Actron GM Code Scanner CP9001 - Manual (Page 31)

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Computer controlled
emission systems
• EGR (Exhaust Gas Recirculation)
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. Various types of EGR
systems are used on different vehicles.
The EGR valve may be operated by
engine vacuum or with a computer
controlled electrical (or vacuum)
signal. See “EGR” in Glossary (Section
8) for more details.
• Air Injection System
This system reduces harmful carbon
monoxide (CO) and hydrocarbon (HC)
emissions. The computer takes outside
air from an air pump and directs it to
the exhaust manifold during engine
warm-up. (The extra air helps partially
burned exhaust gases to completely
burn and reduce pollution.) After
warm-up, depending upon vehicle, the
air pump may send air down to the
catalytic convertor or “dump” it back to
the atmosphere. Various types of Air
Injection systems are used on different
vehicles. Refer to “Air Injection
System” in Glossary (Section 8) 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 opens a connection
between the cannister and the engine
(by energizing the Purge solenoid.)
Then, engine vacuum draws the trapped
fuel vapors into the engine for burning.
Other computer
functions
The computer often controls various
other functions around the vehicle.
Detailed explanations may be found in
your vehicle service manual. Some
typical examples are...
• Air Conditioner (A/C) Clutch
The computer can turn off the air
conditioner to reduce engine loading.
This is desirable during heavy
acceleration, engine cranking or low
speed steering maneuvering. The
computer may also disable the air
conditioner when refrigerant pressure
is too low (or high) to prevent A/C
damage. The computer stops the air
conditioner by using a relay to
disconnect voltage from the A/C
clutch.
• Radiator Cooling Fan
The computer controls engine cooling
fan (electric type) operation on most
vehicles. Usually the fan is turned on
when engine temperature exceeds a
certain level or when the air condition-
ing system is used. The computer uses
a relay to power the fan. Some
vehicles have a second fan for extra
cooling.
• Variable Assist Power Steering
Saturn Vehicles: This system delivers
power steering assist based on vehicle
speed - little assist during high speed
straight steering, increased assist for
low speed turns. The computer
controls fluid flow in the power steering
pump using an Electronic Variable
Orifice (EVO) actuator (more fluid flow
provides more steering assist). Refer
to “EVO” in Glossary (Section 8) for
more details.
• Torque Convertor Clutch
The computer can control the lock-up
clutch in an automatic transmission
torque convertor. The clutch is locked
during steady, warm engine cruise