
The "brains" of a computerized engine control system is the Powertrain Control Module (PCM), which is also called the
Electronic Control Module (ECM), Electronic Control Unit (ECU),
microprocessor, control module or brain box. Regardless of the
name, the unit is really nothing more than a sophisticated calculator.
There's nothing magic about its capabilities, though some can do
some pretty amazing things. Even so, the PCM's basic function is
to crunch numbers. It takes in raw data from its various sensors,
translates the sensor data into numbers, then processes the numbers
to determine what needs to be done next.
The number crunching is controlled by a program. A program
is nothing more than a set of step-by-step instructions that
tells the computer how to manipulate the data that's fed into it.
It's like a road map with many possible routes and paths -- and
all based on a system of logic. "If this, then that" is the
decision making process that's used to determine the sequence of
events that take place as the numbers are crunched.
On GM computers, the program is contained in an "integrated
circuit" chip, a small rectangular black chip with numerous legs.
It's a special kind of chip called a "Program Read Only Memory" chip.
Changing the PROM chip recalibrates the fuel and ignition curves as
well as other functions the computer controls. GM devised the
replaceable PROM chip so the same computer could be recalibrated
for use in different applications. Ford, Chrysler and the imports
do not have replaceable PROM chips.
One of the nice things about having a replaceable PROM chip
is that it allows the computer to be "retuned" for increased
engine performance. Aftermarket performance PROMS have become a
popular item in recent years, a fact that has not escaped the
attention of the Environmental Protection Agency. The only
type of aftermarket PROMS that are emissions legal are ones that
do not increase emissions.
Back to the computer. The end product of all the number crunching
is a control output -- which is usually the switching on or off of
an electronic circuit. The computer does this by grounding the
circuit electronically. This completes the circuit allowing current
to flow through a solenoid or relay (sometimes referred to as an
"actuator" device).
COMPUTER OUTPUTS
When the incoming signal from the coolant sensor tells the computer
that the engine is getting hot, the output command that follows is
to turn on the cooling fan. The computer does this by grounding the relay
circuit that controls the electric cooling fan. When the relay
clicks on, the electric cooling fan starts to spin and cools the
radiator.
Another COMPUTER control function is locking up the torque converter
on automatic equipped vehicles. When input from the vehicle
speed sensor tells the computer the car is going fast enough to
lockup the converter (for better fuel economy), the computer
grounds the lockup solenoid circuit which causes the converter
lockup clutch to engage.
To help the catalytic converter burn the pollutants in the
exhaust, the computer controls the routing of air from the air
pump. When the coolant sensor tells the computer the engine is
cold, air is routed to the exhaust manifolds to help reduce
carbon monoxide (CO) and unburned hydrocarbons (HC) in the
exhaust. But as the engine warms up, vehicles with "three-way
plus oxygen" converters need the air rerouted into a middle
chamber in the converter so it can reduce oxides of nitrogen
(NOX).
Some other functions controlled by the engine computer
include opening the purge valve on the charcoal canister so
accumulated fuel vapors can be siphoned into the engine and
burned, controlling the exhaust gas recirculation (EGR) valve,
turning the electric fuel pump on and off, and regulating the
alternator's output voltage.
FUEL LOOPS
One of the most important control functions that the computer
handles is regulating the fuel mixture. It does this by using
what's called a "feedback loop." The primary sensor input here
comes from an oxygen (O2) sensor (called a "Lambda" sensor in
import applications) located in the exhaust manifold. The O2
sensor's voltage signal changes in response to the concentration
of oxygen in the exhaust. A high concentration of oxygen is
interpreted as a lean fuel mixture, while a low concentration
means a rich mixture.
The computer monitors the constantly fluctuating input from the O2
sensor, and orders the fuel mixture to do exactly the opposite of
what the O2 sensor reads to compensate. In other words, a lean O2
sensor reading makes the fuel mixture go rich, and a rich O2 sensor
reading makes the fuel mixture go lean. The constant readjusting
and flip flopping the fuel mixture allows the computer to maintain
a relatively balanced fuel mixture is maintained for maximum fuel
economy and low emissions.
But all this doesn't happen the instant the engine starts.
It takes a few minutes for most oxygen sensors to heat up and
start producing a signal. So until there's an O2 sensor signal,
the computer stays in what's called an "open loop" mode of
operation. In this mode, the fuel mixture is slightly rich and
does not change. This is necessary to maintain a good idle while
the engine is warming up. Once the computer starts to receive a
signal from the O2 sensor and/or the coolant sensor tells it the
engine has warmed up enough to start leaning out the fuel
mixture, the computer shifts into the "closed loop" mode. In
closed loop, it uses the signal from the O2 sensor to constantly
fine tune the fuel mixture.
So how does the computer actually change the fuel mixture?
On engines with carburetors, it does it by changing the "duty
cycle" (on time versus off time) of a mixture control (M/C)
solenoid in the carburetor's fuel metering circuit. The M/C
solenoid is a little valve that cycles open and shut very rapidly
(you can actually hear it buzz while it's running). Increasing
the on time allows more fuel to flow through the carburetor's
jets, which richens the fuel mixture. Decreasing the on time
restricts the flow of fuel to lean the fuel mix. Mechanics can
read the duty cycle of the solenoid by hooking up a dwell meter
to its test lead, or by using a scan tool to monitor it through
the computer itself.
