Evaporator fans are located in the freezer compartment behind a metal or plastic bulkhead and pull air over the evaporator coil and circulate it throughout the unit. Almost all refrigerators have a damper assembly that controls the amount of air entering the fresh food compartment from the freezer. Normally this damper is manually adjusted, but on some models it is electronically controlled. Another complicated and unnecessary part for the factory authorized repair person to chuckle over when you pay a couple of hundred to replace it. The old knob and plastic damper solution worked as well and rarely had to be replaced.
Evaporator Fan (2) pulls air over evaporator coil (1)
The evaporator fan is wired to run when the compressor is running. If the evaporator fan fails the compressor will run longer since there will be no air flow to the fresh food compartment where most cold control sensing bulbs are located. The temperature will go up in the fresh food compartment and the evaporator coil will ice up causing the freezer temperature to rise.
Sometimes a defrost problem will ice up the evaporator and jam the fan blades. Before assuming the evaporator fan is bad, disconnect power, remove cover from fan and evaporator coil, and ensure that blade is free to turn. With power off, disconnect the wires from fan motor and read windings with volt-ohm meter. A reading of infinity indicates an open motor winding and the motor should be replaced.
If the fan blades are impossible or difficult to turn suspect a seized bearing. An evaporator fan with bad bearings occasionally emit a high-pitched squealing noise that is impossible to ignore.