Defrost Problem Repair
OK, using the 363-11781 parts and assuming yours is the same and this may be the wrong assumption.
First remove the evaporator cover in the freezer so you can see the coils.
Do not let them de-ice.
If they are heavily iced/frosted over you have a defrost problem.
We are a little late on the above but the defrost thermostat can be closed by holding it's face onto an ice cube. Just be very careful to not touch the connections.
Manually force a defrost cycle by turning the defrost timer cam with a screw driver till the fans and compressor turn off.
Often it is under the unit at the front or the back. They are also often in the control console, in this case there is usually a hole in the cover to let you get at the timer cam
Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the timer needs replacing, re: it is probably stalling during it's rotation so is never getting into a defrost cycle.
If not on.
Unplug the unit.
Remove the wire for one side of the heating element from the wiring and measure it for continuity, usually around 20 ohms or so.
If the heater is OK
Remove one wire to the defrost thermostat and measure it, should be 0 ohms when frozen. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.
If both the above are OK then odds are the defrost timer contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.
Here are the parts I found for the 781 unit. You can look them up at AppliancePartsPros. They all have pictures which will let you compare them to what is in your unit.
Defrost Thermostat - AP2071242
Defrost Timer - AP2061693
Defrost Heater - AP2071484
If you do not own a meter, I would suggest you purchase a one. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.
There is a good STICKY at the start of this forum about it's use.