Refrigeration defrost system working procedure?

I want to touch base on how defrosting works on a modern ice box. That's a refrigerator or a freezer. I am old school and we call it an ice box. Back in 1948 when electricity finally made it to my neck of the woods, my dear grandmother Pearl purchased her first electric ice box. At that point her house was 150 years old! When her ice box would start to frost up she would simply unplug it and crack the door open with a slat-back chair. Today's modern ice box has automatic defrost. The defrost system consists of 3 components. A timer or auto defrost control, a heater, and a heater termination thermostat. The timer has 2 modes: compressor run and defrost. For anywhere from 8 to 12 hours the timer remains in compressor run mode. This means for those hours the compressor will run whenever the cold control wants it to. In the defrost mode the compressor and fans are shut down, then the heater and heater termination thermostat are activated for 21 minutes. What do we need a termination thermostat for? Well if the heater stayed on for 21 minutes the ice box would be destroyed! A typical termination thermostat is set to 'close contacts' (activate the heater) at 25f and "open contacts" (kill the heater) at 50f. An ice box with new fangled electronic defrost (ADC automatic defrost control) uses a logarithm to activate defrost when it is deemed necessary by the control to do so. How does it do it? Various inputs to the control include how many times the ice box doors were opened, how many hours the compressor and fans have been going, and how long the last defrost took. When the right combo of events occur the ADC will kick in the defrost mode. The heater and terminator (termination thermostat) is the same as with a timer. So what goes wrong? The terminator and heater both die with age. The heater can be checked with a meter for continuity. Most of the time you can tell it is bad. Some of the time you cannot. The terminator sometimes will swell up. The only correct way to test it for continuity is to put it in a freezer and let it get below 25f, then quickly check it before it warms up to 50f. Timers hang up either in run or defrost modes. Sometimes if you spin the manual advance around several times it will work some more till it hangs up again. Might be years before it hangs up again depending on humidity conditions in your neck of the woods. ADC boards simply burn out due to voltage spike, lightning, corrosion, etc. Sometimes simply removing the board and cleaning the corrosion off the connection can get em working again. Don't make a huge mistake and buy an expensive board when a cheap sensor may be the blame. Remember the control relies on input from sensors to determine defrost. You need to check the sensors with a meter to see if they are within factory specs. Their could be as many as 6 of them also. They all have to be in spec in order for defrost to work correctly. If one of them has fallen out of its mount that is enough to foul up the the whole procedure.