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June 17th, 2011, 07:05 AM
#31
Ok, so this time I saw the evap starting to freeze up after only 1-2 weeks. I got the part in (ordered 2 thermistors) so figured I needed to see what was up. This morning I cut out the old thermistor and did Shawn's science experiment to test. I got some inconsistent readings so I put the thermistors back in the aluminum block to level the numbers out and account for any inconsistencies in the ice water bath. Here is what I found:
New thermistor: 16.18k ohms which is between 0 and +1 C, call it 33 degrees F
Old thermistor: 21.65k ohms which is between -5 and -6 C, call it 24 degrees F
So, the old thermistor seems to think it is 9 degrees F COLDER than the new/accurate one. So in actual use (assuming these values hold - bad thermistor may vary?) my assumption/conclusion is that if the motherboard is going to 'defrost' at say 32 deg F, the real temp with new/good thermistor is 33 deg F so evap is still not(or barely) frozen, but with the old thermistor the real temp of the evap is 24 deg F which is below freezing, so the evap is pretty much always at freezing or below level and never really defrosts.
Shawn: Does that make sense - is that what is happening and in this case the new thermistor *should* fix the problem?
I've taken some pictures along the way and will post them as I progress. My gallery is HERE.
One thing I did see is that the thermistor on this evap is 'special' it has an extension with ribs to hold it in the block, then the wires are on a right angle. I guess the new one I can just hold in place with a blob of silicone. The picture of the old thermistor is called 'Evap Thermistor' in the Gallery.
Last edited by STWD; June 17th, 2011 at 07:22 AM.
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June 17th, 2011, 03:25 PM
#32
Go ahead and change the thermisstor and keep an eye on it. However when I say keep an eye on it understand that you do have to put it back together because if not you will not get proper airflow to cool the refrigerator and without the evaporator cover the coils may frost up regardless.
The way this works is the refrigerator will open the three way valve to allow refrigerant to flow to the coils in the fresh food section to start a cooling cycle and they should start to frost up. Then when the main board “decides” it is cold enough in the fresh food section it shuts off refrigerant flow to the fresh food coils but it leaves the fan on for defrost. Then with the fan blowing on the coils the ice that built up during the cooling cycle will melt off. But lets say the temperature rises in the fresh food section. Now the thermistor in the upper part of the fresh food section will be “calling” for the main board to start the cooling cycle again. However the board has to “check in” with the fresh food evaporator thermistor to see if the ice is off the coils. If the thermistor is faulty it can “say go ahead and start the cooling cycle again all the ice is melted” when really still needs to be in defrost mode. I hope this makes sense!
So we do have other things that can cause this but the thermistor is the most likely and cheapest however not exactly the easiest as you well know.
When you buy the replacement thermistor you will not get the one with the extension with ribs. GE just says to put the thermistor in the block and use silicone to keep it in place.
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