My refrigerator runs constantly.

You have correctly identified the two possible suspects: the defrost timer and the cold control. If you're sure the refrigerator cycles off for defrost, then the cold control electrical contacts are sticking. Replace it.

If you're not sure the fridge cycles off for defrost, then it's easy to test. Locate the defrost timer on your fridge and turn the knob until you hear a big click. This is defrost mode. Then wait approximately 21 minutes and 37 seconds for it to come out of defrost mode and start running again. If it doesn't, then the defrost timer is bad. Replace it.