Long story short, I have a defrost problem.

Short history:
Due to cooling (or lack thereof) issues and excessive frost in the freezer compartment, I suspected a defrost issue. Using a multimeter, I determined that there IS continuity (with some resistance) across the defrost heater. I determined that this unit does NOT have a defrost timer, but is instead computer controlled. I tested the bimetal switch and found it to be CLOSED at 0F, 70F, and when heated by a blow dryer (basically, it appears to be shorted closed all the time).

What I have done:
I ordered a new bimetal switch (DA47-10160F), which arrived today. Before installing, I decided to test it. It also tests CLOSED at 0F, 70F, and when heated by a blow dryer. At this point, I'm very confused as I expect the switch to open at some temperature below 70F.

I do not have a service manual for my model specifically, but I do have a service manual for a similar model. I plugged the unit in (keep in mind it has been unplugged for days, and is completely at room temperature of about 70F) and forced the unit into defrost mode. The defrost heater immediately began to glow red and remained glowing until I unplugged the unit several minutes later. This surprises me, as I would expect the defrost cycle to be very short at room temperature.

There is a defrost temperature sensor (separate from the bimetal switch) as well. According to the service manual, the two are wired in parallel. Once started, the defrost cycle is supposed to end when both:[LIST=1]

The bimetal switch opens (at approximately 54 degrees, at least for the model for which the service manual is written)

The defrost temperature sensor reaches a sufficiently high temperature (I don't know what that temperature is, but I think it's safe to assume it should be less than 70F!)[/LIST]I forced a defrost cycle again. This time, once the heater was glowing, I unplugged the bimetal switch (simulating the switch opening). The heaters immediately stopped. However, the cycle did not end ... when I plugged the bimetal switch back in, the heaters came back on and continued to glow. Odd.

The service manual also have a reference table of resistance values at various temperatures, so that one can test the temperature sensors. Testing both the refrigerator & freezer temperature sensors yielded similar results that, per the service manual, corresponded to roughly 70F. However, testing the defrost temperature sensor yielded a resistance value that indicated a temperature of -15F!

Suspecting a faulty temperature sensor, I swapped the defrost sensor with the freezer compartment sensor. I forced another defrost cycle. Once the heater was glowing, I unplugged the bimetal switch to simulate the switch opening. This time, after a couple of seconds there was a faint "click". Plugging the bimetal switch back in did NOT turn the heaters back on, leading me to believe that the defrost cycle had indeed ended this time.

At this point, I suspect that the defrost temperature sensor is faulty and needs replaced. However, I am completely confused by the fact that the new bimetal switch behaves exactly the same as the old one. Did I get a faulty replacement part? I mean, I know it happens, but I'm looking for validation that my understanding of the system is correct before I go down that path.

All of this, of course, does not really fit with what I expected to find when I dug into the defrost system. Given the excessive frost that we've observed in the freezer compartment, it seems odd to me to find that the defrost cycle is actually running TOO LONG rather than not enough.

Any comments/suggestions at this point?

Thanks in advance!

-Bob