Fost accumilating inside

There are three components that control the automatic defrost cycle.

#1 is the defrost timer, this is a simple clock and set of contacts that switch from cooling mode to defrost mode every 8 to 10 hours. The timer knob can be turned clockwise slowly until a positive 'click' is felt, this is the defrost mode. Let it sit there for about 10 minutes and you should hear the hiss or sizzle, if not then we go a bit deeper.

#2 defrost heater, this heater is located on the bottom of the cooling coils behind the back panel in the freezer, it will run long enough to remove the frost from the evaporator coils and turn off by the defrost terminator or no longer than 25 minutes maximum

#3 defrost terminator or thermostat or bi metal, 3 names same part. This is clipped on to the evaporator coils near the top and senses the temperature, it is there so the defrost cycle does not run too long.