How to Check for Continuity in a Thermostat

jyhill		#1 Posted : Tuesday, May 3, 2011 11:38:04 AM(UTC) Quote Multi-Quote
Rank: Member Groups: Registered Joined: 6/23/2009(UTC) Posts: 9		The evaporator coils (I think this is the term) in my GE side-by-side (GSS25JFMC) have been frosting over repeatedly every several weeks or couple of months, and I have been pulling off the back panel and using a hair dryer to defrost them. I recently used a multimeter to test the heater element and found that it appears to be working OK. Now I think I need to test the defrost thermostat (AP3884317), and my question is this: To test the thermostat, do I cut the two wires leading to the thermostat and touch one multimeter probe to each wire, listening for the beep or looking for a low or near-zero ohms reading? One online repair guide mentioned slip-on connectors and placing the probes on the "terminals" of the thermostat, but I take it this is for a different refrigerator because I see no terminals or slip-on connectors on my thermostat. Am I correct in thinking that I must cut the two wires, touch the probes to the wires to determine continuity and then splice either a new thermostat or the old one if it is still working back into the remaining hanging wires? Thanks very much for any help you can offer.

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magician59		#2 Posted : Tuesday, May 3, 2011 12:56:49 PM(UTC) Quote Multi-Quote
Rank: Advanced Member Groups: Senior Expert Joined: 8/16/2007(UTC) Posts: 3,273		You are correct about cutting the wires (Just make sure the refrigerator is unplugged, first!). When yu're through testing (or if you have to replace the thermostat) splice the wires back together and secure the splices with wire nuts. When you test the thermostat (also called the defrost terminator), note that at room temperature, it should be open (Infinite ohms). Somewhere below freezing (32 degrees F) it should close (zero ohms--or continuity).

jyhill		#3 Posted : Tuesday, May 3, 2011 1:19:49 PM(UTC) Quote Multi-Quote
Rank: Member Groups: Registered Joined: 6/23/2009(UTC) Posts: 9		Outstanding. Your answer is a big help. I will do that tomorrow and then possibly order the thermostat.

jyhill		#4 Posted : Friday, May 6, 2011 1:01:48 PM(UTC) Quote Multi-Quote
Rank: Member Groups: Registered Joined: 6/23/2009(UTC) Posts: 9		Back again after taking out the defrost thermostat and testing it with the multimeter. Tested it immediately after taking it off the evaporator coil (i.e., still seemed to be quite cold) and then put it into ice for a few minutes and tested again. Both readings appeared to be OK -- i.e., 0 and a beep. So now it appears that both my heater element and defrost thermostat are working. What should I do next to address this problem of the coils frosting over after several weeks or a couple of months, causing the entire refrigerator to warm up? Any guidance you could offer would be appreciated.

magician59		#5 Posted : Friday, May 6, 2011 1:16:37 PM(UTC) Quote Multi-Quote
Rank: Advanced Member Groups: Senior Expert Joined: 8/16/2007(UTC) Posts: 3,273		If the evaporator fan works in the freeze cycle, I would suspect a bad thermistor; or a bad connection between the thermistor and the mother board. If you see frost build up in other spots in the freezer, besides the evaporator coil, you might have an air leak. Check the door seal for any openings or tears.

jyhill		#6 Posted : Tuesday, May 10, 2011 7:46:47 AM(UTC) Quote Multi-Quote
Rank: Member Groups: Registered Joined: 6/23/2009(UTC) Posts: 9		Haven't noticed any frost buildup elsewhere (there is ice often in the drain trough directly under the heater element, but I figure that is part of the evaporator freeze problem). I found the two thermistors around the evaporator coils -- one at the left bottom and the other on the top coil, pretty much like the pictures used here Testing A Refrigerator Thermistor On A GE Refrigerator. By the way, my refrigerator is from before 2005, and I think the thermistors do look like the "faulty" type shown in the illustration on that page. Now I need to test the thermistors. Do I test both? Do I have to find and check the ones in the fresh food section? Can I test them without having to cut them out -- instead using the main-board J1 plug method described at the above link? If I have to cut them out (I'm worried about not having enough wire if they are OK and have to be spliced back in), do I test the continuity with by touching one of the multimeter probes to each of the paired wires (is this called SPT?) running into the thermistor? Again, all instructions are appreciated.

magician59		#7 Posted : Tuesday, May 10, 2011 8:04:45 AM(UTC) Quote Multi-Quote
Rank: Advanced Member Groups: Senior Expert Joined: 8/16/2007(UTC) Posts: 3,273		All thermistor tests are resistance tests. The results should vary according to the ambient temperature. They should be done at the mother board connection. Cutting and re-splicing wires can affect the resistance values significantly enough to give false signals to the mother board. I wouldn't suspect any problem within the fresh food section, but if it's convenient for you to also check that one, it can't hurt. In a lot of cases involving the mother board, problems are obvious; i.e. loose or broken connections, burnt or scortched components, etc. Carefully inspect your board before committing to any time consuming removal of components. Let us know what you find.

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