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Idiomas disponibles
Idiomas disponibles
(Sometimes an idle position switch on the
throttle is used.) The computer merel y
watches RPM and adjusts an idle spee d
control device on the vehicle to maintai n
the desired idle condition. Note that this
is another example of "closed loop "
operatio n. The co mputer performs an
action (activating an idle control device) ,
then watches the results of its actio n
(engine RPM) and readjusts as necessary
until the desired idle speed is achieved.
There are two types of idle speed
control de vices. The fir st is an adjust-
able throttle stop that can be positiond
by a computer controlled m otor. The
second method allows the throttle to close
completely, then ha s a co mpu ter
controlled solenoid to pass air around
the closed throttle to set the 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.
A simple form of idle speed adjustment
using a "throttle kicker" actuator is used
on early EEC-IV V-8 engine s. Th is device
is described later in the MCU se ction.
You set spark timing in a non-computer
engine by using a timing light an d
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 timin g
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.
coolant
throttle position,
and manifold pressure or mass air flow
sensors are used.) Then, the computer
adjusts timing according to factory
(RPM ,
temperature
54
programmed instructions. Some vehicles
have a "knock" sensor. The computer
can "fine tune" the spark timing if this
sensor signals an engine kno
timing advance signal is sent by the
compu t er to an ignition module which
eventually creates the spark.
• EGR Valve – The EGR valve lets
exhaust gases re-enter the intake
manifold and mix with the incoming air/
fuel. The presence of exhaust gases
reduces combustion temperatures in
the cylinders and this reduces
poisonous NOx emissions. The
computer controls the flow of gases
through the EGR valve. The EGR
system is only used during warm
engine cruise conditions. A partiall y
open EGR valve at other times can
cause stalling .
• Thermactor Air System – This system
works w ith the catalytic convertor. The
computer takes outside air from an air
pump and directs it to the exhaust
manifold or catalytic convertor as
necessary f or best emission perfor-
mance. Refer to "Thermactor Air
System" in Reference Glossary for
more explanation.
• Fuel Evaporation Recovery System –
A special canister collects vapors
evap orating from the fuel tank,
prevent ing them from escaping into
the atmosphere and causing pollution.
During warm engine cruise conditions,
the computer draws the trapped
vapors into the engine for burning.
(See "CANP" in Reference Glossary.)
The computer controls other odd jobs
like handling "speed control" and
transmission torque convertor lock-up
and shifting functions. Detailed
explanations may be found in your
vehicle service manual.
The Reference Glossary describes the
various sensors and actuators used in
the EEC-IV and MCU systems. You can
learn more by reading these definitions.
ck c ondition. A
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