Actron GM Code Scanner CP9001 - Manual (Page 30)

30
Acceleration, deceleration and idle
conditions
As long as the engine and oxygen
sensor are hot, the computer can
operate “closed loop” for best economy
and least emissions. During the drive
conditions listed on the left, the
computer may have to ignore the sensor
and run “open loop,” relying on internal
programming for fuel delivery instruc-
tions. During idle, for example, the
oxygen sensor may cool down and stop
sending a signal. A different situation
can occur during wide-open-throttle
acceleration. The computer sometimes
adds additional fuel (on purpose) for
temporary acceleration power. The
computer knows it is running “rich” so it
ignores the sensor signal until the wide-
open-throttle condition is over.
How the computer controls idle
speed
Throttle position and RPM sensors tell
the computer when the vehicle is
idling. (Sometimes an idle position
switch on the throttle is used.) The
computer simply watches RPM and
adjusts an idle speed control device
on the vehicle to maintain the desired
idle condition. Note that this is another
example of “closed loop” operation.
The computer performs an action
(activating an idle control device),
then watches the results of its action
(engine RPM) and readjusts as
necessary until the desired idle speed
is achieved.
There are two types of idle speed
control devices. The first is an
adjustable throttle stop that is
positioned by a computer controlled
motor. The second method lets the
throttle close completely. An air
passage bypassing the throttle allows
the engine to idle. A computer
controlled motor adjusts air flow
through the bypass to set idle speed.
Smaller engines can stumble or stall
at idle when the air conditioner
compressor turns on or the power
steering is used. To prevent this,
switches tell the computer when these
demands are coming so it can
increase the idle accordingly.
How the computer controls spark
advance timing
You set spark timing in a non-computer
engine by using a timing light and
adjusting the distributor at idle RPM.
During vehicle operation, timing is
changed by either engine vacuum
(vacuum advance function) or by engine
RPM (centrifugal advance function.)
These spark timing changes are done
mechanically inside the distributor.
Computer controlled vehicles using a
distributor still have you set spark timing
by using a timing light and adjusting the
distributor at idle RPM. The timing
changes which occur during vehicle
operation, however, are controlled
electronically. The computer looks at
sensors to determine vehicle speed,
engine load and temperature. (RPM,
throttle position, coolant temperature
and manifold pressure or mass air flow
sensors are used.) Then, the computer
adjusts timing according to factory
programmed instructions. Some
vehicles have a “knock” sensor. The
computer can “fine tune” the spark
timing if this sensor signals an engine
knock condition. A timing signal (“EST” -
Electronic Spark Timing) is sent by the
computer to an ignition module which
eventually creates the spark. The
computer uses a crankshaft position
sensor to determine piston position, so
it can send the spark timing (EST)
signal at the proper moment.
Newer ignition systems use no
distributor. There are several versions,
such as Computer Controlled Coil
Ignition (C3I), Direct Ignition System
(DIS), Integrated Direct Ignition (IDI)
and Opti-Spark. These systems use
multiple ignition coils. (2 spark plugs
are wired to each coil.) Sensors for
crankshaft position or camshaft
position (or both) are used by both the
ignition module and computer to fire
the correct coil at the proper time. The
computer provides spark advance
timing as before - by looking at vehicle
speed, engine load and temperature.
(RPM, throttle position, coolant
temperature and manifold pressure or
mass air flow sensors are used.) Refer
to vehicle service manual for detailed
ignition system descriptions.