A transport temperature control unit and methods of defrosting an evaporator coil of a transport temperature control unit. The transport temperature control unit includes an evaporator coil, an ambient air temperature sensor for sensing an ambient air temperature, a return air temperature sensor for sensing a return air temperature, a discharge air temperature sensor for sensing a discharge air temperature, an evaporator coil temperature sensor for sensing an evaporator coil temperature, and a controller. The controller initiates a defrost cycle when a large temperature differential occurs over the evaporator coil.