[richappy]

Per. the South Jersey Appliance data sheet, you can go to the motherboard in the back and measure both resistance and voltage and frequency at the J2 connector. This schematic is generic so refer to the fridg's schematic to confirm wiring and double check with wire colors at the evaporator motor.
J2 pin 3 to J2 pin 4 about 12.3 vdc @ high speed,8 vdc at medium speed,4vdc at low speed. J2 pin 3 to J2 pin 1, 220 hz high speed, 160 hz medium speed, 100hz low speed. I would assume opening the fridg door would give a high speed signal. Resistance should be about 2K. I believe the signal is a square wave with a dc component. A analog meter will not give the same readings. A digital meter reads RMS value, analog an average.
For units with the annoying fan noises, changing frequencies/voltages will confirm a bad motherboard.
This evaporator motor is more complex than any other and it is recommended to replace whenever the motherboard voltages fail. Simple resistance checks of the motor are probably not useful. Usually a zero volts output cause is a blown resistor near the J2 connector.
For the South Jersey schematic crunch in Google" junk GE GSS20IEMbww new motherboard" Make sure to enter it exactly as shown with the same capital letters.
Also, you can remove the three pin connector for the compressor/defrost power and jumper the power input pin J11 to either the compressor J9 or the defrost circuit J8 to activate independent of the motherboard.
With this schematic, you can measure all the thermistor resistances and input/ output voltages.
Also, I just realized this board contains the auger motor and auger solenoid drivers and the water dispenser solenoid. So, if these fail, it's the motherboard. The triac drivers for the condenser motor and the evaporator motor are rather small and rated less than 1/2 amp so they blow real easy whenever these motors go bad.
Also, I found out the dispenser water solenoid and auger drivers are small relays on this board, but the encoder signal inputs are never defined anywhere!