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jim4568  
#1 Posted : Monday, August 20, 2012 6:13:27 PM(UTC)
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jim4568

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I am stumped! A little over a week ago our refrigerator (no freezer--it's an all refrigerator unit) started freezing everything when set a the same thermostat position as it has been for the last 8 years. Turning the thermostat warmer didn't help--the compressor just kept running all the time and thermometers I put inside indicated that the temperature was around 28deg. I looked at the schematic on the back and determined that a failed thermostat (stuck with the contacts closed) could be the only cause for it to continue to run continuously so I ordered a new thermostat. It came and was simple to install but nothing changed--it just keeps running forever until things freeze. I talked to an "expert" on one of those ask a question web sites and he had no other ideas except that the replacement thermostat was bad or the wrong part type. I finally called a local repair guy who came out the next day and spent an hour looking at everything--the compressor, the evaporator coils, etc. He said everything looked normal so it must be the new thermostat. He then put a temporary thermostat in place (bypassing the normal one)--a Supro SC1002 plus a temperature monitor to monitor the temperature over the next two days. Three hours later the temperature was again below freezing (as determined by two thermometers in the fridge and frozen water in a cup) so I turned the temporary thermostat to its lowest setting and it's still running at 32deg inside. I can make it turn off by moving the control on the SC1002 to the OFF position but when I turn the control back on at the warmest setting it comes on again after a few minutes and runs forever. It acts like the thermostat contacts are being welded by each turn on event but they are rated for many times the actual current I can measure (the compressor draws 1.04A when running) so that seems unlikely. For the last week I have been running the refrigerator using a simple on/off timer that I can set in 30 minute increments. That keeps the inside temperature between 35deg and 45deg with 60 minutes on and 90 minutes off. When running the temp falls by 10.7 degrees per hour and when off (at about 78deg ambient) the temp rises by 7.2deg per hour. The good cool down rate says the compressor and evaporator coils and fan are running fine and the defrost cycle happens as expected about once per day. Does anyone have an explanation as to how this can happen?
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richappy  
#2 Posted : Tuesday, August 21, 2012 1:54:39 AM(UTC)
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richappy

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Unit may be low on refrigerant, measure the stabilized freezer temperature and post.
jim4568  
#3 Posted : Tuesday, August 21, 2012 5:37:34 AM(UTC)
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There is no freezer compartment in this refrigerator--it's a Whirlpool Sidekick unit. The repair guy looked at the evaporator coils and saw them pretty uniformly frosted and he touched them all over and seemed to be satisfied with the "feel". Then, using thermocouples, he measured the evaporator coil temp at the input from the compressor (7F) and at the end of the coil it was 12F if I remember correctly. Then he stuck the end of the thermostat probe right inside the coil (instead of it being in the path of the airflow a little over an inch above the coil) and it still didn't turn off. That is probably not a valid thing to do since he said that at least the last six inches of the thermostat probe needs to be in the cold sensing area. Also, if the unit was low on refrigerant would it be able to maintain 40F running only 40% of the time using the timer?
richappy  
#4 Posted : Tuesday, August 21, 2012 2:00:34 PM(UTC)
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Go to Google and search for "slow leak in a WP sidekick" Very good info there. Your refrigerator is running as a chiller with low side system pressures around 18- 20 lbs, but in your case, probably less than that.
Your system probably has a refigerant leak with a larger than normal capillary line. Apparently, if you have a leak, Super seal stop leak will work.
You have a diferential control in this fridg. Probably trip point around plus 10 degrees. If the evaporator temperature is too high, say 15 degrees, your fridg temperature will follow a log curve and never reach the control trip point. In the limit, will try to reach 15 degrees, but thermal losses will limit the temperature.
Be on top of this problem, if it has an evaporator leak, at some point, it may get worse, suck in noncompressables and moisture and damage the compressor.
jim4568  
#5 Posted : Tuesday, August 21, 2012 6:05:05 PM(UTC)
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jim4568

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Thanks--your explanation sounds reasonable. I still have one question though. The repairman left the Supco SC1002 temporary thermostat in place which senses the air temperature at the thermostat instead of the air coming off the coil. This thermostat doesn't seem to turn the compressor off when set to its warmest setting (1) even when a thermometer right by it is registering 30F or below. According to the spec on the SC1002 it looks like it should turn off the compressor at 40F at the lowest setting. Any idea why that would be?
jim4568  
#6 Posted : Tuesday, August 21, 2012 6:27:21 PM(UTC)
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jim4568

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I looked at many posts from various forums on Google and it does look like a slow leak would explain the problem. I'm probably in the early stages of the leak now where the airflow coming off the evaporator coils is just barely above the temp that would turn the thermostat off. Most end up replacing the whole refrigerator because of the cost to repair a leak being so high. I took the items out of the fridge that will be ruined by freezing and will let it run overnight to see what happens. I will let you know what we end up doing and, again, thanks for your advice.
richappy  
#7 Posted : Wednesday, August 22, 2012 12:14:59 AM(UTC)
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If you do not fix the item, I would use a ambient controller on this a GC505. I have used this on a small chiller. The cap tube on this must be exposed directly to the ambient air in the unit.The repair person did not know what he was doing. Lots of times a universal control will not work properly as the sensing tube cannot be inserted properly in the evaporator air stream.
If it has a very slow leak, it should be charged up to the pressures mentioned and put on watch for indications of a leak.Failure to do this will ensure reduced efficiency. The leak may be a very small one and stop leak will work, but not if the leak is internal to the compressor. All evaporator leaks I have seen are rather bad and end up damaging the compressor, so vigilence is needed. I have never used stop leak for that reason, but you might seek a repair person who has used it before.
richappy  
#8 Posted : Wednesday, August 22, 2012 3:40:49 AM(UTC)
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The verification of a bigger diameter cap line and a "normal" low side pressure of 18 - 20 degrees would verify you have a "chiller".
So, if true, you might have an airflow problem due to a bent evaporator fan. All chillers I have worked on were display units with glass doors that were opened often so high evaporator air flow was needed to chill the box down fast. Also, the units had a relatively high hp compressor to handle the increased enegy use.
jim4568  
#9 Posted : Thursday, August 23, 2012 6:38:59 PM(UTC)
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jim4568

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After two days with a temperature logger in the refrigerator here is what I found:
1. If you just let it run the temperature will get down below 20F--the thermostat never turns it off and things start to freeze.
2. With my little timer set to turn power on for 60 minutes followed by off for 90 minutes the internal temp cycles between 30F and 40F.
3. Your explanation must be correct--the coils get cold enough to really cool the refrigerator but not cold enough to shut off the thermostat.
4. I tested the original thermostat in a freezer and it seems to work fine.

So, assuming I have a slow leak, it is not a simple fix because 90% of the coils are inside the refrigerator side walls and inaccessible. I have decided to get a "temporary" thermostat that mounts over the opening for the normal thermostat but senses the air inside the refrigerator (not near the evaporator coils). If it is a very slow leak it may work for months or years so I will have to monitor it frequently. When it can no longer hold a reasonable temperature I will buy a new refrigerator.

Thanks again for your advice.
richappy  
#10 Posted : Friday, August 24, 2012 12:40:22 AM(UTC)
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richappy

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Sounds like a good plan as a bad evaporator leak should have ben observed by now so you probably have a microcrack leak, or internal compressor leak.
It might be worthwhile to re-charge to get the efficiencies up, or you will still have efficiency problems with the new controller.
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