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Modern hydromechanical transmissions are equipped with electronic-hydraulic (mechatronic) control systems having much more functional capabilities compared to classic hydraulic systems. However, automatic hydraulic systems are widely used not only as the basic transmissions' control device and a mechanical part of mechatronic systems but also as an important facility providing reliability and safety of such systems. At the same time such an application has quite a number of restrictions, especially for transmissions with a great number of gearstages. One of the main reasons is the inability to control more than 3 or 4 stages of gears while using the vehicle's speed as a control parameter. It deals with the parabolic characteristic of the output pressure of centrifugal governor used as the vehicle speed sensor. This restriction can be eliminated by using a hydraulic detector of torque converter's turbine (i.e. input shaft of gearbox) speed.

Modern vehicles have a number of safety systems, main of which are basic control systems - steering and braking systems. Requirements to these systems, defined by the vehicles design safety, are regulated by the system of the international and national standards and rules. Mechatronic control systems of the hydromechanical transmissions pertain to the “yongest” automotive systems, and questions dealing with their safety don't have yet the complete theoretical and normative base. The connection between these mechatronic systems safety and their structure is investigated in this paper. The method of the safety analysis with help of the “refusal graphs” is proposed, which permits to bring out and to eliminate the possibility of the dangerous refusals capable to create the crash situations during automobile motion.