My maytag side by side refrigerator will not cool down properly. The freezer cools o.k. I unplugged the refrigerator and let it sit overnight. I cleaned the coils in back and checked the fan motor. After plugging in the refrigerator, it will cool down good for a day and then the temperature goes back up. Is it the defrost timer? If so, where is it located? Please help

Yep. It's the defrost timer. To be sure you need to test this with a multitester to be sure it's bad. You can download a schematic of your machine from the net if you get the model # off the machine and enter that into your search engine.