A non-destructive impact sensor to measure on-line fruit firmness was evaluated. This sensor is an adaptation of a static model used in the laboratory to measure fruit quality and was installed in an experimental fruit packing line with a commercial sizer chain. The firmness index is related to the acceleration-time curve supplied by an accelerometer attached to an impacting arm. The main objective of this study was to evaluate sensor performance and sources of variation. We made classification trials on three fruits: peaches (Prunus persica (L.) Batsch), apples (Malus domestica Borkh.), and pears (Pyrus communis L.), as well as working trials, such as placing the fruit, orientation, and others. The sensor works correctly at a speed of 7 fruits s-1 (0.63 m s-1) and allows fruit classification at three levels of firmness using specific software. Good discrimination was obtained only for soft peaches. There were variations in results between different fruits and different parts of the same fruit mainly due to the non-uniformity of fruit shape and lack of ripeness homogeneity of each one.