Problem could be caused by one of three items. A defective defrost timer; a defective defrost heater; or a defective defrost thermostat. Defrost timer checked OK. Had to remove back wall of freezer which required removing the ice maker. When the back was finally removed I found a 4" thick wall of ice covering the full width of the evaporator coils. After melting the ice I removed the defrost thermostat and ran a continuity check and found it defective. Also checked the heating element which was good. Replaced unit and all worked great.