Embodiments of a powertrain cooling system and embodiments of a radiator module are provided for deployment onboard a vehicle. In one embodiment, the powertrain cooling system includes a first airflow path, a second airflow path, a primary radiator core positioned in the first airflow path, an auxiliary radiator core position in the second airflow path, and a first airflow valve positioned in the second airflow path upstream of the auxiliary radiator core. The first airflow valve is movable between a closed position wherein the first airflow valve impedes airflow to the auxiliary radiator core and an open position.