Freezer/refrigerator compressor running constantly. I think I might have a defrost timer problem

Question

I think I might have a defrost timer problem the freezer will freeze and than defrost. The compressor never seems to shut off and I searched for a defrost timer but, I only found listed in the parts manual a bi-metal sensor is this how they control and time the defrost on a Whirlpool model ET25DMXVN01? The refrigerator seems cold enough it is just that the freezer keeps freezing things and then defrosting.