A cooling system for a motor vehicle comprising: • a cooling circuit for the cooling of a combustion engine and a hydrodynamic retarder that is arranged to exert braking influence onto a drive shaft of the motor vehicle, by means of a cooling fluid that flows in the cooling circuit • a cooling fluid pump for the circulation of the cooling fluid in the cooling circuit, • a cooling fluid cooler that is connected into the cooling circuit for the cooling of cooling fluid, whereby this cooling fluid cooler comprises a cooling fluid inlet that is connected through the said retarder to a cooling fluid outlet of the combustion engine through a first line of the cooling circuit and a cooling fluid outlet that is connected to a cooling fluid inlet of the combustion engine through a second line of the cooling circuit, whereby the said first line is connected at a point that lies between the retarder and the cooling fluid cooler to the said second line through a third line of the cooling circuit such that it allows through this line return of cooling fluid to the combustion engine without passage through the said cooling fluid cooler, and • a thermostat arrangement with a first thermostat device arranged to regulate the flow of cooling fluid to the cooling fluid cooler and to the said third line, depending on the temperature of the cooling fluid that is led into the thermostat device from a pilot line that is a component of the cooling circuit, which pilot line is connected at its upstream end to the said second line. characterised in that the retarder is designed to use the cooling fluid flowing in the cooling circuit as retarding medium, wherein the thermostat arrangement comprises a second thermostat device arranged to receive cooling fluid in the first line from the retarder and to regulate the flow of cooling fluid to the cooling fluid cooler and to the said third line depending on the temperature of the cooling fluid received, and that the second thermostat device is designed to open for flow in the cooling circuit of cooling fluid that arrives at it onwards to the cooling fluid cooler with effect from a second temperature that is at least 5 K higher than a first temperature of cooling fluid arriving at the first thermostat device in the said pilot line and with which this first thermostat device is designed to open for flow of cooling fluid that arrives at this thermostat device from the retarder in the first line of the cooling circuit onwards to the cooling fluid cooler. • a cooling circuit for the cooling of a combustion engine and a hydrodynamic retarder that is arranged to exert braking influence onto a drive shaft of the motor vehicle, by means of a cooling fluid that flows in the cooling circuit • a cooling fluid pump for the circulation of the cooling fluid in the cooling circuit, • a cooling fluid cooler that is connected into the cooling circuit for the cooling of cooling fluid, whereby this cooling fluid cooler comprises a cooling fluid inlet that is connected through the said retarder to a cooling fluid outlet of the combustion engine through a first line of the cooling circuit and a cooling fluid outlet that is connected to a cooling fluid inlet of the combustion engine through a second line of the cooling circuit, whereby the said first line is connected at a point that lies between the retarder and the cooling fluid cooler to the said second line through a third line of the cooling circuit such that it allows through this line return of cooling fluid to the combustion engine without passage through the said cooling fluid cooler, and • a thermostat arrangement with a first thermostat device arranged to regulate the flow of cooling fluid to the cooling fluid cooler and to the said third line, depending on the temperature of the cooling fluid that is led into the thermostat device from a pilot line that is a component of the cooling circuit, which pilot line is connected at its upstream end to the said second line.