A vehicle includes an engine, a transmission with a shift valve, an alternator, an auxiliary battery, a voltage sensor operable to measure an auxiliary voltage, and an electronic transmission range selection (ETRS) system. The ETRS system includes a controller, and also an electronic range selector device that transmits an electronic range request signal to the controller to request the shift of the transmission to a requested operating range. The controller executes a method to predict a loss of auxiliary power aboard the vehicle, and to control an action of the vehicle in response to the predicted loss of auxiliary power. The electronic range request signal is received from the range selector device and auxiliary voltage. The controller calculates a slope of a decrease in the measured auxiliary voltage. A control action is executed with the alternator is in the charging state and the calculated slope exceeds a calibrated slope threshold.