[COLOR="Blue"]A while ago my freezer started turning ice cream rock hard, then the fridge section started getting warm. [/COLOR]
Your symptoms seem weird. Usually when you have a defrost problem the first sign is that the fresh food side warms up and later the freezer warms up.
This is due to the evaporator fan being unable to pull air through the coils due to the ice/frost so the unit cannot cool the air properly.

[COLOR="Blue"]And the test sheet is essentially gibberish.[/COLOR]
Looks like you have the wrong tech sheet.
I went to the Sears parts site got the timer number and then on AppliancePartsPros did a cross reference on this number and then looked at a number of units to see if the parts in the control section matched yours.
I ended up with MSD2258KES00 being close to yours. Here is the tech sheet for that unit.
http://www.servicematters.com/docs/wiring/Wiring%20Sheet%20-%20W10142176.pdf
A bit of a guess but the best I could do.

[COLOR="Blue"]I went ahead and replaced the defrost control, because it seemed like a good idea at the time. [/COLOR]
Try running a forced defrost using the procedure in the above tech sheet.
[COLOR="Blue"]
I also tested the defrost heater for continuity, and got a reading that suggested it's fine. I forgot what it was, but it wasn't 0 or infinity.[/COLOR]
Tech sheet shows the heater should be about 25 ohms.
Be sure to remove one side of it before checking it, this ensures that you do not read an alternate/parallel circuit path.

[COLOR="Blue"]So I'm stuck. It seems like the heater works, the adaptive control is new, and I can't test the bi-metal strip. In fact, I can't even find it.[/COLOR]
Bi-metal should be clipped to the evaporator coils.
Should measure 56,000 ohms when it is frozen.
Infinite ohms when at room temperature.
http://www.searspartsdirect.com/partsdirect/showPart.pd?partNumber=2188824&productGroupId=0046&supplierId=106&brandId=x&documentId=K0404002&keySuffixId=NA&modelNumber=x&keyId=0035&pageId=00008&productTypeId=x&searchModelNumber=10644553401&subCompDesc=Unit%20parts,%20parts%20not%20illustrated&brandDesc=KENMORE&modelDesc=SIDE-BY-SIDE-REFRIGERATOR&pathTaken=partSearch&blt=11
It may be all metal or plastic with a metal face.


I hope tha above helps some.

[COLOR="Blue"]That is in fact the test sheet I have. [/COLOR]
You sort of lost me. You said in your first post "the schematic shows only 4 wires to the adaptive defrost control, but there are actually 6, and none of the wire colors match the schematic." That is why I tried to find a wiring diagram for a similar unit but with 6 wires to the adaptive defrost board.

[COLOR="Blue"]But I don't know how to tell if the bi-metal strip is closed,[/COLOR]
It should measure 56,000 ohms when frozen, infinite ohms when at room temperature.

[COLOR="Blue"]Do you think the test sheet means I should jumper the test terminal to "close" the bi-metal strip?[/COLOR]
You are not really closing it but you are shorting across it.
In other words if you get into a defrost mode and the heater does not come on, you can use the test terminal to see if the defrost thermostat is the culprit that is preventing the heater from coming on.

[COLOR="Blue"]If so, how do I turn the thermostat on and off?[/COLOR]
Looks like your cold control is a slider, usually moving it to the far left is off.
Easy enough to check if unit is running and you slide the control to off the compressor and fans will turn off.

[COLOR="Blue"]And does this test the defrost control, the bi-metal, the heater, or...?[/COLOR]
If the heater comes on it is OK.
If the heater goes off once all the ice has melted off then the bi-metal is OK.
If both the above check out OK then odds are it is the adaptive defrost board that is shot.

[COLOR="Blue"]I assume the ice cream getting hard was due to the vents getting frosted over, and the freezer staying too cold. The fridge was probably getting warmer at the same time, but I just didn't notice it until later. But my trouble shooting assumes that a properly running defrost cycle keeps the vent door and vents from icing over and plugging up. Is that correct?[/COLOR]
I guess it is possible. My problem with it is if you have a defrost problem the freezer will get warmer not colder.

Read more: http://forum.appliancepartspros...sting.html#ixzz0wrOPVdbS
http://forum.appliancepartspros.com


When I tested the defrost heater, I probed the end of the unplugged connector, so I think that was the only circuit I was reading. And I think it may have been either 24 or 42 ohms, so that sounds about right.
Yes it does sound OK

[COLOR="Blue"]Sorry, I wasn't being very precise. I should have said that my defrost control unit has 6 terminals where the schematic only shows 4 (and the wire colors don't match).[/COLOR]
Yes we do seem to have a communication problem on this.
The wiring diagram I included does have a 6 terminal adaptive defrost board.
But onward...

[COLOR="Blue"]From the first test, it seems like the heater works. If I understand you correctly, this points to the bi-metal strip as the culprit, right? (I just replaced the defrost control, so it's brand new.)[/COLOR]
Yes if the heater works then all that is left in the defrost system is the defrost thermostat. Note that there is alwauys an outside chance that the replacement board you received is no good but I would replace the thermostat first.

[COLOR="Blue"]But shouldn't the fan go on when it's in defrost mode? Otherwise, how does the warm air get circulated through the vents to keep them clear of frost?[/COLOR]
No.
The fan should turn off during defrost. You just want the ice to melt off the coils. You do not want the warm air to be blown into the freezer compartment as this will also melt your food.
Defrost does not effect the vents.

[COLOR="Blue"]Is there any chance the fan motor is getting weak?[/COLOR]
Yes it is possible that the fan is slowing down.
I went to the Sears parts site. It looks like the amount of air that enters the fresh food side is regulated by the left hand temperature control. I think this is just a mechanical control that opens/closes a flap in the diffuser assembly.
I would check that the door is fully open on the coldest setting.
Also check that the lower vent between the freezer and fresh food is not blocked by food.
Note that some units have a thermostat controlled vent to regulate the amount of air coming into the fresh food side but I could not tell from the Sears parts breakdown if your unit has this.