refrigeration system 40 but also reduces the discharge air temperature when operating in modulation with return air control the reduced air flow caused by the ice build up will result in a rise in return air temperature simultaneously the reduced air flow paths will produce a drop in discharge air temperature at some point these temperature changes will transition the system 40 into discharge air control once in discharge air control the controller 135 will manipulate the suction line throttle valve 125 to maintain the discharge air temperature at the floor limit t 2 however as the air flow path continues to shrink the discharge air temperature will continue to drop the continued drop will cause the controller 135 to move the suction line throttle valve 125 to a more closed position even as the return air temperature rises it is this combination of a reduction in discharge air temperature coupled with an increase in return air temperature and the movement of the suction line throttle valve 125 toward the closed position block 190 in fig3 that signals the need for a defrost cycle block 195 the controller 135 senses these conditions and initiates the defrost cycle most systems also include an evaporator coil temperature sensor 200 that can also be used to indicate the need for a defrost cycle and the end of the defrost cycle as discussed there are various ways to defrost an evaporator 105 e g passing hot engine coolant or refrigerant through the third heat exchanger 130 electric heat etc the particular system or method used is not important to the invention described herein after the defrost cycle is complete the.