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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 merely
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 adjust-
able throttle stop that can be positioned
by a computer controlled motor. The
second method allows the throttle to
close completely, then has a computer
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 compres-
sor 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 engines. This
device is described later in the MCU
section.
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 advance signal is
sent by the computer 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 partially
open EGR valve at other times can
cause stalling.
• Thermactor Air System – This system
works with the catalytic convertor. The
computer takes outside air from an air
pump and directs it to the exhaust
manifold or catalytic convertor as
necessary for best emission
performance. Refer to "Thermactor Air
System" in Reference Glossary for
more explanation.
• Fuel Evaporation Recovery System –
A special canister collects vapors
evaporating from the fuel tank,
preventing 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
54
vehicle service manual.
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