function of timer in frost free refrigerator

There are many separate components in a refrigerator's defrost system that must work in concert for a frost free system to work properly. We will attempt to explain the workings using simplified electrical schematics.
The heart of the defrost system is the defrost control. The most common control is a mechanical defrost timer which is a motorized device that opens and closes several electrical contacts. Each contact can be thought of as a simple light switch but instead of a light, one connects the defrost heater circuit, another connects the cooling system. When one of these is switched on, the other is switched off. A motor on the timer (NOT illustrated) turns a cam that opens and closes these contacts at set intervals (see below for other types).
Cooling Cycle

During the cooling mode, the defrost timer closes a contact to the compressor circuit so it will run. The circuit to the defrost heater is open.
While in this mode, the thermostat (a.k.a. cold control) cycles the compressor and fan motors on and off to maintain an appropriate temperature.

How To Test the Defrost Timer

A mechanical defrost timer controls the defrost cycle of the freezer. In older models, the timer runs continuously and roughly every six hours, cuts power to the cooling system and sends power to the defrost heater. In newer models the timer advances only while the compressor or defrost cycle is running - an improvement for efficiency. As the timer continues to advance, power to the heating element is cut and power is restored to the cooling system. If the timer does not advance, the appliance will be stuck either in defrost or refrigerate mode, resulting in, respectively, no cooling or frost build-up.
Caution: Please read our safety information before attempting any testing or repairs.
 Before testing the defrost timer, unplug the refrigerator to avoid an electrical shock hazard.
The defrost timer is usually found behind the front grill "toe kick" of the refrigerator. It may also be found behind a cover plate inside the refrigerator or freezer, in the temperature control console, or behind the refrigerator near the compressor.
To test whether the defrost timer is simply failing to advance, locate the advance screw and turn it clockwise until you hear it click. This advances it to the next mode. If it was cooling before, it is now in defrost mode. Simply wait about 35 minutes and check whether it has left defrost mode and has resumed cooling (listen for the compressor). If it does not advance, the timer motor is probably bad and the entire timer needs to be replaced. If it advances as it should, then you can follow the steps below to test the switch electrically.
The timer is usually held in place with one or more screws. Remove the screws and gently pull the timer out far enough to disconnect the wiring connector. The connector can be removed by firmly pulling and rocking it left and right. It is not necessary to note the position of the wires because the connector plug is keyed so that it can be replaced in only one way.