should my fan kick straight in ?

I think you mean:

"I've taken the back off freezer. It was ice blocked. Defrosted with hair dryer put back together. Should the fan in freezer kick straight back in or whe it reaches its temperature?"

May one respectfully suggest you learn about punctuation and use it? Your question, as you wrote it, was almost meaningless.

The fan inside the freezer will circulate air through the chiller and wi run while the freezer is cooling. Depending on the make and model(which, for some reason you decided not to tell us), the internal fan may shut down when the door is open.

xxx
July 2016