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Fridge WRS321SDHZ08 "Adaptive Defrost Controller" function not working, anyone understands its inner workings?


huron

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This fridge was bought from a tent sale clearance new with some cosmetic damages.  It was sold as is with no warranty.

The refrigerator operated fine for a month (used infrequently) and then I noticed that there was ice buildup on the evaporator coil.  The defrost cycle was not coming on.  Got into the service mode and I was able to manually activate the defrost cycle and saw the heater come on.  This tells me the defrost activation function, relay and wiring were OK.  The service sheet said the defrost heater should be about 600W, that translates to roughly 24 ohms.  I measured my heater and found it to be 120 ohms (about 120W) and thus concluded that the heater was not getting hot enough to melt the ice during the allotted defrost cycle time.  I bought a new defrost heater and installed it.  Several weeks later, I found ice buildup again so the new defrost heater did not fix the problem.

I verified again that the service mode activates the defrost heater and it does.  I verified that the defrost thermistor (2.7K@room) and it's resistance matches its sensed temperature when cold.  If you see the attached schematic, there are no defrost timer or defrost thermostat, it's all microprocessor controlled.  Running out of ideas, I bought a new identical controller board (W11321510 ) and put it in.  Still the same problem, no defrost cycle!  I have put a data logger on the defrost heater element and it never sees any voltages at all, that is to say that the defrost heater is not shutting off prematurely.

I checked the freezer door switch in case the adaptive defrost controller is counting the number of door openings, the switch's NO/NC contacts both work.  I have run of ideas other than some wiring harness problems, even then, what wiring could be wrong?

Anyone knows in more details how the adaptive defrost controller looks for in terms of environmental conditions that it needs to see in order to trigger the defrost cycle?  BTW, the FC and RC both regulates temperature fine.

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Replace the defrost control the exact logic of the board is not documented nor does it matter. 
 

You have proved it’s not initiating defrost on its own. It needs replaced. 

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18 hours ago, huron said:

I bought a new identical controller board (W11321510 ) and put it in.  Still the same problem, no defrost cycle!

@16345Ed Looks like he already tried that

I haven't looked up anything on this so don't know if there are more than one board and maybe wrong board replaced.

Edited by Budget Appliance Repair
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 Junk that plastic clip and zip tie that thermistor straight to the evaporator.

Edited by MrApplianceMatt
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