It does sound like the defrost timer might be bad. When did you first notice that your fridge was always running?
Keep in mind that your refrigerator has a 16 hour timer. This means that it stays on for 16 hours, then off for 35 minutes to defrost. Most timers are 6-8 hours, which makes it easier to notice when the fridge turns off (goes into defrost). Try measuring the temperature inside both the fridge and the freezer section with a
refrigerator thermometer and post your results here.
Breakdown diagrams for
Model TPX21PRXDWW