OUTPUT
ACTUATORS
SENSORS
INPUT
BRAINS OF THE
COMPUTER
Some examples of sensors: thermistors
(for temperature readings), potentiom-
eters (like a throttle position sensor), re-
lays(for voltage and signalreadings),and
signal generators (such as an 02 sensor).
The network of sensors has the job of
deliveringinformationthecomputerneeds
to know by converting it into electrical
signalsthecomputercanunderstand.Sig-
nals running from sensors to the PCM are
referred to as “inputs.”
Sensors monitor key things such as:
• Engine Temperature
• Intake Manifold Vacuum
• Throttle position
• RPM
• Incoming Air Temperature
• Volume of Incoming air
• Air Fuel Ratio, in percentage ( % )
Switches and Actuators are electric de-
vices energized by the computer to allow
commands to perform a specific function.
Switches are often called relays (such as
the coolant fan switch). 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 switches
and actuators. Sometimes information is
sent to other system computers like trans-
mission, brakes, ignition modules, and trip
computers. Signals running from the PCM
to other components are called “outputs.”
How the computer controls fuel deliv-
ery
Engine operation and emissions perfor-
mance depend upon precise fuel delivery
and ignition control. Early computer sys-
tems controlled fuel by electronically ad-
justing the carburetor metering and jet
systems. Soon, however, this was re-
placedby themoreprecisefuel deliveryof
fuel injection.
In an electronically carbureted system,
the computer simply controls fuel flow
basedon howfar the throttleis 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 somewhat more sophis-
ticated in the way it delivers fuel. The
computerstilladdsanappropriateamount
of fuel to the entering air, but now it uses
fuelinjectors(either in a throttle bodyorat
each intake port). Fuel injectors are far
more precise than carburetor jets, and
create a much finer fuel “mist” for better
combustion and increased efficiency. In
addition,mostfuelinjection systems have
ways of measuring exactly how much air
is entering the engine, and can calculate
the proper air/fuel ratio using lookup
tables. Computers no longer have to “es-
timate” how much air the engine is using.
In many modern systems, the computer
also uses information provided by sen-
sorstogiveitanideaofhowwellitisdoing
itsjob,andhowtodoitbetter.Sensorscan
tell the computer how warm the engine is,
how rich or lean the fuel mixture is, and
whether accessories (like the air condi-
tioner) are running. This feedback infor-
mation allows the computer to “fine tune”
the air/fuel mixture, keeping the engine
operating at its peak.
What the Computer needs to know:
•Engine operating condition. Sensors
used are: coolant temperature, throttle
14 15
position, manifold pressure (vacuum),
air flow and RPM.
•Air intake. Sensors used are: mass air
flow,manifoldabsolutepressure,mani-
fold air temperature and RPM.
•Air/fuel mixture status. Sensors used
are: oxygen sensor(s).
NOTE: Not all engines use every sensor
listed above.
Open and Closed Loop Modes:
Open or closed loop operation refers to
the way the computer is deciding how
much fuel to add to the air entering the
engine. During cold start and other low
demand, low temperature situations, the
computer operates in open loop mode.
This means that it is relying on a set of
internal calculations and data tables to
decidehowmuch fuel to add to the incom-
ingair.Itusessensorssuchasthecoolant
temperature sensor (CTS), the throttle
position sensor (TPS), and the manifold
absolute pressure sensor (MAP) to deter-
mine optimum mixtures. The important
difference here is that it
does not
check to
seeif the mixturesarecorrect, leaving the
computer adjustment loop open.
In closed loop mode, the computer still
decideshowmuchfueltoaddbyusingthe
sensors listed above, and by looking up
the appropriate numbers on a data table.
However,itnowchecksitselftodetermine
whether the fuel mixture is correct. It is
able to check itself by using the informa-
tion provided by the oxygen sensor(s)
(O2S) in the exhaust manifold. The oxy-
gen sensors will tell the computer if the
engine is running rich or lean, and the
computer can take steps to correct the
situation.Inthisway,thecomputercloses
theadjustmentloopbycheckingitselfand
making necessary corrections. It should
be noted that the O2 sensors must be at a
veryhighoperatingtemperature(approxi-
mately 650° F) before they will begin to
feed information back to the computer.
This is why open loop mode is neces-
sary—togive the O2sensors time towarm
up to operating temperature.
As long as the engine and O2 and Coolant
TemperatureSensorsareatoperatingtem-
perature, the computer can operate in the
closed loop mode. Closed loop mode in-
suresthattheair/fuel mixture is at theideal
14.7:1 air/fuel ratio needed for efficient
combustion. But in stop and go cycles, the
O2 sensor may in fact cool down enough
that the computer will need to rely on a set
of internal parameters and go into open
loop mode again. In some cases, this may
also happen during extended periods of
idling. Many newer vehicles now use
heated O2 (HO2S) sensors to prevent this
condition.
In many vehicles, the computer controls
other systems related to open and closed
loop modes, including idle speed, elec-
tronicspark control, exhaustgasrecircula-
tion, and transmission torque converter
clutches.Inopenloopmode,someofthese
systemswillbeadjustedtospeedthewarm-
ingof the engine andget the computer into
closed loop mode as quickly as possible.
OBD II: The Next Horizon
In1994,manymanufacturersbeganequip-
ping cars with a new class of computer
technology which puts more processing
powerunder your dashthan everbefore. It
is called On-Board Diagnostics, Second
Generation, or OBD-II. It is required on all
vehicles sold in the US beginning January
1, 1996 (though most domestic manufac-
turers introduced it earlier than required),
and offers increased system monitoring
and diagnostic information. This new sys-
tem will store a library of 400 general
trouble codes and another 400 manufac-
turer-specific codes. These codes cover
Body Systems (B-Codes), Chassis Sys-
tems(C-Codes),andPowerTrainSystems
