An automatic door opener for delivery trucks may include a door opener motor, clutch, and transmission which are connected to a door opener rail. The automatic door opener is attached to the roof of a delivery truck cargo bay and the door opener motor is operable to open and close a cargo door. The clutch and motor are controlled by a motor control computer which may be programmed to automatically operate the clutch, close the cargo bay door, and disengage the clutch as desired. A rail carriage may be connected to the cargo bay door by a door connector arm which includes a quick release connection to the carriage and which may be operated from the outside of the cargo bay door. A sensor may be connected to the door opener rail and to the motor control computer such that the motor control computer tracks the position of the carriage along the door opener rail independent of the operation of the motor.