After some troubleshooting to ensure the thermostat and heating element were working, determined that the defrost timer was the problem. With the defective part, I was able to manually advance the timer to activate the defrost cycle, but it wouldn't do so on its own. Unplugged the refrigerator from the wall. Timer is located in the refrigerator compartment at the very top. The compartment houses the cooling thermostat, the light bulb and the defrost timer. Using 1/4" nut driver, remove 4 screws holding compartment cover to top of fridge. Lower a bit to access electrical wiring. Wiring harness can be unplugged to work on the timer away from the fridge which makes it a bit easier. Timer is mounted with 2 phillips head screws. Remove screws and unplug timer from wiring. Install new timer, plug in wiring, secure with the phillips head screws and remount entire compartment.
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