In most older refrigerators and some newer ones, a motor-driven timer is used to stop the compressor and turn on the defrost heater. This timer controls how often the cycle occurs, and how long the compressor stays on. With a timer, this is a fixed cycle; for example, the refrigerator might be in the cooling cycle for 10 hours, then spend 20 minutes in the defrost cycle.
ADC - ADAPTIVE DEFROST CONTROL
If your refrigerator is operating in the tropics, in really warm, humid air, it will need to run a lot and defrost often. And if the door is opening and closing a lot, letting in even MORE warm humid air, it will need to cycle even more often.
In a cold climate, it probably will not need to run nearly as long, or defrost nearly as often.
Compressors and defrost heaters use a lot of energy, so to make refrigerators more efficient, refrigerator designers are minimizing the total amount of time that they stay energized.
To adapt to these different conditions, designers are using electronics...solid state circuit boards with microprocessor logic. Such techniques are called Adaptive Defrost Control, commonly abbreviated as ADC.
To make decisions about how long and often the refrigerator cools and defrosts, the control board must have input about the conditions that the fridge is operating under. Each manufacturer uses a different logic scheme and different inputs, not just temperatures, but things like how long the refrigerator door stays open, how long the compressor runs, and the duration of the previous defrost cycle.
For example, door open info is provided to the logic board by the door switch - the same one that controls the refrigerator's internal lights. If the light bulb is burnt out, the ADC will still accumulate door open time. However, if the door switch has failed, the next defrost will occur either too quickly or too slowly.
Some designs even have a "vacation mode;" for example, if the door has not been opened in 72 hours, the refrigerator will not defrost as often.
How often defrost occurs may also be shortened under certain circumstances. For example, the ADC is programmed with a maximum amount of time that the heater can stay on; say, for 16 minutes. If the heater stays on for the maximum amount of time, without being stopped by the terminating thermostat, the microprocessor will assume that not all of the frost melted, and it will initiate the next defrost cycle much sooner.
DO YOU HAVE A DEFROST TIMER, OR AN ADC?
How can you tell whether you have an ADC or a defrost timer without searching for it on your refrigerator?
The easiest way that I've found is to go to a website such as PartSelect.com. Type in your refrigerator's model number and search for the words "board" and "timer." They have an exploded parts diagram for most model numbers; this can tell you where to search for the board or timer.
Where to find your refrigerator's model number
Some ADC and other control boards in the refrigerator control other functions, too, such as icemaking and electrical air damper door opening and closing. The easiest way to tell the difference between the ADC and other boards is that the harness connection is always labeled with the compressor, defrost heater, L1 and L2. Often the leads are labeled for the defrost thermostat, too, and a sometimes even test connections.
DEFROST TIMER AND ADC BOARD LOCATIONS
Mechanical defrost timers can be a bit difficult to find. They come in many different styles. Often they are mounted under a cover plate or in a bracket that hides all but the advancement pinion. The pictures above show some different styles of timers and what the timer might look like installed, and also some typical mounting locations.
ADC boards are mounted in similar locations to defrost timers. You don't really need to find yours just yet; you only need to be able to initiate a defrost cycle for diagnosing. If, after diagnosis, you need to replace it, then you will need to find it.
