Abstract: Apparatus for controlling the return air flow to the evaporator coil of an air conditioning unit that is mounted on the roof of a vehicle wherein the return air flow enters the unit vertically and must be turned 90.degree. in a limited amount of space before being delivered to the coil. A plenum is mounted over the return air duct that has a back wall containing an arcuate segment that curves inwardly over the entrance toward the entrance face of the coil. A stabilizer is mounted along the inner edge of the entrance in front of the coil which coacts with the contoured rear wall of the plenum to evenly distribute the return flow across the coil and prevent unwanted turbulence in the flow that would inhibit flow through the coil.