Failures of friction bearings of the crank mechanism comprise from 5 to 25 % of engine failures. The analysis of the main reasons for failures shows that the dominant reasons are the following: excess of loading conditions; severe operating conditions; non-observance of the periodic maintenance of the lubrication system; violation of the procedure and conditions of maintenance (contamination, oil residues, etc.); use of poor-quality oils and filters, etc. It is possible to prevent the growth of failures of friction bearings by a continuous monitoring of their complex technical state. For that purpose, we have supposed that the technical state of the crankshaft main bearings of the crank mechanism can be determined by measuring the pressures in the central oil line and calculating their difference in the cycles with the maximum load and without it at different engine crankshaft rotation frequency. As a result of the experimental work, we developed a method for in-place diagnostics of the state of friction bearings of the internal combustion engine, as well as an instrument that provides loading conditions for the bearings of the crank mechanism. We obtained an experimental dependence for determining the wear degree of the crankshaft main journal by the difference in the minimum pressure amplitudes of two adjacent cycles during the operation of the diagnosed cylinder in various modes.