I suspected that the defrost heater was not coming on. I 'ohmed' out the circuit. The timer was working OK. The temperature switch (attached to the evaporator coil) was measuring 'open' even when it had ice all over it (very cold). It should be 'closed' (conducting) at that temperature. I found the model number of my refrigerator and googled it and found 'AppliancePartsPros.com' where they had a very nice exploded view of different parts of my refrigerator's systems, and I identified the temperature switch in the illustrations. Their web site gave me the part number, so I ordered a new temperature switch from them on-line and received it within two business days via FedEx ground. Excellent service!