A cooling system for a heavy road vehicle comprising a hybrid propulsion system , said hybrid propulsion system comprising - a first, mechanical propulsion system comprising a mechanical drive train including an internal combustion engine which provides a traction force to a first traction wheel via a gearbox , - a second, hydraulic propulsion system comprising a hydraulic pump unit for powering a hydraulic motor in order to provide a traction force to a second traction wheel , wherein said cooling system further comprises a pump and a first cooling circuit including a gearbox cooling circuit , a hydraulic propulsion cooling circuit and a first radiator for cooling of a coolant flowing in the first cooling circuit, said radiator being connected in series with and located upstream of the gearbox cooling circuit and the hydraulic propulsion cooling circuit , characterized in that - said hydraulic propulsion cooling circuit and the gearbox cooling circuit are connected in parallel, wherein - a temperature dependent thermostat is positioned downstream said hydraulic cooling circuit , said temperature dependent thermostat being configured to increase the coolant flow through the gearbox cooling circuit relative the coolant flow through the hydraulic propulsion coolant circuit if a measured temperature of the coolant flow downstream said gearbox cooling circuit exceeds a predetermined threshold temperature, and wherein the cooling system further comprises a control unit configured to turn off the hydraulic propulsion system if a measured temperature of the hydraulic propulsion cooling circuit and the gearbox cooling circuit is above a predetermined temperature threshold limit. - a first, mechanical propulsion system comprising a mechanical drive train including an internal combustion engine which provides a traction force to a first traction wheel via a gearbox , - a second, hydraulic propulsion system comprising a hydraulic pump unit for powering a hydraulic motor in order to provide a traction force to a second traction wheel , wherein said cooling system further comprises a pump and a first cooling circuit including a gearbox cooling circuit , a hydraulic propulsion cooling circuit and a first radiator for cooling of a coolant flowing in the first cooling circuit, said radiator being connected in series with and located upstream of the gearbox cooling circuit and the hydraulic propulsion cooling circuit , characterized in that - said hydraulic propulsion cooling circuit and the gearbox cooling circuit are connected in parallel, wherein - a temperature dependent thermostat is positioned downstream said hydraulic cooling circuit , said temperature dependent thermostat being configured to increase the coolant flow through the gearbox cooling circuit relative the coolant flow through the hydraulic propulsion coolant circuit if a measured temperature of the coolant flow downstream said gearbox cooling circuit exceeds a predetermined threshold temperature, and wherein the cooling system further comprises a control unit configured to turn off the hydraulic propulsion system if a measured temperature of the hydraulic propulsion cooling circuit and the gearbox cooling circuit is above a predetermined temperature threshold limit. <EOS>