This paper introduced the influence of changing the excitation chopper of auxiliary generator to DC input on the circuit of diesel locomotive, and expounded the phenomena of faults, the search for faults, the causes of faults and the solutions of faults. It was determined that the faults were caused by the instantaneous connection of the supporting capacitor in the excitation chopper after the circuit design changes. Pool voltage drops, resulting in digital signal acquisition errors, which led to logical control disorder. Finally, the solution to the problem was put forward, and the correctness of the analysis conclusion and the solution was verified by the test.