[byrddog]

My freezer ices up. lower freezer above the freezer. could this be the defrost timer? orsomething else?

[woodchuck]

Post your model number. Some subzero models don't have defrost heaters etc.

[byrddog]

the model # is 550 1992 model I have changed the defrost timer already and a thermostat

[byrddog]

model is 550 year 1992 with freezer drawer

[woodchuck]

You can check your defrost thermostat, defrost heater and defrost timer here-Refrigerator Repair Guide: How To Fix a Refrigerator - ACME HOW TO.com

can you view your evaporator coils in the freezer and observe the ice pattern?
An irregular ice pattern would point to low freon or defective compressor. Solid all over would be defrost problem.

[byrddog]

One question: the freezer fan is not running. should it run all the time? could a defective fan cause iceing up of evaporator?

[woodchuck]

The fan won't cause an icing problem. It pulls the air across the evaporator circulating the cold through the freezer and fridge. The frost forms when the moisture in the air is frozen when it hits the super cold evaporator coils. Excess moisture from leaky door gaskets and excessive door opening can increase it. But the defrost system will melt the frost every 8 to 10 hours. The fan should run when the compressor is running. It won't run when the set temp. has been reached or during defrost cycle.

[byrddog]

I changed the defrost timer. now the freezer seems to work properly. but the frig on top doesnt get cold enough. I've had it running for a week or so and ice is not building up at freezer condensor anymore and the fan is working. the motors are running Ok, Maybe the thermostst for the frig is not right or might it be possible that freon is low?