Chapter 7 Computer Control Fundamentals 135
Electric Motors
Electric motors are sometimes used as output devices.
These motors are usually dc motors, similar to the starter
motor. Some motors are reversible and can be driven for-
ward and backward, according to the current delivered to
them. The motor for an electric cooling fan is an example
of an ECM-controlled motor, Figure 7-30. Some vehicles
use an electric motor to operate the throttle plate. There
is no mechanical linkage between the accelerator pedal
and the throttle plate. This type of system is known as an
electronic throttle control.
Small electric motors are often attached to the throttle
linkage to precisely control idle speed. They open and
close the throttle through a system of gears, that allow the
motor to extend or retract a plunger. These motors are often
called stepper motors or servo motors. They are designed to
operate through an exact amount of travel, or “step.”
Electronic Output Devices
Electronic output devices can be ignition modules or
other solid-state components. These devices are similar to
relays, since low current from the ECM controls high cur-
rent flowing in the electronic component. Many of these
electronic components are input sensors as well as output
devices. An example is the ignition module used on many
vehicles. The module controls the timing and strength of
the output from the ignition coil based on commands from
the ECM. It also sends engine speed and crankshaft position
signals to the ECM. See Figure 7-31.
X-by-Wire
X-by-wire is the term used for an electronic system that
performs a job formerly done by a mechanical or hydrau-
lic system. For example, drive-by-wire is an electronic
system that moves the throttle plate, where older systems
used cables and linkages. Shift-by-wire is the replacement
of hydraulic transmission valves with computer-controlled
solenoids. Each new generation of vehicle uses more and
more X-by-wire systems.
Electronic Throttle Control
A common and increasingly used type of X-by-wire
system is electronic throttle control. This system replaces
the cable or rod-and-lever linkage of earlier systems with
a system of electronic devices driven by the ECM. Refer
to the schematic in Figure 7-32. The throttle control is
usually an enclosed motor that operates the throttle plate
through gears, Figure 7-33. The electronic throttle control
has a fail-safe spring that returns the throttle to idle if con-
tact between the throttle and ECM is lost. This prevents a
dangerous situation from developing should any part of the
control system fail.
Control Loop Operation
A control loop can be thought of as a constantly recur-
ring cycle of causes and effects. The purpose of a control
loop is to maintain a certain condition, even when other
conditions are constantly changing. Many control loops are
relatively simple. An example of a simple control loop is
the regulator valve that controls engine oil pressure. When
oil pump output exceeds the spring setting of the pressure
regulator, the spring is compressed and the valve opens.
Oil escapes past the valve into the oil pan, lowering oil
pressure. When pressure is lowered, the spring closes the
valve. Refer to Figure 7-34 for the oil pressure control loop.
This action keeps oil pressure at the same level no matter
how fast the oil pump is turning.
In the computer control system, the control loop is
much more complex, but the basic principle is the same.
The control loop tries to keep the air-fuel ratio as close to
14.7:1 as possible under all engine operating conditions.
It does this by receiving inputs from the sensors, process-
ing them, and issuing commands to the output devices.
The control loop is from input sensors, to ECM, to output
devices, to engine/system, and back to input sensors.
Examples of simple loops include:
p
Oxygen sensorECMfuel injectorsengineoxygen
sensor
p
Knock sensorECMignition modulespark
plugsengineknock sensor
p
Engine coolant temperature sensorECMradiator
fanengineengine coolant temperature sensor
The ECM combines all of these simple loops into
one complex loop that considers the input from all sen-
sors, makes decisions, and sends commands to all output
devices, Figure 7-35. When a vehicle is first started, it oper-
ates using preset parameters in the ECM while the ECM is
monitoring sensor input. This is called open loop. Once
the vehicle reaches its normal operating temperature, the
Figure 7-30. Electric cooling fans can be controlled by the ECM.
The ECM can turn the fan on and off and determine whether it
runs at high or low speed.
Electric
fan motor