The Automotive Vacuum System - Craftsman 20930 Operator's Manual

THE AUTOMOTIVE VACUUM SYSTEM

This manual deals with vacuum, how it is used in
various automotive systems and how the vacuum
pump can be used to test and diagnose these
systems. This section discusses what vacuum is,
how it is measured, where it comes from on an
automobile, the system for distributing and using
vacuum, and some troubleshooting basics.
WHAT IS VACUUM?
Put simply, vacuum is empty space, and may exist
as either a total or partial vacuum. Vacuum does
not, of itself, create power. Rather, power for
vacuum devices depends on the presence of
atmospheric pressure. The atmosphere exerts a
pressure of 14.7 pounds per square inch (psi) on
everything at sea level. If a portion of the air is
removed from one side of a diaphragm (partial
vacuum), the atmospheric pressure will exert a
force on the diaphragm. The force is equal to the
pressure difference times the diaphragm area
(FIGURE 1). Generally, the less air (greater vacuum)
in a given space, the more the atmosphere tries to
get in and the more force is created.
HOW IS VACUUM MEASURED?
In the United States, vacuum is commonly measured
in inches of Mercury (" Hg). It may also be
measured in centimeters of Mercury (cm Hg) and
kiloPascals (kPa). Atmospheric pressure will support
a column of Mercury in a manometer gauge about
30 inches high or about 76 cm high. This is the
barometric pressure in " Hg which varies as the
weather changes. Vacuum readings in " Hg are
really negative pressure readings. For example,
30" Hg vacuum would be a complete vacuum. Half
of a complete vacuum would be 15" Hg. A gasoline
engine at idle usually pulls about 16-22" Hg
vacuum. On deceleration, because the throttle is
closed, the vacuum will increase. The pump will pull
about 25" Hg as indicated on its vacuum gauge
which is calibrated in both " Hg and kPa.
WHY ENGINES CREATE VACUUM
Vacuum is created when air is withdrawn from
a given volume, or a sealed volume is increased.
That is why vacuum is available in an engine.
On the intake stroke, the piston moves down, this
creates a partial vacuum because the volume of the
cylinder is increased. Air cannot rush through the
intake system fast enough to totally fill the space
created when the piston moves down (FIGURE 2).
This is the most common automotive vacuum
supply source.
GASOLINE VS. DIESEL VACUUM
Because a diesel engine does not produce as much
vacuum as a gasoline engine, a mechanical vacuum
pump must be employed to operate vacuum
devices. The pump is useful in testing devices
on both types of engines.
14.7 - 10.7 = 4 PSI
FIGURE 1:
VACUUM VS. ATMOSPHERIC PRESSURE
Intake Stroke
FIGURE 2: THE ENGINE AS A VACUUM SOURCE
4
Piston area 10 sq. in.
14.7 PSI
10.7 PSI
40 Pounds
Vacuum Port
Vacuum
Air