Hello Caroline42,

( Caution ) Unplug refrigerator before doing anything.

It sounds like you may have a defrost problem if the complete evaporator is getting an ice build up. ( Possible sealed system problem if the lower half is frosted up only )
The defrost sytem consist of three parts:
1. Defrost timer (Located inside the freshfood section with the cold control on the ceiling )
To test the defrost timer, place a screwdriver in the timer cam slot, and turn it until you hear the first "snap". The defrost timer is now in the defrost cyle. At this point and time you must read the wire diagram to determine which numbered terminals are for the defrost circuit.

For example we will say:

Term#1 is Ground
Term#2 is Defrost
Term#3 is Common
Term#4 is Compressor

Set the ohmmeter scale on RX1, and place the probes on #2 Defrost and #3 Common terminals. You should get continuity.

Next, cycle the timer until you hear the second "snap". The meter will show no continuity, indicating that the defrost cyle is over and the refrigeration cycle begins.

Now, place the meter probes on the terminals #3 Common and #4 Compressor. The ohmmeter will show continuity indicating the refrigeration cycle is activated. Turn the timer cam once again, untill you hear the first "snap". The meter will show no continuity.

At no time should there be continuity between terminals #2 and #4. (If so, the switch contacts are burned and welded together. The defrost timer must be replaced)
The timer motor runs continuisly cycling off. The timer motor itself is somewhat difficult to diagnose but to make it easier you can rule out the defrost thermostat and heater before replacing the timer.

2. Defrost Heater (Located below the evaporator coil behind the back wall of the freezer)
The heater is easy to diagnose. Its like a light bulb. It either works or it doesnt. Glass heaters fail more often than the calrod type heaters and both can be tested for continuity.

3. Defrost thermostat (Located behind the freezer back wall attached to the evaporator inline with the heater )

The defrost thermostat will have continuity when it is at it coldest. If it doesnt then it is defective.

If you have any other questions feel free to write back.