122 Auto Engine Performance and Driveability
codes, is stored in RAM and constantly updated. Some
ECMs can compensate for wear and changes in sensor out-
put and output device response.
Keep Alive Memory. The ECM stores basic operating
parameters in what is known as keep alive memory (KAM).
KAM is sometimes referred to as keep alive RAM or nonvol-
atile RAM. The ECM can access this information and use it
to adjust the output devices to compensate for the sensor’s
reduced output. This is usually referred to as an ECM’s
adaptive strategy. The basic operating parameters in KAM
allow the ECM to maintain vehicle operation even though
sensors are providing abnormal readings. The vehicle can
be driven to a service facility in this limp-in mode.
Note: Some newer ECMs can retain volatile
memory for several days without battery
power.
Central Processing Unit (CPU)
The central processing unit (CPU) actually controls
ECM operation. It directs the operation of the other circuits,
telling them which tasks to perform. It can be thought of
as the ECM’s manager, directing the flow of information
through the various circuits. The CPU also determines the
overall status of the engine/vehicle and compensates for
short- and long-term changes in engine operation.
The CPU takes the information from the input pro-
cessor and compares it with information in memory. If the
input signals match the data in memory, the CPU takes
no action. If the inputs do not match the preset data, the
CPU instructs the output processor to change the opera-
tion of the output devices until the input signal matches
the data in memory. Some inputs are simply for reference,
such as intake air temperature and atmospheric pressure.
The CPU compensates for these input readings by modi-
fying the operation of other systems. Some outputs, such
as air-fuel ratio, are directly and aggressively adjusted by
the CPU.
Output Processor
The output processor, or output driver, takes the digital
ECM commands and converts them into analog electrical
signals for the output devices. Since most output devices
are operated by simply turning them on and off, the output
signal (pulse width) is varied in length. The output proces-
sor often acts like a relay, using the small voltage inputs
from the main processor to control the flow of high current
into solenoids or motors. On a few vehicles, the output
processor is a separate unit, sometimes called the output
or power computer.
The ECM makes use of quad drivers. These are output
chips containing power transistors and other components
that can absorb high current in the event that an output
device or connection is shorted to ground. This prevents
damage to the ECM internal components. The output pro-
cessor also protects the ECM from voltage spikes or shorted
output devices.
Other ECM Components
All ECM’s have a chip that acts as an internal clock.
This internal clock times engine operation and is used by
the ECM to coordinate its internal operations. Some ECM
clocks track the actual time of day as long as they are con-
nected to battery power. Most ECMs have a backup proces-
sor that has preset engine operating information. Should
the ECM malfunction, this chip, often called a mem-cal,
allows the vehicle to operate in limp-in mode. While the
vehicle will not run efficiently, it can be driven to a shop
for service.
Specific Functions of Computer Control
Systems
While monitoring input sensors and adjusting output
devices, the ECM performs many functions within its cir-
cuits that are important to vehicle operation and safety. The
following sections describe some of these specific ECM
functions.
Short- and Long-Term Fuel Trim
The ECM interprets sensor readings and adjusts the
fuel system to compensate. The ECM makes adjustments
to the injector pulse width, which is how long the injectors
stay open. This is done by constantly checking the input
sensor data in short term memory and making adjustments
based on this information. The process is called short-term
fuel trim. Some manufacturers refer to this as an integrator.
Short-term fuel trim adjustments are made in response to
temporary changes in engine operation, such as increased
load when driving up a hill.
The ECM can also make longer, semipermanent adjust-
ments to the air-fuel ratio in response to engine operation.
This process is called long-term fuel trim, sometimes
referred to as block learn. Long-term fuel trim adjustments
are made when the ECM determines that the vehicle is
operating under set conditions for an extended period of
time, such as hot or cold weather, high altitudes, or with an
on-going driveability problem.
Scan tools can monitor short- and long-term fuel trim.
The scan tool interprets both fuel trim readings as a count
number or percentage. The ECM in some systems can
monitor the fuel trim to detect problems.
Idle Speed Control
The ECM controls the amount of air flowing into the
engine at idle by controlling a throttle positioner or air
bypass valve. The rate of airflow will not be less than a
set minimum, which is called minimum idle speed control
or minimum idle air rate. On most engines, minimum idle