A vacuum switch is an automotive component that can be used to regulate the flow of manifold vacuum to various devices and accessories. In the same way that electrical signals are used to operate most engine components and various accessories today, vacuum was once the most easily controllable power source. Vacuum has been used to operate everything from windshield washer pumps and motors to choke pull-offs, and a switch of some sort is required in each one of these cases. A vacuum switch may be operated mechanically, by an electrical impulse, a change in temperature, or various other means.
Manifold vacuum is one of the common byproducts of operating internal combustion engines. This vacuum is a result of the way that internal combustion engines operate, due to piston movement during the intake stroke and the way that air is throttled when it is allowed into the manifold. The pressure differential between the intake manifold and the outside atmosphere results in a relatively steady amount of vacuum, which can then be used to power various devices. If a leak develops in a vacuum switch or line, excess air may enter the manifold and the engine will tend to run poorly.
In order to harness vacuum to power various accessories and devices, a number of different types of switches can be used. The simplest type of vacuum switch is a check valve, which can allow air to flow in one direction but not the other. This type of switch is essential to maintain the integrity of manifold vacuum in some cases. Delay valves are slightly more complicated, in that they contain external orifices and are able to stop the propagation of vacuum until certain conditions are met.
Other vacuum switches can use the engine temperature to change a vacuum supply. Temperature Vacuum Switches (TVS) are one example of this, in that they include a temperature sensitive component. When the coolant temperature rises over a certain threshold, a TVS can switch a carburetor's vacuum supply from ported to manifold. This increase in vacuum can then alter the operation of the carburetor to better respond to the driving conditions that resulted in the heated up coolant.
Heating, ventilation, and air conditioning (HVAC) systems in automobiles can also make use of vacuum switches. These switches are often mechanical in nature, in that an operator can turn a dial or move a lever which activates the switch directly. Cables, chains, and other devices often connect the HVAC controls to vacuum switches in older automobiles, allowing direct control of the vacuum flow. A vacuum switch in this case may open or close a coolant valve to the heater core or operate a blend door within the ducts.