Refrigerator Thermistor Troubleshooting

I have a Norcold N611v, 1 year old now.  My interior temps have been too cold, 26 - 30 degrees on average.  I have moved the thermistor around on all different fins, according to the manual, and up, down and sideways!  This does not make the fridge run any warmer.  Some food freezes, but not too much.  I could live with it the way it is, but I know something is wrong as the fridge should not be running this cold.  Today I tested the thermistor with my multimeter.  With the multimeter probes inserted into the thermistor connector, and the thermistor in ice water, I did not read any ohms...

The metal contacts are very small and hard to see, but I'm fairly certain that I had the positive and negative multimeter probes making contact with the correct pins.  The multimeter ohms setting is working correctly as I did a continuity test on a piece of wire and it beeped.  Is it possible that my thermistor has failed, yet the fridge is not freezing all my food?  I was under the impression that the fridge will run continually and freeze all food if the thermistor is bad.

I want to understand the problem better before I correct it.  If the thermistor is really not functioning, I need to understand why the fridge is only too cold by about by 10 degrees.  Is this considered normal.  If I need to replace the thermistor, I am considering the snip the tip by Attica.  Any thoughts on my problem and the Attica?

Welcome to the RV Forum Oroman1

First thing I will address is how are you loading the refrigerator. My wife was cramming all kinds of stuff right on top of the fins. this was causing most of that stuff to freeze.

Second I will say that the RV gas refrigerators are not the most accurate when it come to maintaining a constant temperature.

As far as your testing of the thermistor ..Sorry I can't help with that

From your diagnosis it sounds like you have a bad thermistor, given their relatively low price (around $15 online on ebay, etc) I would probably just try replacing it at this point.

If your thermistor is bad then the display panel green LED will flash On/Off every 3 seconds and the control board will operate in a backup mode or fixed cooling cycle on the Norcold model you have. Is the light flashing? If not, it is possible that the thermistor has failed in a high resistance mode but not really an open condition (rare) where the resistance is fixed and does not change and this would hold the cooling cycle in a narrow range I think.
Here is a link to the service manual. It's possible the control board is locked up and you can try a reset. The procedure is found in the manual on page 27.

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

You said it was hard to get contact on the tiny pins. Are you disconnecting the coming from the thermistor or leaving them hooked up? They need to be disconnected to get a reading on the thermistor itself, otherwise you are reading the circuit through the board on the fridge.

regval said:

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

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Thanks so much for that link.  I have a Norcold 842 and have been searching for info on the thermistor and fin placement.  This tells me it should be located on the 10th fin but I have a lot more reading to do to figure out my problem - I have to use and 8 to get good cooling when 5 was the max needed.  Anyway, this manual will help.

Bill

Bill N said:

Thanks so much for that link.  I have a Norcold 842 and have been searching for info on the thermistor and fin placement.  This tells me it should be located on the 10th fin but I have a lot more reading to do to figure out my problem - I have to use and 8 to get good cooling when 5 was the max needed.  Anyway, this manual will help.

Bill

Click to expand...

You're welcome. Good luck with your issue.

Thanks regval:

Ends up I was not getting enough contact with the tiny pins inside the frige.  Soo, I measured ohms where the thermistor wires connect to the power board.  Bingo.  Got readings way to high at 45 degrees, when I turned the unit on, and at 32 degrees where the fridge is currently turning off the heat at.  Ohms should be about 31,000 at 32 degrees.  My thermistor is reading 40,800, 33% too high!  The RV is only 1 year old, just past warranty.  How can the thermistor go bad so quickly?  Bad thermistor?  Is it that simple?  Knowing what I know now, I wish I had gotten a refrigerator with a temp adjustment knob independent of the fin adjustment.  My Norcold 611 does not have any temp adjustment capabilities other than the fin, which did nothing for my problem.  I have looked at the Attica snipthetip product and it seems like it works from all the videos I have reviewed.  I did ask the Attica supplier for the ohm range associated with the adjustment knob before I purchase.  This product will supposedly give me about a 10 degree, or more, adjustment range, which I would not get it I replace the thermistor with an OEM part.  What do you think?

research "Snip the tip"

Oroman1 said:

Thanks regval:  I have looked at the Attica snipthetip product and it seems like it works from all the videos I have reviewed.  I did ask the Attica supplier for the ohm range associated with the adjustment knob before I purchase.  This product will supposedly give me about a 10 degree, or more, adjustment range, which I would not get it I replace the thermistor with an OEM part.  What do you think?

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I've never used the Attica, so I can't offer an opinion on it's effectiveness or reliability. The standard thermistor replacement may fit your needs and solve your problem but it requires you feed the wire of the new one back out the refrigerator to the control board. If you choose to stay with the same type of thermistor you have, the easiest way to install it to detach the cooling fin clip from the bad thermistor and tape the new thermistor's connector end to the old thermistor wire inside the frig and go out to the refer compartment and pull on the old thermistor wire, pulling the new thermistor's wire back out the rear of the refer using the old thermistor as a fish wire. Don't use a lot of tape connecting the two wires or it may get stuck. Connect the new thermistor to the control board and test operation.
Good luck.

regval said:

the easiest way to install it to detach the cooling fin clip from the bad thermistor and tape the new thermistor's connector end to the old thermistor wire inside the frig and go out to the refer compartment and pull on the old thermistor wire, pulling the new thermistor's wire back out the rear of the refer using the old thermistor as a fish wire. Don't use a lot of tape connecting the two wires or it may get stuck.

