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The internal combustion engine in a common automobile produces almost 20 inches (51 cm) of vacuum, and this pressure differential may be harnessed for many uses. Engine vacuum is also the best direct source of information on the engine's load. Most delay valves have a one-way function, where there is either no restriction or no movement in one direction.
There are several common types of vacuum switches.
A valve that only allows the vacuum signal to move in one direction. Often used with vacuum reservoirs.
A vacuum delay valve is a valve with a small orifice, which delays a vacuum signal. These are commonly used in automobiles to alter the behavior of a vacuum signal. Delay valves are usually color-coded to their function.
Coolant temperature override
Also referred to as a Temperature Vacuum Switch (TVS) or Thermal Vacuum Valve (TVV). The CTO switch measures coolant temperature, usually from a passage in the intake manifold. It was commonly used starting in the 1960s for switching the vacuum advance from ported vacuum to manifold vacuum at high engine temperatures to promote cooler idling. As emissions controls expanded, it gained many other uses.
They commonly have between two and five vacuum ports.
Thermostatically-controlled air cleaner system
The factory air cleaner on carburetored vehicles has a switch controlled by a bi-metallic strip that ultimately selects cool or preheated air. This valve is usually open on one side to the air space within the air filter, and is both temperature controlled, and bleed style. Its function is to keep the air going in to the carburetor above a certain temperature to prevent icing.
Non-linear vacuum regulator
A NLVR regulates vacuum signal so it is between two vacuum source levels. As one signal increases the other regulator valve switches to another vacuum source.
A bleed valve controls a metered orifice that adjusts to allow air into the line to weaken a vacuum signal. They are commonly controlled by coolant temperature or under hood air temperature, but may be part of the HVAC system.
A vacuum booster uses one vacuum signal, usually ported, to control another signal, usually manifold, that is stronger. It may also use a venturi effect.
Positive crankcase ventilation valve
The positive crankcase ventilation (PCV) is a one-way valve that ensures continual refreshment of the air inside the engine's crankcase.
Vacuum electrical switch
A vacuum switch that turns an electrical connection on or off. Used for early electronic control modules to determine engine load or function.
An electrical solenoid that switches a vacuum signal on or off.
Manual vacuum switch
Operator controlled switches were often used to control vacuum powered devices before the advent of robust automotive electrical systems or reliable mechanical linkages. They consisted of a variety of shapes, sizes, and configurations much like electrical switches.
They are commonly found as window and HVAC controls.
The term "vacuum switch" can also refer to a specialized type of electrical switch in which the contacts are enclosed in a vacuum envelope. This allows the switching of very high voltages with a compact switch, while preventing arcing across the contacts.