Auto A/C Terms - Glossary
Vacuum Reservoir

Typically a small canister mounted on or near the evaporator case under the hood of the vehicle, the vacuum reservoir is designed to do nothing more than hold vacuum for use by the mode doors of the temperature control system.

Mode doors direct the air flow inside the vehicle. Located in the plenum, the mode doors will direct air to the defrost, dash or floor vents. When controlled by vacuum, those doors rely on the supply of vacuum from the vacuum reservoir.

The vacuum reservoir is required so that mode door positions are maintained when the vehicle’s engine vacuum is reduced, such as times when the vehicle is under hard acceleration.