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 > Moving fridge thermistor up = warmer??

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wopachop

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Posted: 06/24/20 06:09pm Link  |  Quote  |  Print  |  Notify Moderator

I wonder which cooling fin Dometic wants the thermistor attached to? The instruction manual does not show a picture or mention it. On Dometic website now and not seeing much.

Im using a surface temp gun and i realize its a wet shiny surface but hoping its somewhat accurate enough. Main goal is to determine differences in fin temp. That far right fin sure seems to be colder than its brothers and sisters 4 inches to the left. Agrees with what the link was saying.

wopachop

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Posted: 07/15/20 10:24am Link  |  Quote  |  Print  |  Notify Moderator

dougrainer wrote:

wopachop wrote:

Now if I pull the thermistor completely off the cooling fins, shouldnt I see fridge temp get very low before it cycles off and allows temps to rise?


Possibly, Some Control boards react differently. There is a resistance/Ohm signal from the Thermister and if out of normal range may cause malfunction of the Cooling cycle. On Norcolds, if you disconnect(unplug) the Thermister, it will cause a BOS cooling cycle. This causes the refer to alternate between cooling and shutting off at timed intervals. But, for a lot letting it hang down will cause 24/7 operation, until the refer Thermister gets cold enough if that is possible. Remember, The Thermister senses the FIN temp, not the air temp. The fin temp will be 10 degrees colder than the actual refer temp. No harm in trying, post what happens. Doug
Im still confused on how the control board is reacting to the thermistor. I was taking surface temp readings of the fins and moving the probe to different locations. I cant seem to develop any patterns besides the dang fridge being 42-44F.

Ive tried pulling the thermistor off the fins several times. I was hoping that would cause 24/7 cooling, since the probe would be reading air temp. But that doesnt seem to be the case, as my graph of the temps show it cycling off and on.

In desperation i installed a larger computer fan into my center ductwork. Im still fiddling around with the speed control to see if there is such a thing as "too much air". Im guessing that running it full blast would blow out the propane. Im on electric.

Also put a larger fan inside the fridge. I have 2 fans now that are double sided tape to the roof. With the probe laying in the drip tray im finally cycling between 36-38F.

That part im confused about is why isnt my fridge iceing up with the probe completely off? I was just gone for a few days and worried i was going to come back to frozen stuff. That happened weeks ago when i first pulled the thermistor off the fins. It did a big long cooling cycle down to 36F. But then rose into the 40s and cycled there. With the thermistor hanging in the air. Just didnt make sense.

mchero

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Posted: 07/15/20 10:52am Link  |  Quote  |  Print  |  Notify Moderator

I would think you would want to see ice on the fins. Our Norcold 4 door runs 24/7 on AC.
Temp set on panel at 4, 9 being the highest. Outside temps during day are between 80 and 90 deg with hummidity around 75%. Our fridge temp hovers at 32 deg. We have a two fan blower on the fins. Ice on end fins.


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dougrainer

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Posted: 07/15/20 11:50am Link  |  Quote  |  Print  |  Notify Moderator

1. Norcolds have a "defrost" cycle built in for the bottom part of the refer. What Norcolds do is, every 48 to 54 hours the refer shuts OFF for about 2 hours to allow the evap fins to defrost any ice build up
2. ICE is NEVER a good thing on Evap fins. ICE is a insulator and will yield warmer temps in the lower section.
3. There is NO reason to attempt to move the Thermister probe end around. It does not make any difference at all
4. AS to hanging in the air. Remember I told you when you DISCONNECT a Norcold Thermister it puts the refer in BOS mode. As I stated, I am not sure how the refer sensor system would react with the Sensor OFF the fins. I suspected with out a normal OHM return signal it would go into BOS mode. Your testing seems to confirm that.
5. At this point if I had your refer in the shop. I would verify the 2 cooling fans operate and would hot wire both 120 elements for 24 hours. After 24 hours with the fans running, if the refer failed to get below 36 to 38 degrees-----Your CU is bad. Doug

PS, The defrost cycle only happens when the refer is left ON continously. If you turn the refer OFF and ON before the first 48 hours is reached the timer resets. Some fulltimers(that know the defrost cycle) get in the habit of turning the refer OFF and ON every day to prevent that defrost cycle.

