An automated clutch between the engine and the variable-speed transmission in the power train of a motor vehicle is operated by a control unit which receives signals from several circuits and/or sensors via one or more conductors. The control unit shifts from a normal operating mode to an emergency mode when one or more sensors and/or circuits and/or conductors are defective. When in emergency mode, the control unit disengages the clutch when the driver starts the engine while the vehicle brake or the parking brake is applied. This ensures that the engine can drive the vehicle as soon as the transmission is shifted into a forward gear or into reverse gear and the application of the brake or brakes is terminated because the control unit is then again free to engage the clutch. Furthermore, such mode of operation enables the driver to shift the transmission into or from a selected gear in response to the application of a relatively small force to the gear shifting lever.