Abstract: A defrosting control for a heat pump system utilizes a first temperature sensor mounted to the outdoor coil fan motor windings, a second sensor mounted to sense the outdoor ambient air temperature and a third sensor to sense the refrigerant temperature in the outdoor coil. The control circuitry determines the temperature difference of the first two sensors, representing the fan motor temperature rise, which increases dramatically when the outdoor coil is frosted; if the difference exceeds a selected abnormal temperature rise and the temperature sensed by the third sensor indicates frosting is likely to have occurred, the defrost cycle is initiated. The cycle is terminated when the temperature sensed by the third sensor indicates that all frost is likely to have melted.