A method of thermodynamic diagnostics for solid hydraulic machines of construction excavators is presented. When working with solid hydraulic machines of construction excavators, the temperature of the working fluid increases, which can reduce the viscosity and efficiency of the system, as well as lead to component wear. It is shown that the method of thermodynamic diagnostics for solid hydraulic machines of construction excavators makes it possible to estimate energy losses, heat generation and the effect of temperature on the efficiency of the hydraulic drive system. Equations of the energy balance of a hydraulic pump with drainage and drainage-free mains are given. The results of modeling the thermophysical parameters of working fluids used in a hydraulic drive are obtained for known values of the thermophysical parameters of the fluid. The efficiency of solid hydraulic machines is determined depending on pressure and temperature differences. It is found that in the presence of undissolved gas in the hydraulic system, the hydrodynamic method for determining total efficiency provides a lower probability of rejection of serviceable hydraulic pumps than the method for diagnosing high volumetric efficiency. The higher the pressure drop and the lower the pump efficiency, the higher the measurement accuracy. The error in determining the total efficiency of a hydraulic pump by thermodynamic methods is influenced by the following factors: instability of the characteristics of the working fluid, the presence of undissolved gas in the working fluid, heat exchange between the hydraulic pump case and the environment. Recommendations are made to determine the efficiency of volumetric pumps in real-world operating conditions by measuring pressure and temperature differences.