Modern vehicles require a low aerodynamic drag to minimize fuel consumption. A not negligible share of the overall CD-value of a vehicle is produced by the engine compartment air flow. Therefore this share has also to be optimized. Furthermore, customer wishes for higher powered engines as well as for more safety and comfort result in more tightly packed engine compartments. Even the reduction of pass-by-noise required by legal reasons is often achieved with the help of underbody covers which in turn affect the engine compartment flow.All these items may lead to rising underhood temperatures. To reduce the development time of new vehicles, numerical simulations of engine compartment air flow are more and more used to predict high temperature fields and to show ways to develop suitable remedies in the concept phase of the vehicle development. The experimental basis for such codes is provided by aerodynamic investigations in a wind tunnel. At least the three components of local velocities and the volumetric air flow at the inlet and the outlet plane have to be determined.This paper first reports the development and the use of a modern aerodynamic “Test Radiator” for the investigation of velocity distribution and for the volumetric air flow measurements through a car cooling package. For this purpose air vanes of small diameter, able to detect even reverse flows, are used.Furthermore, the development of various LDV-techniques, suitable for the investigation of complex flow fields, for instance behind a rotating fan are reported. And a newly developed 3-D miniature-probe is shown.The influence of various cooling configurations, with/without condenser in front of the radiator, and with stationary/rotating puller fan and with/without vehicle speed is also shown.