The most effective way of non-destructive control of the technical condition of a complex hydro mechanical system such as a steering drive is diagnostics by dynamic characteristics. The diagnostic signs considered in this article are: dynamic values of the pressure and flow rate of the working fluid, as well as the speed of the spool movement and the shift of the rod under disturbing influences. On the example of the steering drive KAU-30B, the possibility of diagnosing the wear of elements of hydro mechanical systems based on the analysis of transients is shown.