In common with other owners and operators of large fleets of vehicles, the U.S. Army is faced with a major maintenance task. A significant part of this task is the correct and timely diagnosis of the vehicle faults which cause down time and those which can propagate into secondary, and usually more serious, damage. The Army, of course, has two operational situations which differ in type and degree from those of commercial fleets. First, the duty cycle for Army vehicles is less predictable and instant availability is essential. The second is that Army vehicles are armored, shielded and waterproofed (fordable) to an extent that test point access is severely limited in comparison to commercial vehicles. To grapple with this situation, the U.S. Army Tank-Automotive Command has placed a series of contracts for feasibility studies with RCA's Aerospace Systems Division to investigate the practicality and the means for preinstrumentation of vehicles and for devising appropriate control and readout test units compatible with this built-in instrumentation.In the process of this work, several new transducers have been developed, and a concept for testing for engine power has been applied.Two new instruments have been designed and tested for feasibility. A “vehicle readiness unit,” suitable for use in on-board monitoring of road tests and sample runs, with provision for a simple engine power test and for detecting marginal ignition systems, is also described. A “vehicle test meter,” which permits a diagnostic survey to be performed at better than a 10:1 time savings over conventional maintenance procedures, is discussed.In addition, a computer-controlled unit developed by the Army for detailed diagnostic tests has been adapted and software developed to permit operation with the built-in instrumentation (diagnostic connector).The basic objectives and operating principles of these devices are presented.