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Debugging GE GSH25JFXB Defrost Problems


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16 replies to this topic

#1 mpetersen

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Posted 21 July 2013 - 12:52 PM

Should the GSH25JFXB enter a pre-phill mode prior to defrosting? Everything I've read about the GE Adaptive Defrost mentions a pre-chill mode where the temperature dives 15 degrees before the defrost heater is activated. However, my fridge is showing no signs of this. See attached temperature plot, purple trace. Should this fridge model be performing a pre-chill before defrost?

Background: I am debugging a defrost problem. The defrost cycle is melting things, including ice in the ice drawer and causing ice buildup. I've seen temperatures as high as 80F during defrost at the evap thermistor and at the top of the freezer near the ice maker. The bottom of the freezer, near the thermistor, has also gone as high as 40F during defrost. Both thermistors in the freezer test correctly.  I have also tried terminating the defrost early by shorting the evap thermistor at the control board when the temperature rises to ~45F - this appears to work.  Details:

 

http://blog.iascaled...rator-problems/

So the reason things are melting is clear, but the reason it is getting so hot is not. The lack of pre-chill seems possibly a contributing factor, but I have not found anything conclusive that my model should do this.  If it shouldn't, then I'll keep looking elsewhere.  If it should pre-chill, then I'm guessing the control board may be faulty.

 

Thanks in advance,

Michael

 

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#2 mpetersen

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Posted 21 July 2013 - 12:55 PM

Does the GE GSH25JFXB have electronic dampers between the frdige and freezer? The user manual that came with it mentions them (though I suspect the manual is a superset for all models), as do Technical Service Guides for similar models (I have yet to find one for this specific model, though). However, there is no electronic damper that I can find between the freezer and fridge sections. The lower opening is just that - an opening. The upper one by the light does have a clear plastic damper, but it is free swinging - no electronic control.

What confuses me, however, is where the wires connected to J3 pins 1-4 on the control board go. From everything I can find, these are listed as Damper control wires. However, if there is no damper, why are these installed on my fridge? There are many other wires not installed in the connectors for features not present in this model, so clearly there was some thought given to only installing what was necessary. Having these wires present makes me think they must do something...  Yet I can't figure out what that something is.

I am mainly curious at this point, but would like to understand it as I debug a defrost problem:

 

http://appliantology...frost-problems/

Thanks in advance for your help.

Michael



#3 Samurai Appliance Repair Man

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Posted 21 July 2013 - 02:11 PM

Does the GE GSH25JFXB have electronic dampers between the frdige and freezer? 

 

 

No.  Your unit uses the infamous, yet elegant mylar damper assembly.  

 

The damper would not affect the defrost problem.  

 

The evaporator thermistor would.  

 

And does.  

 

Often.  

 

Almost aways.

 

Count on it.  

 

Replace it.  http://www.repaircli...55X10025/914093

 

Thermistor-WR55X10025-00855831.jpg



#4 mpetersen

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Posted 21 July 2013 - 03:16 PM

I've contemplated replacing the thermistor, but since it tested good over the range from 32F to 90F (within 5% of the expected values), I figured it probably wasn't the problem.  Still might replace it anyway just to be sure...

 

What about the pre-chill mode?  Should I be concerned that I don't see the 15 degree drop in freezer temperature before the unit goes into defrost mode?

 

Thanks,

Michael

 


Also, what do the wires connected to the damper terminals on the control board do since there is no electronic damper?  Do they go to the damper/thermistor/light connector in the fridge section but just aren't used?  (just curious at this point)

 

Thanks,

Michael



#5 Samurai Appliance Repair Man

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Posted 21 July 2013 - 04:43 PM

The prechill operation is part of the algorithm burned into the muthaboard.  It does this based on information it gets from the freezer and evaporator thermistors.

 

You can test prechill in diagnostic mode.  

 

But based on your temperature graph, I'd bet my third nad that the evap thermistor hath shat the bed.  



#6 DurhamAppliance

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Posted 23 July 2013 - 02:10 AM

It is also economically smart to replace thermistors before replacing a board .

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#7 DurhamAppliance

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Posted 23 July 2013 - 02:16 AM

Also, what do the wires connected to the damper terminals on the control board do since there is no electronic damper? Do they go to the damper/thermistor/light connector in the fridge section but just aren't used? (just curious at this point)

Thanks,
Michael

Haven't you heard the phrase "curiosity kills the tech?" More accurately, do you really want to delve into the mind of a GE engineer if you don't have to? If so, do it at your own peril.

