Abstract: An air flow control mechanism for use in a radiator system of the type having a radiator core constructed from a plurality of parallel core sections which are equally spaced apart and aligned in a single row with the opposing faces of adjacent core sections being open to provide air flow passageways through each core sections at right angles to the cooling air directed at the front of the radiator core. The radiator air flow control mechanism comprises gate means operatively connected in each of the spaces between adjacent core sections to pivot between a first and second position, where the gate means in the first position directs the cooling air through each of the core sections' air flow passageways in one direction and where the gate means in the second position reverses the direction of flow through each of these passageways. A drive means is provided for reciprocally moving the gate means between the first and second positions.