Studies indicate that a significant portion of all mining machine failures are associated with electrical system problems. As more highly automated machines are developed, the complexity of a machine's electrical control system can be expected to increase considerably, requiring a greater degree of skill and experience on the part of mine maintenance personnel. Knowledge-based expert diagnostic systems can provide a powerful method of improving equipment maintenance by quickly diagnosing control circuit malfunctions and recommending appropriate repair procedures. To investigate the applicability of expert systems in the area of control circuit diagnostics, the U.S. Bureau of Mines has developed a knowledge-based system to diagnose component malfunctions in the electrical control circuit of a continuous mining machine. The system leads the user through the appropriate procedures required to quickly identify the faulty control circuit component. Graphical displays are incorporated within the system to assist the user in locating the various components and test points. Once the faulty component has been isolated, the system is capable of accessing a database which can provide mine personnel with information concerning specific component part numbers and the availability and location of spare parts. This paper discusses the expert system development tool used to build the system, the structure and development of the knowledge base, and the software used to implement the graphical displays and equipment database.