A defrost Timer opens the heater circuit after 7HRs Compressors work time , it works to defrost ice on the evaporator for 7 min , actually mealting ice take more than 20 min in the refrigerator , but the  circuit of the refrigerator makes the extra delay time 

The defrost timer has four pins labeled from 1 to 4. Each pin has its own function in the operation of the timer.

Pin 1: The main input power is attached to pin 1.Pin 2: When in defrost mode, the power (pin 1) connects to pin 2, which activates the defrost mode. Most defrost timers stay in defrost mode for 30 minutes.Pin 3: This is your ground / neutral wire. It will be separated from the other three pins.Pin 4: When the timer is not in defrost mode, the power (pin 1) connects to pin 4, which activates the compressor and fan. Most timers stay in this mode for 10 hours.

To test if the defrost mode is working, apply voltage to pin 1. When the power supply is turned on, you should be able to measure the voltage on pin 2.

Once the timer advances past 30 minutes, the compressor and fan should turn on. You will know that the defrost timer is working if you measure voltage on pin 4 when the timer is not in defrost mode.

The compressor and fan pin should be active for 10 hours (or however long your defrost timer is designed to run). Once the 10 hours is over, the defrost timer will switch back to pin 2, activating the defrost.