The purpose of this work is to prove the effectiveness of a practical study of the temperature regime for diagnosing bearing assemblies of mechanical transmissions. Using the example of a two-row roller bearing of the rear support of the intermediate shaft of the vehicle transmission, the temperature in the friction zone under different transmission operating modes on a KAMAZ-5320 was calculated. Finite element analysis of temperature fields made it possible to determine the relationship between the temperature on the outer surface of the bearing assembly and the temperature in the friction zone through the coefficient k=0.327. The zone for temperature measurement -the bearing cover is determined and the maximum temperature of its heating is set. Thermal imaging observations confirmed the effectiveness of the developed method of technical diagnostics of bearings.