Some repair / testing tips to verify this is the part you need:
If your defrost timer is bad or going bad the usual signs are frosting up on the back panel in the freezer section that blocks the air flow to the refrigerator section or the refrigerator won't turn on (no fans running, not cooling) , but the lights are on inside. 1st we will cover the frosting up in the freezer. You will need to locate the defrost timer. You can find the timer either in the refrigerator section mounted inside the same panel as the temperature contol, behind the kick panel on the bottom of the refrigerator, or mounted on the back of the refrigerator around the area of the compressor. Once you've located the defrost timer the first test want to do, using flat head screw driver, is to turn the dial clockwise slowly until the timer clicks. At this point the refrigerator fans and compressor will turn off. If after a few minutes you hear water dripping on the heater inside the back panel that means that the defrost thermostat and heater are okay and the defrost timer is not advancing into defrost mode. You can order the timer and it will solve your problem. If you do not hear water dripping on the heater you will need to take the back panel off and run a continuity test on the heater. If the heater tests out okay then your defrost thermostat is bad replace it. If you get no continuity through the heater replace both the defrost thermostat and the defrost heater.
Now we will cover the other sign, the refrigerator fans and compressor won't turn on and the refrigerator/freezer are getting warm. 1st locate your cold control and using a multi-meter test for 120vac at the 2 wires running through it. If you get 120vac then the power is stopped by the thermostat replace it. You can verify this by removing the 2 wires and jump them with a wire stripped on both ends. If the refrigerator kicks on then you know the thermostat is bad. If you don't get 120vac ther the quickest test is to locate your defrost timer and hit in the are of the timer relatively hard with the handle of a screwdriver. If the refrigerator fans and compressor comes on then the timer needs replaced. You can also manually turn the dial on the defrost timer. If the fans and compressor come on manually turning the dial this means that the timer is getting stuck in defrost replace it.
Note: The timer listed on this page is a universal part. If you look at the picture above there is a flying lead that will have to be placed onto 1 of the 3 terminals that are off to the right of the 1 terminal mounted by itself. When you go to replace the timer you will need to take the old timer apart to find our which terminal to attach the flying lead to. On this timer the terminals are labeled with 1, 2, 3, and 4. Number 3 will be left alone as it is line 1 to the motor. Number 1, 2, or 4 will need the flying lead and you will be able to find that information out by looking inside you old timer to find which of these terminals has a small copper wire running to. These timer we find hold up very well, most of the time better thand the original that is placed when manufactured.