Brand: Maytag Model Number: MBF2258XEW1 Main Symptom: Iced up evaporator, warm fridge, repeatedly failed defrost thermostats What happens & when: Three times this year, have had to replace failed defrost thermostat due to iced up evaporator and warm fridge. Two of those thermostats were disassembled and severe pitting was noted on the contacts.
Error Code (if any): None
Parts or tests already tried: Three times this year have replaced failed defrost thermostat. Disassembled two of them and the contacts had severe pitting.
Question: After three failures of defrost thermostats this year with severe pitting of the contacts in the two that have been disassembled, I am wondering if there is something additional causing these failures.
Is it likely that the repeated failure of these defrost thermostats is due to something outside the thermostat itself? For instance, can a malfunction of the control board cause the thermostat contacts to cycle too often and result in severe pitting? Is there something else that could be suspected?
It is possible that the defrost heater circuit could be grounding out in the line after the defrost heater. The defrost thermostat is supposed to open up to stop the heater which then signals to the main control that the heater has done what needs to be done. However, if there is a grounded section or excess current from an improper resistance, it can cause the defrost thermostat to arc and pit in the contacts. If you test the heater for resistance with a multimeter, what reading do you get?
First, an answer to your main question – the defrost heater resistance tested at 30.3 ohms.
After defrosting and before disassembly, we found and accomplished the “Service Tests” on the appropriate “Tech Sheet” for this refrigerator at Whirlpool’s servicematters.com :
Service Test 1 was accomplished in both a warm and cold condition and checked good for each condition (Open and Closed when expected).
Service Tests 2 through 6 were all good results and Service Test 9 showed the defrost cycle set to “Adaptive”.
Despite Service Test 1 showing Open and Closed when expected, we removed the thermostat for testing separately. It showed 55.6k ohms in a warm condition. We chilled the thermostat in a deep freezer for a while and then its resistance tested at 1.0 to 1.5 ohms. My boyfriend/mechanic considered 1.5 ohms to be too much resistance in the closed position and he installed a new thermostat.
Problem is that this is the 4th thermostat this year. On the surface, it seems like an excessive failure rate unless something outside the thermostat is also failing and we are wondering what other tests or procedures we can do.
I am not sure where we would look for the other thing you mentioned – “grounding out in the line after the defrost heater”. Also, am wondering if grounding before the defrost heater would also be suspect and additionally whether the control board itself can cause any of these problems. Not sure of how to diagnose any of those potential problems.
The defrost heater certainly checks good. The wire on the opposite side of the heater to the wire from the defrost thermostat can be tested for continuity to the evaporator. If there is a continuity reading, then there is a ground in the circuit. If there is not continuity, the next consideration is how long the compressor is operating. If the defrost heater circuit checks good, does the compressor operate very frequently?
I have to ask where we would place the multimeter probes to do this test you mentioned: “The wire on the opposite side of the heater to the wire from the defrost thermostat can be tested for continuity to the evaporator.”
Regarding the compressor, no, it does not seems to operate “very frequently”, though that is hard to quantify with the refrigerator being in use throughout the day. It has long periods when it is not operating.
If you trace from the defrost bi-metal thermostat to the heater, that will be considered wire side A for testing purposes. The other end of the defrost heater will be considered wire side B. Testing from wire connection on wire side B with one probe to the evaporator coil with the other probe, will identify if the heater might be grounding out. If there are long periods where it is not operating, it should be holding temperature well. Excessive time spent with the compressor on can cause more frequent defrost cycles which can affect the life span of a defrost thermostat.