I have a Hotpoint side/by/side that is having problems. The unit runs constantly, the freezer frosts up and doesn’t freeze. The Fridge portion seems to be functioning properly. After manually defrosting, the freezer will work for a few days, but then the problem recurs. I’ve replaced the motherboard, thermostat and defrost heater assembly, to no avail. I need to know what to try next! 
Hi.
You should replace the defrost sensor, clipped on the top of the evaporator coil and connected to the main control board. It actually sensing the temperature of the evaporator and based on that the control board initiates and terminates the defrost cycle.
The defrost sensor [PART]AP3185407[/PART]
Good luck. Please keep us posted of the results.
Thank you.
Simon.