:40am Link  |  Quote  |  Print  |  Notify Moderator

"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.

* This post was last edited 02/14/20 09:09am by BFL13 *   View edit history

BFL13

Victoria, BC

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Posted: 02/14/20 09:56pm Link  |  Quote  |  Print  |  Notify Moderator

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! [emoticon]

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