A system for blocking an output shaft of the engine of an automobile, of a type that includes a toothed wheel coaxially mounted on the shaft, and a first finger mobile between a shaft releasing position and a shaft blocking position in which the first finger is engaged in a gap formed between two teeth of the wheel. The system includes an alignment mechanism capable of aligning a gap between two teeth of the wheel and the first mobile finger, and the first mobile finger is configured to be inserted into the gap without any substantial circumferential clearance.