They use different seals to keep the air going through the radiator and not AROUND it... Then fan shrouds and fans are tested for best performance...
They test the Jeeps parked one foot from a wall and make them idle for over 1/2 hour to make sure that the fan, shroud, and seal packages direct the air to go through the radiator and the wind doesn't sway the results...
The climate control and engine cooling groups work together to develop the cooling module (radiator and condenser with the seals)... They take road test trips every winter and summer to extreme temperatures to validate that their systems work properly... There is lots of testing and validation on the components to make sure that they work properly... They do lots of lab testing before the road trips to make sure that they will survive the extreme temperatures... There are many people working on it to get it right...
