134 Auto Engine Performance and Driveability
ECM that the air conditioning system is low on refrigerant.
The ECM then deactivates the compressor clutch to prevent
it from engaging.
Virtual Sensors
A virtual sensor is not a physical sensor at all, but ECM
programming that calculates a vehicle operating value. This
value is calculated based on input from physical sensors
that monitor different engine and vehicle conditions. An
example of this is the ECM program used on some early
computer control systems to determine mass airflow. The
program reads throttle opening, manifold vacuum, engine
speed, and intake air temperature to calculate mass airflow.
This eliminated the need for an actual MAF sensor. Now,
virtual sensors are often used to measure conditions that
would be impossible or expensive to measure with actual
physical sensors. Examples include the amount of alcohol
in gasoline, transmission slippage, exhaust system tempera-
tures, and airflow through the radiator.
Output Devices
Output devices are electromechanical or electronic parts
that carry out the commands of the ECM. Output devices can
be solenoids, electric motors, or electronic devices. These
are explained in the following sections. Output devices are
discussed in more detail in later chapters.
Solenoids
The most common output devices are electric sole-
noids. A solenoid consists of an iron plunger surrounded
by a coil of wire. When the coil is energized by passing
current through it, a magnetic field is created and moves
the plunger. When the coil is de-energized, a spring
returns the plunger to its original position. In many cases,
the solenoid plunger is attached to a flow control valve.
The valve may control the flow of fuel, air, exhaust gases,
or transmission fluid.
Some solenoids are pulsed on and off many times per
second. Examples are fuel injectors and idle air control
solenoids. Other solenoids operate less frequently, remain-
ing on or off for seconds or minutes. Examples are the air
pump switching solenoids, evaporative emissions canister
solenoid, digital EGR solenoid, and torque converter con-
trol solenoid.
Idle Air Control (IAC) Solenoids
The idle air control (IAC) solenoid, or valve, allows
air to bypass the throttle valve into the intake, Figure 7-28.
The IAC valve is open as long as the throttle plate is closed.
When the throttle plate opens, the ECM instructs the IAC
valve to close the bypass port, since air through this port is
not needed while driving. All of this happens very quickly.
Due to the small size of the passages in the bypass, this
valve is at times subject to sticking or clogging.
Relays
Electrical relays are also solenoids. Relays are used
when an electrical component consumes more power than
the ECM can safely deliver, Figure 7-29. In these cases, a
small amount of current from the ECM energizes the relay
(solenoid), closing a set of contact points. A large amount
of current then flows through the points to the electrical
component. Electrical components controlled by the ECM
through a relay include the air conditioner clutch and
radiator fan motor.
Figure 7-28. The idle air control (IAC) solenoid allows air to
bypass the throttle valve when the throttle valve is closed. When
the throttle valve is opened, the valve shuts off.
From air
cleaner
Intake air
chambers
To
cylinders
Throttle valve
IAC valve
ECM
Figure 7-29. The vehicle uses many relays to control high-
amperage power. This relay controls the air conditioning com-
pressor clutch.