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#8 mpetersen

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Posted 23 July 2013 - 07:59 AM

It is also economically smart to replace thermistors before replacing a board .

 

Agreed completely and that's what I shall do.

 

 

Haven't you heard the phrase "curiosity kills the tech?" More accurately, do you really want to delve into the mind of a GE engineer if you don't have to? If so, do it at your own peril.

 

My inner engineer strikes again.  :)

 

Michael



#9 mpetersen

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Posted 08 August 2013 - 10:55 PM

OK, I replaced the thermistor.  No luck.  If anything, the problem got worse.  The defrost stays on longer and now it is long enough to trigger the automatic 8-hour defrost cycle (i.e. no adaptive defrost).  So, I broke down and replaced the control board thinking the off threshold for the evap thermistor input might have drifted over time, allowing it to stay on too long.  No luck.  Same as the old board.

 

In the attached temperature plot, the first defrost is with the new thermistor, old board.  The second defrost is one I forced (via the 1 4 test mode) after replacing the control board.  Same results.

 

Any ideas what might be happening?

 

I have noticed that the evap coil temperature (green line), measured right at the evap thermistor, reaches a plateu right below 70F and stays there for a long time.  It then starts spiking up, shortly after which the defrost cycle is terminated.  Presumably because the temperature crosses the threshold on the control board.  Any thoughts on what could be causing the temperature to flatten out for a time?  If it didn't do that, I think the defrost cycle might be just fine.  I know the defrost heater is on (can see the orange glow) and the temperature at the top of the fridge (red line) right at the top vent is continuing to rise above the evap coil temperature.  The coils must be acting as some sort of temporary heat sink, but is that normal?

 

Thanks again for your help,

Michael

 

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#10 DurhamAppliance

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Posted 09 August 2013 - 06:28 AM

Don't tell me you only replaced one little ol' thermistor? You are having issues with prechill and I think the freezer thermistor is involved in the prechill process. I would change all thermistors in your fridge. I have no support for this but I have seen with my own peepers an apparently good thermistor interfere with board communications. Everyone else thinks I must be on crack. Anyhow, there must be something causing your boards to react the way they do. It could be a remaining thermistor, shorted wire or your temp controller/encoder board. Do the easy/cheap thing first.. ie change thermistorS <----meaning all.

btw you could also replace your defrost limit with one the opens at 55°. Yours open at 140 but there are some GE's that uses your same board that requires the limit to act as a defrost terminator despite the presence of a thermistor. I have not seen this but one of GE's manuals cautioned about one particular model that has this configuration. I don't think it's your model but what the heck, give it a try. At least it will turn off the heater. But don't change too many things at once. Btw pt 2....did you check your board installation instructions carfefully to determine if you need to cut a thermistor wire?

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#11 Budget Appliance Repair

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Posted 09 August 2013 - 07:33 AM

The picture of the evaporator thermistor, which controls the defrost, is still the old style failure prone thermistor.

 

Change the thermistor that is attached to the evaporator coil that you have in the above picture and it may solve all your problems.


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#12 mpetersen

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Posted 09 August 2013 - 07:47 AM

The picture of the evaporator thermistor, which controls the defrost, is still the old style failure prone thermistor.

 

Change the thermistor that is attached to the evaporator coil that you have in the above picture and it may solve all your problems.

 

Oops...  that was the wrong photo.  Sorry for the confusion.  The new thermistor I put in is one of the new style thermistors, all white plastic encapsulated.



#13 mpetersen

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Posted 09 August 2013 - 08:01 AM

Don't tell me you only replaced one little ol' thermistor? You are having issues with prechill and I think the freezer thermistor is involved in the prechill process. I would change all thermistors in your fridge. I have no support for this but I have seen with my own peepers an apparently good thermistor interfere with board communications. Everyone else thinks I must be on crack. Anyhow, there must be something causing your boards to react the way they do. It could be a remaining thermistor, shorted wire or your temp controller/encoder board. Do the easy/cheap thing first.. ie change thermistorS <----meaning all.

