TED (Turbine Engine Diagnostics) is a diagnostic expert system to aid the MI Abrams' mechanic find and fix problems in the AGT-1500 turbine engine. TED was designed to provide the apprentice mechanic the ability to diagnose and repair the turbine engine like an expert mechanic. This paper discusses the reasoning method used in TED, called Procedural Reasoning System (PRS), as well as various design considerations throughout the life of the project. The expert system was designed and built by the U.S. Army Research Laboratory and the U.S. Army Ordnance Center. TED has been fielded to both the active Army and the National Guard.