For 1990-2009 cars

The vacuum solenoid module converts electrical signals from the A/C control head into vacuum signals. These vacuum signals position the mode, the A/C defrost, and the outside air valves in the air distributor.

The vacuum is constantly applied to the module, which distributes the vacuum to four solenoid-controlled valves in the module. When the A/C control signals the module to change an air distributor valve position, the associated solenoid energizes and the vacuum is applied to the proper acutator.