On fuel injected engines, the computer switches the injectors
on and off in little pulses (once every revolution of the
crankshaft or just before the intake valve opens depending on the
type of system). The longer the injector is on, the more fuel it
squirts into the engine. Thus the computer can make the fuel
mixture richer or leaner by increasing or decreasing the duration
of the injector pulses.
On many engines, the computer also regulates idle speed. It
does this on carbureted engines by driving a little electric
motor (the idle speed control motor) in and out to change the
throttle opening. This keeps the idle speed within the
programmed rpm range. On fuel injected engines, idle speed is
regulated by opening or closing a small passageway that allows
air to bypass the throttle.
CONTROLLING IGNITION TIMING
Another vital control function handled by the computer is
ignition timing. To alter the amount of spark advance, the
computer monitors input from two important sensors: the throttle
position sensor (TPS) and manifold absolute pressure (MAP)
sensor. The TPS tells the computer how far open the throttle is
and how quickly it is being opened. The MAP sensor reads intake
vacuum, which the computer uses to determine engine load (reduced
vacuum indicates increased load).
When the engine is accelerating or under load, the computer
backs off the amount of timing advance to prevent the engine from
pinging (too much spark advance causes detonation). The computer
does this by retarding (delaying) the timing signal that goes
from the ignition module to the ignition coil.
The computer can also alter ignition timing to compensate for
changes in altitude by using input from a barometric pressure
(BARO or BP) sensor.
WHEN THINGS GO AMISS
Most computers have what's called a "limp in" or "failsafe"
mode that takes over when a vital sensor signal is lost. When
this happens, the computer substitutes an approximate value for
the missing signal. The engine will still run, but not very well
because the computer doesn't have the right information to keep
things in proper balance.
The computer also has a certain amount of built-in
self-diagnostic capability. But this ability is currently
limited to detecting faults within the computer system itself,
not in components that are "outside" the system. For example,
the computer can recognize there's a problem when it loses a
sensor signal or a sensor signal doesn't make sense (engine
idling but the TPS says the throttle is wide open). But it
can't detect a fouled spark plug, leaky exhaust valve, worn
timing chain or vacuum leak. This limitation, however, has been
overcome with the arrival of OBD II on 1996 vehicles. Ignition,
fuel and compression related misfires can now be detected. For
more information, return to the main menu and go to the screen
on OBD II.
Because of its complexity, the computer ends up being blamed
for many unrelated driveability problems. Consequently, a lot of
computers are needlessly replaced -- which leads to needless
comebacks because the new computer failed to cure the problem.
According to Delco Electronics, a very high percentage (well
in excess of 50%) of the supposedly defective computers that are
returned under warranty have nothing wrong with them.
So what does that mean? It means a lot of computers are
replaced unnecessarily because technicians jump to conclusions
and blame that which they understand least. So make sure you've
done your diagnostic homework before you replace anything: be it
the computer or a sensor.
REMAN ECMS
The engine computer, sensors and other emission controls on 1994 and
older cars are all covered by a federal 5 year/50,000 mile emissions
warranty -- which changed to 8 years and 80,000 miles in 1995 on the
computer and catalytic converter only, and 2 years/24,000 miles on
the sensors and other emission components. So if the computer or any
other emission control component fails while under warranty, the vehicle
can be returned to the new car dealer for free repairs. Once the
vehicle is out of warranty, however, defective parts can be replaced
with aftermarket parts.
Remanufactured engine computers are more like a reconditioned TV or
VCR than a rebuilt starter or alternator. The computer's various input
and output circuits are exercised, and then repairs are made as needed.
Some electronic components with a high failure rate may be replaced
whether they test good or bad to improve reliability. The computer may
also be subjected to vibration and thermal tests to make sure it doesn't
have any intermittent problems.
Computers don't wear out in the same respect that mechanical
components do. Either the circuits perform properly or they don't.
There seems to be no correlation between the number of miles driven and the
frequency of repairs for onboard computers. Failures are often
sudden and unpredictable. But many computers are damaged by
voltage overloads and shorts.
HANDLE WITH CARE
Computers are expensive, so handle with care! Fragile connectors
and delicate electronics can be easily damaged by careless handling.
Static electricity can damage a computer, so leave it in its packaging
until it is ready to be installed. The vehicle's battery should be
disconnected before installing the unit, and a grounding strap should
be worn on your wrist to protect against static discharges.
On GM computers, the PROM from the old computer has to be
swapped to the replacement computer. Care must be taken not to
damage the PROM (use a PROM removal tool and don't touch the
PROM's pins).
The replacement computer will take awhile to adjust itself to its
new situation. The time this takes will vary from one vehicle to
another, but generally takes about four or five miles of driving.
Accelerating the car from 35 to 55 mph using progressively more
throttle each time can speed up the process.