Click to expand...

Having a 2nd person inside pushing will also help.

Snip the tip and you'll be ok and very happy with your fridge.  Snip the tip and add a couple of cooling fans in the roof vent and you'll have a  fridge that works as well as a home one! 

Thanks Drewd:

I understand from your previous posts that you installed one.  Can you tell me what the actual ohm and/or temperature range is on the adjustment knob?

Well I got my Dr YouTube education today on the Norcold thermister and saved myself a bit of money.  I had decided that it was possibly defective because I am having to jack up the temp to the 8 setting when 5 used to be normal.  I watched a couple of YouTube videos on checking the thermister out with an ohmmeter and a cup of ice and mine works fine (after I changed the ohmmeter setting and orginally thought it was cutting out)  Anyway the part checked out good but in the video and on my coach the thermister is clipped onto one of the fins near the right side but the service manual clearly states that it should be clipped on the 10th fin FROM  the right.  The guy in the video stated that it should be on the 10th fin and then started counting from the left.  It also appears that the thermister was originally installed on a fin that is much closer to the right side than the 10th fin. So I will try that and see if it makes a difference.  I will also clean the burner but the #8 setting seems to be common for both AC and LP.

Oroman1 said:

Thanks Drewd:

I understand from your previous posts that you installed one.  Can you tell me what the actual ohm and/or temperature range is on the adjustment knob?

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I'm sorry I can't.  My wiring is soldered and sealed up with heat shrink and my Fluke digital multimeter is at my primary residense-I"m a fulltimer in my RV working out of state.

Update on My Norcold Thermistor:

I obtained new thermistor resistance measurements by taking the thermistor out of the frige and onto a bench.  Used water in a jar with a thermometer and a correctly operating multimeter.  At 32 degrees, I get a value of 31,000 ohms...right where it should be.  However, at 40, 45, 50, 55 and 60 degrees, my measurements are about 2,500 ohms higher than the midrange values.  Not sure if this means that the thermistor is causing the excessive cooling since the ohms are correct at 32 degrees.  As I said, the refrigerator controls are not shutting off the heat source at say 37 degrees, as it should, but continues to cool, shutting off heat at about 28 degrees.  Since the thermistor is wired to the power boardboard, and I think the power board is responsible for shutting off the heat at the pre-set temperature, maybe the power board is bad? 

New Info Confusing:

Looking over the Norcold Service Manual, Publication No. 636355B, 2016. Under one of the Theory of Operation sections, it indicates that the power board measures the thermistor value and communicates the thermistor value to the optical display board.  The optical display board compares the thermistor value obtained from the power board and compares to the factory pre-set cut-in and cut-out values.  The optical display board will then request that the power board turn on or off the heat source as needed to maintain temp within cut-in and cut-out values.  Now it seems that the problem with the frige running too cold could be either power board or the optical display board?  Anybody know how to proceed with this info?

Oroman1 said:

New Info Confusing:

Looking over the Norcold Service Manual, Publication No. 636355B, 2016. Under one of the Theory of Operation sections, it indicates that the power board measures the thermistor value and communicates the thermistor value to the optical display board.  The optical display board compares the thermistor value obtained from the power board and compares to the factory pre-set cut-in and cut-out values.  The optical display board will then request that the power board turn on or off the heat source as needed to maintain temp within cut-in and cut-out values.  Now it seems that the problem with the frige running too cold could be either power board or the optical display board?  Anybody know how to proceed with this info?

Click to expand...

I don't see where you've stated If there are any fault codes on the panel, or that the green or Amber light is flashing. The green light flashing every 20 seconds (my previous post said 3 seconds) is if refer is running on AC volts and indicates the optical board sees the thermistor as faulty and goes into a fixed cooling cycle. If there is no blinking green light on the panel (or blinking yellow light if in the propane mode) then I don't think your power board or optical board has failed but it has the symptoms of being stuck in the BOS cycle.
With both AC power and DC power disconnected from the refrigerator, access the optical board and the power board and remove and reconnect each electrical connection between the the two boards in case you have a marginal connection on one of the harnesses. That would be my next trouble shooting step, but that's your call.
Good luck.

I have a simple request. Would anybody who has a Norcold model 842 refrigerator look at the fins inside and tell me how many fins from the right is the thermistor mounted on.  The service manual says it should be on the 10th fin from the right but it does not appear that mine has  ever been on that one judging by the rust marks left by the clip holding the thermistor.  Thanks.

Again Thanks Regval:

I will try your suggestion.  Do you agree with my thinking that the thermistor is operating within specs?  No there are no lights of any color blinking on the frige front.  My model has no numerical or letters diagnostics displays, only one light next to the power on/off button.  Apparently the frige controls thinks that the thermistor is good, since on my model, a bad thermistor creates a blinking light on the optical panel.