Thermo King V500 20 defrost issue

[tkfreezer]

I have a defrost question, i purchased a used truck box with this unit intact ( V500 20 ) i am using it as a stand alone freezer. Unit works great on electric power, however for some reason it wont go into auto defrost. I can however initiate a manual defrost. Setpoint is 20F, the evap does freeze up at least once per day. Does anyone have any ideas as to what the problem may be.