The problems connected with the diagnostic testing of modern automatic transmissions (AT) have been presented. The applicable diagnostic methods have been reviewed and evaluated. The systems and components subject to accelerated wear and tear have been indicated. The reasons for AT damage have been analysed and the consequences of specific defects have been discussed. A test system of unique design, specially engineered and made for automatic transmission diagnostics based on signals obtained from the hydraulic AT control system, has been presented. Example results of diagnostic tests have been given and analysed.