This section deals with the inner process of the ECM’s
software operation. A generic approach has been taken in explaining each of
the operational modes, regardless of manufacturer. This is done in an effort
to prepare the technician for implementation of the diagnostic techniques
offered throughout this book. By the simply “thinking like the vehicle’s
ECM”, the technician can follow various clues in order to reach a diagnostic
conclusion. The technician should always ask the primordial question, “what
does the ECM needs to see in order to do what it’s doing ? ”
Modern engine control systems are digital systems. By
digital ECM we means that it uses a series of mathematical calculations,
using 1’s or 0’s (On or Off switches), to perform its job of controlling the
engine. A typical ECM employs a microprocessor (main computer chip) and the
needed operational circuitry, such as injector driver transistors and
related electronic components. In essence, a modern ECM is a dedicated
computer, with specific programming (software) that allows it to operate.
Early ECMs were slow 8 bit systems, which contrasts with today’s 32 bit or
even faster systems. The bits are the amount of ON/OFF switches that the ECM
can process at a time. A Pentium-4, for example, is a 32 bit system. This
tremendous amount of processing power is making possible the further
integration of every possible automotive electronic system. By means of
modern vehicle networks, any vehicle system from entertainment to electronic
power steering is being integrated to one another. However, by far the
biggest catalyst for the heavy use of electronics in modern vehicles has
been the increasingly stringent exhaust emissions and fuel economy
regulations. Such level of efficiency and environmental cleanliness would
not be possible without the modern digital ECM.
Virtually all computers are divided into two separate parts,
the hardware and software. The hardware is the actual module, with all its
electronic parts, including transistor drivers, memory chips and related
circuitry. The software is the actual programming residing in the hardware
or inside the ECM’s memory chips. The ECM’s memory is further subdivided
into the ROM (read only memory) and RAM or KAM (random access memory or keep
alive memory). The ROM memory chips, also called keep alive memory, are
permanent storage chips used to store the programming for the different
operational modes, as well as the look-up tables for all the sensors and
actuators. The RAM memory chips are used to store any manufacturing or aging
variations (adaptive memory) of each fuel control component, whether it is
an injector that is 10 % clogged, a slower O2 sensor or a new crank sensor
that has just been installed. This adaptive feature gives the ECM the
ability to adapt as operating and environmental conditions change.
A modern module use multiple modes to achieve a desired
output. A multi-mode ECM operates in one of many possible modes of
operation. At the same time, the ECM always refers to the look-up tables to
determine injector pulse and ignition patterns at any given condition. An
example would be a cold engine that has just been started, in which case,
the ECM would refer to the coolant temperature sensor and adjust injector
pulse accordingly until it has reached the proper warm-up temperature level.
In this example, the ECM’s programming is constantly referencing the look-up
tables stored in ROM memory to determine the injector pulse throughout the
entire coolant sensor range. Furthermore, any smaller variations in the
operation of each sensor/actuator due to manufacturing differences, aging,
etc are stored in RAM or adaptive memory, which the ECM also references
continuously. The RAM/KAM memory is volatile, which means that it gets
erased in the event of any power disruptions, hence the term KAM (keep alive
memory). The digital multi-mode capability allows engine controllers to hold
the engine as close to the stoichiometric ratio for as long as possible. So
it can be argued, that the biggest thrust for the wide use of electronics in
today’s automobiles are government emissions regulations. It would be
virtually impossible to manufacture clean and fuel efficient automobiles
without a massive array of electronic components.
There are seven main modes of operation built into
the software of any vehicle ECM found today, as well as several other system
specific sub-modes. The use of an operational mode is dictated by the
particular conditions under which the engine has to work. Each of these
conditions is significantly different to warrant a different control
software or mode for it. The ECM must determine the operational mode from
the existing sensor data. The seven main fuel control modes are –
cranking enrichment, engine warm-up, open-loop, closed-loop, acceleration
enrichment, deceleration enleanment and idle speed control. Several
other modes of operation exist and apply to specific systems or conditions.
These are – clear flood, limp-in-mode, variable valve timing, selective
fuel injector cut-off and synchronous and asynchronous fuel injection.
These modes are explained in the following pages.
• Cranking enrichment mode – The ECM provides the
extra fuel needed to start the engine. This is done by increasing the
injector pulse width to as much as 40 mS and using injector group firing, as
opposed to sequential. This mode is activated whenever the ignition key is
switched on. As soon as the engine starts and its speed goes above a certain
RPM value, the ECM switches to the next logical mode, whatever that may be.
On certain vehicles, a start/crank input from the starter relay or solenoid
is provided to the ECM. This signal may reach the ECM through either a fuse
or a direct wire connection. A failure in this circuit may cause severe
performance problems, such as excessive fuel delivery – if this signal is
shorted to power. Or it may also cause long cranking time – if this signal
does not reach the ECM (the ECM does not know it has to go into cranking
mode). During cranking mode, the ECM constantly compares the ECT sensor
value with a look-up table stored in ROM memory to determine the correct
injector pulse width. Normal cranking mode A/F ratio is between 1.5:1 to
12.0:1 depending on engine coolant temperature. The main reason for this
mode is that at colder temperatures the fuel tends to form into large
droplets, which do not burn as efficiently. For this reason, manufacturers
are constantly looking for ways to reduce the cranking and warm-up mode
times as much as possible, since they are the greatest cause of automotive
air pollution in modern engines. As a side note, the low voltage sub-mode
also plays a role while at cranking enrichment mode. The low voltage
submode program code instructs the ECM to further increase injector pulse to
compensate for the lower fuel pressure, brought about by the lower available
fuel pump voltage from the starter motor cranking the engine.
Regardless of whatever mode is active, a low supply voltage will increase
injector pulse.