 

You might be right...  yes, I only changed the evap thermistor.  That was based on an exerpiment I did earlier in the week.  During a defrost cycle, I monitored the evap thermistor voltage at the control board.  It consistently rose to ~3.4V or so, flattened off for a while (just like the temperature), and then spiked.  Once it rose above ~3.47V the system shut off the defrost.  Then, during a subsequent defrost cycle, I put a 12k resistor in parallel with the evap thermistor.  If 3.47V was the threshold, then the defrost should terminate at about 40F (evap):

 

70F thermistor = ~6k

40F thermistor = ~12k

40F thermistor || 12k = ~6k

 

Sure enough, the defrost terminated early once the evap temperature rose above ~40F.  This confirmed to me that the freezer thermistor played no role in terminating the defrost cycle - just the temperature at the evap thermistor was sufficient.  Plus, I had seen no reference to anything else in any of the GE technical literature.

 

So, I have simply been trying to address why the defrost was staying on so long.  However, you have a good point that there still is no sign of pre-chill.  I am going to let the new control board run for the day and see what happens.  I may have been premature last night in dismissing it because I did force a defrost soon after installing the new board, not giving it any chance to run a full cycle.

 

btw you could also replace your defrost limit with one the opens at 55°. Yours open at 140 but there are some GE's that uses your same board that requires the limit to act as a defrost terminator despite the presence of a thermistor. I have not seen this but one of GE's manuals cautioned about one particular model that has this configuration. I don't think it's your model but what the heck, give it a try. At least it will turn off the heater. But don't change too many things at once. Btw pt 2....did you check your board installation instructions carfefully to determine if you need to cut a thermistor wire?

 

Interesting idea.  Might be a fallback plan.  Yes, I did read the instructions carefully.  My model is not one that needs a thermistor wire cut, at least according to the instruction sheet I received.



#14 mpetersen

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Posted 01 September 2013 - 10:51 PM

Just an update.  Replacing the thermistors did not work.  Nor did replacing the control board.  Ugh.

 

So, I installed a 24k resistor in parallel with the evap thermistor.  Based on previous experiments this causes the defrost to terminate earlier.  So far, so good.  No melting ice.  No excessive temperatures during defrost.  And no glacier on the evap coils.

 

Although I haven't found and fixed the root cause, this seems good enough.



#15 DurhamAppliance

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Posted 01 September 2013 - 11:37 PM

check your evaporator from top to bottom with a laser thermometer regardless of how your frost pattern appears. Your prechill may not be cold enough and you may think your defrost is on too long simply because your evap is not cold enough from top to bottom. Especially the top 1/4 of your evap. Had to condemn a similar model for this reason 2 days ago.

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#16 mpetersen

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Posted 01 September 2013 - 11:48 PM

When is this best checked?  After the compressor has been running for a while?

 

What should I expect for a temperature on the coils?  Or is the problem that there is a major difference from top to bottom?  I've wondered if this might be the problem - no getting cold enough before entering defrost, but have not found a good way to prove it, nor any data tell me what to expect.

 

If this turns out to be the problem - not getting cold enough - is there any hope to repair it?



#17 DurhamAppliance

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Posted 01 September 2013 - 11:58 PM

Check it after it has been running for at least an hour.


here is the frost pattern i saw... i did wipe some light dusting off the top few coils but the top coils never got below 35f.. Before i wiped the dusting off, the frost pattern initially looked good although the frost at the bottom appeared heavy. The freezer occasionally get down to the proper temps and the fridge was about 4 degrees too warm.

20130830_122759_zps132ab720.jpg

Here is some info from Budget Appliance from another thread that may shed some light... btw he suggests running compressor for 2 hours before you check it... also make sure to put the evap cover in place.

Now he is referencing a different fridge and not the above picture nor is his comment directed at this particular thread...

I think it's a sealed system problem from the looks of one of the earlier and the most recent picture of the frost pattern on the evaporator.

The last evaporator frost picture shown has way to much frost impeding airflow through evaporator so refrig section is warming up. The frost build up is happening because it never thinks it's cold enough so that defrost works properly, the top section of evaporator isn't frosting so the defrost thermistor is too warm so defrost ends early or never occurs and frost continues to build up then blocks airflow.

Need to completely defrost the evaporator again, put evaporator cover panel back on with a few screw and let it run for about 2 hours then take the cover off and post another picture of the frost pattern after running the 2 hours starting with a freshly/completely defrosted evaporator. If the bottom 2/3 to 3/4 of the evaporator are frosted good and again you see the top 1/3 to 1/4 of the coils just wet and sweaty then it's going to be low on freon.


Durham Appliance Thrift & Repair, LLC

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