Worldwide interest in secondary braking systems on buses and trucks is increasing. The causes of this interest are the needs to extend service brake life and to help meet upgraded European braking standards. To satisfy this demand, Detroit Diesel Allison has developed an integral output retarder for the Allison MT600 transmission with future adaptability to the V730 and HT740 transmissions. This retarder provides sustained high power absorption at high output speeds as well as braking capability down to very low output speeds.The unique configuration of the Allison retarder includes both friction clutch and hydraulic retardation. Development of the retarder required combining these two methods through use of a control system which provides quick response leading to smooth braking and suitable operator controllability.