Follow up: I took the evaporator coil cover off this morning. The fan was running, but the coil was frozen solid. After running my wife's hair dryer for an hour and a half, the coil was clean and dry. Freezer temp's down to 2 degrees and dropping. Problem solved (unless it freezes up again.)

OK, I have a defrost problem. The evaporator coil freezes up every couple weeks and requires manual defrosting. I followed the troubleshooting guide at acmehowto.com - How To Fix a Refrigerator. The defrost heater appears to be good (resistance = 22 ohms). I can't measure the resistance of the defrost thermistor or defrost thermostat because they are hard wired into the wiring harnesses and I can't trace the wires to the plug without major disassembly. By then the thermostat is warm and may be properly open. Is there any way to test resistance of these components at the motherboard? Is there a wiring diagram available anywhere? (Couldn't find one behind the toe grill or anywhere else on the fridge.)

Rich, Excuse my caution, but I don't want to make a dumb mistake and fry something. With the connector unplugged, do I jumper between the pins on the motherboard or between the corresponding contacts on the connector? (Without a wiring diagram, I'm flying blind.)

I found the "junk GE GSS20IEMbww new motherboard" thread, but the schematic seems to have been removed. There's a red X in post #8 where it used to be. Is there another way to get this schematic?

For the GE GS and PS models and all ones with the AP4297272 motherboard with damper door problems.
I believe the damper is controled by a simple on/off application of voltage cycling on the thermistor resistance.So, in normal operation, the door will open and close periodically.If the fridge is very warm, the door will remain open till the fridg. gets down to the control setting temperature.Precise control of the fridg. temperature is done by control of the fan speed; higher frequency, more airflow, lower frequency, less airflow. All thermisters are the same; 65kohm=-
-13 degrees, 12kohm=+40 degrees, 5kohm=+75 degrees. These are just typical readings. You can measure these at the J1 connector of the motherboard, upper right. Measure between common J1-pin5 and either J1-1 fresh food, J1-2 fresh food, J1-3 freezer and J1-4 evaporator, used to terminate defrost.
If your thermisters are reading close to these sort of normal values and the damper door remains open, or closed, and the damper motor is not blown out, you probably have a bad motherboard. The 12 volt dc motor resistances are red- yellow 430 ohm, blue-white 430 ohm. If either of these are shorted or way low in value, you will need to also buy a new damper assembly as it will blow out the motherboard. To check the motherboard output voltages to the damper, go to the upper middle connector J3. Measure between J3-1 and J3-2, then between J3-3 and J3-4 should be around 12 volts.
The South Jersey schematic was scubbed off the Samurai site, only available to pros, just refer to your service schematic. If it is missing, contact me for pin info.

Very good work, I appreciate the feedback, you solved a "difficult" problem.
And I again thank the South Jersey group for publishing the info and schematic; this publication and customer complaints forced GE to include the schematic in future refrigerators.