If you discover that your refrigerator is either cooling too much or not at all, there can be a number of reasons why this is happening. The most likely cause is the defrost timer has gone bad. The defrost timer helps regulate the temperature in the refrigerator and freezer to prevent ice buildup. Rather than purchasing a brand new refrigerator or calling an expensive repair person, try troubleshooting the problem first. You just may save a few hundred dollars!
Turn off the breaker that supplies power to the refrigerator or unplug it from the wall outlet. Either way is good, as long as you do not try working on your refrigerator while it is powered.
Find the defrost timer. Depending on the make and model of your refrigerator, it may be found in one of the following areas: behind the front grill in the freezer; behind the cover plate in the freezer; behind the freezer near the compressor; or inside the temperature control console.
Turn the advance screw clockwise until you hear it click.
Turn the power back on. If the refrigerator was previously stuck in cool mode, it will now be advanced to defrost mode.
Wait about 30 to 40 minutes and check it again to see if it switches back to cooling mode. You will know when it begins cooling again, as the compressor will turn on.
If the defrost timer is not advancing, turn the power off again.
Remove the timer. The plug that connects to the timer can only be connected one way, so it's not important to draw a wiring schematic for this job.
If the timer did advance as it was supposed to, then you will now have to test the timer electronically. With the power off, remove the defrost timer. Take the timer to a clean workspace.
Set your multitester, available at any home improvement or electronics store, to read ohms. This is usually the RX1 or X1 setting, depending on the model.
Locate the four terminals on the defrost timer. Find the terminal labeled "C" or "3." If the timer is not labeled, look at the connector plug to determine which terminal gets connected to the white wire; that is the common terminal.
Touch one of the tester's probes to the common terminal. Touch the other probe to each of the remaining terminals on the timer. As you touch the other terminals, one or two of the terminals -- depending on the timer's current setting -- should read "zero" and the remaining one or two should read "infinity."
Turn the advance screw again until you hear the click and then probe the terminals again. This time, the results should be reversed. The terminal(s) that read "zero" earlier should now read "infinity" and the other(s) should now read "zero."
If the timer does not pass this test, replaced it. If it does operate correctly, return it to its place in the refrigerator. The problem with your refrigerator is caused by another issue, like a bad defrost heater or thermostat.