The lines running from the transmission to the radiator contain transmission fluid (oil) which runs through a heat exchanger sealed inside a side tank of the radiator. When you start out with a cold car the radiator warms up much faster than the transmission and the heat exchanger help bring the automatic transmission up to operating temperature sooner. On a long trip in the summer, the transmission runs hotter and the heat exchanger in the radiator helps keep the transmission cooler. As long as the grille block and belly pan don't cause the engine to overheat, you should be fine. About the only time you'd ever need worry is long trips (100+ miles oneway) with a heavily loaded vehicle in the heat of summer.
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