Cooling circuit and method for transmissions having an integrated retarder and a separate oil arrangement for transmission and retarder

The cooling circuit comprises an oil/coolant heat exchanger (3) located inside the retarder (1) and an oil/oil heat exchanger (4) in the transmission unit (2). When the retarder (1) is not activated the oil cooled down inside the oil/coolant heat exchanger (3) can be guided through the oil/oil heat exchanger (4) in the transmission unit in order to deflect the heat from the transmission sump. An actuator for switching the coolant circuit on or off including the switch off valve of the retarder (1) can be part of the system.