Hi @bostondave ,
The evaporator coils icing up excessively is usually caused by a defrost problem.
According to the parts supplier shown below the part number for the tech sheet for your model freezer is 297246001. It shows that the freezer is auto defrosted for 30 minutes every 12 hours.
During this time the compressor is turned off and the defrost heater is turned on to melt the ice build up. The bi-metal defrost thermostat releases when the temp on the evaporator unit reaches 40°F to signal the end of the defrost cycle by turning off the heater and to restart the compressor to cool things back down again to the set temp. When the temp reaches 25°F the defrost thermostat re-operates in readiness for the next defrost cycle.
Even though the temp gets that high at the end of the cycle the thermostat is close to the evaporator and heater and normally the overall air temp in the cabinet doesn’t go above 32°F as you don’t want the food to start defrosting
Here’s a link to the parts (supplier example only) that may help as it gives a rough idea of where everything is located. If you find a faulty part, there are other suppliers online that may suit you better. Just search for 253.28459807 parts to get results.
Check that the defrost heater (part #14) and the defrost thermostat (part #20) are both OK.
Also check that the drain tube going from under the evap unit to the evaporator pan (part #31A) under the cabinet near the compressor is not blocked preventing the defrost meltwater from draining away.
Here’s a video that shows how to test the bi-metal defrost thermostat that may also help.