Microprocessor based automatic test equipment has made it possible for maintenance personnel to conveniently make sophisticated tests of vehicle engines. In some cases, the most difficult part of the process is no longer the data evaluation, but rather the data acquisition or sensing. This has made sensor development an important part of improving vehicle testing. This paper discusses the development and operating principles of a diesel engine speed sensor which improves substantially the mechanic's access to the speed signal, thereby augmenting the use of existing automated tests based on speed signal processing. Developed by the U.S. Army Tank Automotive Command, this Variable Reluctance Sensor (VRS) overcomes problems of diesel speed signal accessibility. Conventional speed signal sources, such as tachometer drives, are difficult for a military mechanic to access due to the presence of engine accessories and armor. The VRS senses the motion of a rocker arm by projecting a strong magnetic field around it and detecting the field variations caused by its motion. Thus the mechanic can measure engine speed without tools by simply placing the VRS on the valve cover. Then using Simplified Test Equipment for Internal Combustion Engines (STE/ICE) the mechanic can rapidly assess engine speed adjustments and power. This paper describes the technical problems encountered in VRS development and reviews the development sequence. It then presents the final mechanical configuration of the VRS and details the signal processing logic and circuitry. Finally the intended VRS applications are discussed, with mention of potential future uses with Army automated test equipment.