Auto A/C Terms - Glossary
Mode Door

The plenum of the automotive air conditioning system works to direct incoming air through the dash vents, defroster or floor vents. This air distribution is effected by the position of the mode doors in the plenum.

Typically, the mode doors are operated by vacuum (manifold vacuum from the engine) or by small electrical motors. Simply by selecting a change in vents (like from dash to defroster) on the control head, the mode doors move to close and open air flow passages within the plenum.

The most common failure that occurs with vacuum operated systems is that no matter what position is selected on the control head, air distribution does not change. Typically it will be directed through the defrost or floor vents. This problem is caused by a leak in manifold vacuum or in the vacuum reservoir.

Usually located under the hood on the evaporator case, the vacuum reservoir stored manifold vacuum for use by the mode doors. That is so under hard acceleration, mode doors will be able to maintain whatever position they are in.

When the reservoir leaks or there is a serious vacuum leak in other vacuum lines under the hood, the mode doors will not have enough vacuum to ‘hold’ any other position but their default position. The default position will vary depending on the vehicle, but typically it is defrost or floor.

This condition is easily corrected by finding the vacuum leak and repairing the line or replacing the vacuum reservoir.