Cooling system for a hybrid traction train of a motor vehicle

The system has a branch connecting an output of a heat engine (1) to a radiator (5), and a main thermostatic valve disposed between a spur of a derivation branch (21) and the radiator on a conduit. A thermostatic mixing control valve (20) is connected between an additional output (12) of the radiator and the derivation branch and is connected to a traction chain through its output (37).