Actron CP9135 OBD II AutoScanner User Manual
Page 31
About OBD II
B-2
What
does
the
computer
control
The
main
control
areas
of
the
vehicle
computer
are
fuel
delivery,
idle
speed,
spark
advance,
and
emission
controls.
Some
on-board
computers
may
also
control
the
transmission,
brakes,
and
suspension
systems
as
well.
What
has
not
changed
A
computer-controlled
engine
is
very
similar
to
the
older,
non-computerized
engine.
It
is
still
an
internal
combustion
engine
with
pistons,
spark
plugs,
valves,
and
camshaft(s).
The
ignition,
charging,
starting,
and
exhaust
systems
are
very
similar
as
well.
You
test
and
repair
these
systems
just
as
before.
The
technical
manuals
for
these
components
show
you
how
to
perform
the
tests.
Additionally,
compression
gauges,
vacuum
pumps,
engine
analyzers,
and
timing
lights
will
continue
to
be
useful.
The
engine
computer
control
system
The
vehicle’s
on-board
computer
is
the
“heart”
of
the
system.
It
is
sealed
in
an
enclosure
and
connected
to
the
rest
of
the
engine
by
a
wiring
harness.
The
computer
is
located,
in
most
cases,
in
the
passenger
compartment,
behind
the
dashboard
or
in
the
“kick
panel”
position,
although
some
manufacturers
locate
the
computer
control
module
in
the
engine
compartment
area.
Most
computers
can
withstand
a
lot
of
vibration
and
are
built
to
live
in
a
rugged
environment.
The
computer
is
programmed
by
the
manufacturer.
The
program
is
a
complex
list
of
lookup
tables
and
instructions
telling
the
computer
how
to
control
the
engine
based
on
various
driving
conditions.
To
do
its
job,
the
computer
uses
sensors
to
know
what
is
happening
and
then
provides
instructions
back
to
a
network
of
switches
and
actuators
throughout
the
vehicle.
Sensors
(computer
inputs)
Sensors
are
devices
which
measure
operating
conditions
and
translate
them
into
signals
the
computer
can
understand.
Some
examples
of
sensors:
thermistors
(for
temperature
readings),
potentiometers
(like
a
throttle
position
sensor),
and
signal
generators
(such
as
an
02
sensor).
Relays
and
actuators
Relays
and
Actuators
are
electric
devices
energized
by
the
computer
to
perform
a
specific
function.
A
relay
is
an
electromagnetic
device
(or
switch)
for
remote
or
automatic
control
that
is
actuated
by
the
computer
or
another
device.
Actuators
might
include
solenoids
(such
as
fuel
injector
valves)
and
small
motors
(such
as
the
Idle
Speed
Control).
Not
all
of
the
computer’s
outgoing
signals
are
routed
to
relays
and
actuators.
Sometimes
information
is
sent
to
other
system
computers
like
transmission,
brakes,
ignition
modules,
and
trip
computers.
These
signals
are
also
called
“outputs.”
How the Computer Controls Fuel Delivery
Engine
operation
and
emissions
performance
depend
upon
precise
fuel
delivery
and
ignition
control.
Early
computer
systems
controlled
fuel
by
electronically
adjusting
the
carburetor
metering
and
jet
systems.
Soon,
however,
this
was
replaced
by
the
more
precise
fuel
delivery
of
fuel
injection.
In
an
electronically
carbureted
system,
the
computer
simply
controls
fuel
flow
based
on
how
far
the
throttle
is
opened
by
the
driver.
The
computer
“knows”
how
much
air
can
flow
through
the
carburetor
at
various
throttle
openings,
and
adds
the
appropriate
amount
of
fuel
to
the
mixture
at
the
carburetor.
Fuel
injection
is
some
what
more
sophisticated
in
the
way
it
delivers
fuel.
The
computer
still
adds
an
appropriate
amount
of
fuel
to
the
entering
air,
but
now