Wishbone51

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Posted: 07/15/20 11:54am Link  |  Quote  |  Print  |  Notify Moderator

Deleted ... Nevermind


2017 Jayco Jay Feather 25BH
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wopachop

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Posted: 07/15/20 12:12pm Link  |  Quote  |  Print  |  Notify Moderator

I was thinking about the out of normal OHM range. But even if that existed for my model it would be towards the benefit of cooling.

When you first start the fridge it will be ambient temps. So the probe will send that OHM signal. Fridge cools to 38F. Now with the probe off the fins, should the cooling continue to get much cooler? Since the probe is normally reading the fin temp that Doug described as being 10 degrees cooler. I would think a probe hanging in the air would have to read around 28F to send the correct OHM reading to shut down.

My particular dometic fridge does not have the temperature setting dial. But i still enjoy reading about other models and how they act. I want to find the OHM specs and test my probe.

dougrainer

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Posted: 07/15/20 01:42pm Link  |  Quote  |  Print  |  Notify Moderator

wopachop wrote:

I was thinking about the out of normal OHM range. But even if that existed for my model it would be towards the benefit of cooling.

When you first start the fridge it will be ambient temps. So the probe will send that OHM signal. Fridge cools to 38F. Now with the probe off the fins, should the cooling continue to get much cooler? Since the probe is normally reading the fin temp that Doug described as being 10 degrees cooler. I would think a probe hanging in the air would have to read around 28F to send the correct OHM reading to shut down.

My particular dometic fridge does not have the temperature setting dial. But i still enjoy reading about other models and how they act. I want to find the OHM specs and test my probe.


Norcolds, unlike Dometic, actually MONITOR the slow progression of the cooling cycle. Lets say 70 degrees is the Interior start temp. The system watches the Temp drop. A Correct CU operation will yield 1 degree temp drops. So, 70 then 69 then 68 and so on. The usual fault point is around 50 degrees. If at some time, the refer while trying to continue cooling down reaches a plateau where the temp fail to continue dropping(way above a correct full cool down), the Norcolds will throw what is Called a NC or NO CO code. NO COOLING! At this point you can disconnect the Thermister and the Norcold enters BOS mode (Back up operating). This causes the refer to run for a set time then shut off for a set time then start again and continue the start and stop. Now, as I stated, I do not know what the parameter is when you have the Thermister sense AIR only temp and it CANNOT see a steady progression of a cool down. Dometics operate slightly different and usually the hanging Thermister causes full cool down. Doug

wopachop

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Posted: 07/15/20 07:34pm Link  |  Quote  |  Print  |  Notify Moderator

What can we conclude from this graph? I had been watching the cycles this morning and it seems to turn off and on every 1 hour. So i started to wonder if the fridge was in some type of timed mode.

But then you can see later in the day the cycles took a longer period of time. So i dont think its doing a timed cycle anymore.

I dont get why the thermistor is telling the computer to end the cooling cycle before it goes down like 36F like it did earlier in the day.

[image]

dougrainer

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Posted: 07/15/20 08:03pm Link  |  Quote  |  Print  |  Notify Moderator

wopachop wrote:

I was thinking about the out of normal OHM range. But even if that existed for my model it would be towards the benefit of cooling.

When you first start the fridge it will be ambient temps. So the probe will send that OHM signal. Fridge cools to 38F. Now with the probe off the fins, should the cooling continue to get much cooler? Since the probe is normally reading the fin temp that Doug described as being 10 degrees cooler. I would think a probe hanging in the air would have to read around 28F to send the correct OHM reading to shut down.

My particular dometic fridge does not have the temperature setting dial. But i still enjoy reading about other models and how they act. I want to find the OHM specs and test my probe.


I missed part of your post. I believe you stated you do NOT have a Temp adjustment button or knob. That means you have a AUTO Temp Dometic. That means you can slide the thermister up and down on the fin, but that will only change the interior temp 4 degrees at best. A BAD Thermister will still OHM out correctly. Doug

dougrainer

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Posted: 07/15/20 08:06pm Link  |  Quote  |  Print  |  Notify Moderator

Dometic Thermister OHM chart. Doug


https://www.fixya.com/support/t19425444-domatic_rm_2852_refrigerator_compartment

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