The hardware and software for a prototype computer controlled cooling system for a diesel powered truck has been designed and tested.The basic requirements for this system have been defined and the control functions, previously investigated in a study using the computer simulation model, were incorporated into the software.Engine dynamometer tests on the MACK-676 engine, comparing the conventional cooling system and the computer controlled system, showed the following advantages of the computer controlled system: 1. The temperature level to which the engine warms up to at low ambient temperature, was increased. 2. The faster shutter response reduced the temperature peaks and decreased total fan activity time. 3. The faster fan response reduces fan engagement time which should improve truck fuel economy.