drops below a specific value the fan 15 can firstly be switched off the switch over valve 44 is not switched to bypass mode again unit the retarder is completely switched off in one developed embodiment there may be provision for the rotational speed of the fan to be controlled as a function of the respective retarder braking level for example the rotational speed of the fan can be increased at a high retarder braking level i e high braking torque request and thus a requirement for a large amount of heat to be conducted away while it is reduced when low braking torque is requested in addition to the fan 15 and the switch over valve 44 a rotational speed regulated coolant pump 7 is provided in the embodiment of a vehicle cooling system illustrated in fig1 the rotational speed regulated coolant pump 7 is also actuated by the control regulating device 24 it is possible for example when a retarder on instruction is present to adjust the rotational speed of the coolant pump to a predetermined significantly higher value than during normal driving mode the delivery capacity in the coolant circuit is increased and more heat can be conducted away than during the normal mode if the braking torque or the requested braking power drops below a specific value the rotational speed of the pump 7 can be reduced to the value required solely by the engine cooling system when the retarder is switched off by reducing both the rotational speed of.