A system is provided for controlling a grain cart relative to a grain truck. The grain cart includes a grain tank and an unload auger configured to transfer crop material out of the grain tank. The grain truck includes a truck box extending from a first end to a second end. The truck box includes a top edge extending around the top of the truck box. The system comprises a ranging device and a controller. The ranging device is configured to identify a distance to the top edge of the truck box and identify a distance to an area in the truck box. The controller is configured to determine a position of the grain cart relative to the grain truck, and determine whether the grain cart is positioned near the first end of the truck box. If the controller determines that the grain cart is positioned near the first end of the truck box, the controller is configured to determine a fill level in the area based on the distance to the top edge of the truck box and the distance to the area in the truck box, determine whether the fill level exceeds a threshold, and if the controller determines that the fill level does not exceed the threshold, the controller is configured to start the unload auger.