Abstract: A cooling system in a vehicle (1) powered by a combustion engine (2). A first line circuit (4) for cooling the combustion engine (2) and a second line circuit (5) which receives coolant from the first line circuit (4) for cooling at least one medium in a heat exchanger (14a, 14b). The second line circuit (5) includes a line (5b) with an extra radiator (9) and a bypass line (5c) with a valve (12) by which it is possible to distribute the coolant flow between the parallel lines (5b, 5c). A control unit (13) controls the valve (12) so that it leads at least a major part of the coolant through the line (5b) with the extra radiator (9) when there is need for extra cooling of the medium in the heat exchanger (14a, 14b) and so that it leads at least a major part of the coolant through the bypass line (5c) when there is no need for extra cooling of the medium in the heat exchanger (14a, 14b).