Recently, application of electronics to construction machinery has proceeded rapidly. And various electronic control systems have been developed for off-highway dump truck.In this paper, we explain an automatic retarder control system, as an example of effective use of on-board computers. This system controls the cruising speed at a constant set value while monitoring the brake's cooling oil temperature. Two of the most important functions of this system is prevention of brake trouble, and fine speed control. The former is automatic restriction of maximum descending speed by feeding information of brake's cooling oil temperature into the logic control unit. The latter is an adaptive control of brake force under various operating conditions. This control system was equipped to our newly developed dump truck, HD1600M-one of our exhibit at CONEXPO ‘81.