Chapter 7 Computer Control Fundamentals 141
able to receive and store information from the sensors. The
volatile memory also stores information about malfunctions
in the form of diagnostic trouble codes (DTCs).
Inputs to the ECM can be generated from switches,
resistance units, magnetic fields, chemical-reaction sen-
sors, pressure-sensitive materials, light-emitting and light-
collecting devices, and transducers. Input sensors and
switches monitor conditions in the engine and drive train,
outside atmospheric conditions, and direct or indirect
driver inputs.
Engine sensors are installed on the engine and mea-
sure the actual conditions in the engine, such as tempera-
ture, engine speed, crankshaft position, manifold vacuum,
intake airflow, exhaust gas oxygen, and engine knock.
Other engine sensors may be used to measure the tempera-
ture of the incoming air, camshaft position, or exhaust gas
temperature.
Drive train sensors monitor conditions in the transmis-
sion/transaxle and other drive train components. Accessory
equipment sensors are used to monitor such conditions
as alternator output voltage, whether the air conditioner
or cruise control is on or off, and whether the wheels are
being turned. Atmospheric sensors measure conditions
outside of the vehicle, such as air temperature, barometric
pressure, and sometimes humidity. Driver-operated sen-
sors are installed on the throttle linkage, brake pedal, and
shift lever. Virtual sensors determine vehicle conditions by
monitoring inputs from two or more sensors dedicated to
other purposes.
The ECM receives inputs from the input devices,
processes these inputs, and sends commands to the output
devices. Output devices are those electromechanical or
electronic parts that carry out the commands of the ECM.
They cause changes in engine operation. Output devices
can be solenoids, electric motors, or electronic devices.
Functions that were formerly performed by mechanical or
hydraulic systems are called X-by-wire systems.
The first computer control systems were known as
on-board diagnostics generation one (OBD I). The current
generation, OBD II, is a more sophisticated system that can
more closely monitor engine conditions and tell when a
part or system is deteriorating before it actually fails. The
shape and location of the OBD II diagnostic connector is
standardized, as are the diagnostic trouble codes set by the
system.
Review Questions—Chapter 7
Please do not write in this text. Write your answers on
a separate sheet of paper.
1. Electronic engine control systems are composed of
______, ______, and ________ devices that control
engine operation.
2. All information processing in an ECM is a(n) _____
operation.
3. List the four main circuits in an ECM.
4. In many sensors:
(A) a reference voltage from the ECM is passed
through a resistor unit in the sensor.
(B) a voltage is modified as the resistance of the
sensor varies with conditions.
(C) varying voltage from the sensor is passed to the
ECM as an input signal.
(D) All of the above.
5. A 7X speed sensor sends seven signals to the ECM
when the crankshaft rotates _____ time(s).
6. A knock sensor produces an electric current in
response to a change in:
(A) temperature.
(B) speed.
(C) pressure.
(D) Both A & C.
7. An oxygen sensor uses a(n) ______ reaction to
produce a voltage.
8. Describe a virtual sensor.
9. The most common output devices are electric
solenoids. Solenoids:
(A) consist of an iron plunger surrounded by a coil
of wire.
(B) are energized by passing voltage through them,
creating a magnetic field that moves a plunger.
(C) may control the flow of fuel, air, exhaust gases,
or transmission fluid.
(D) Both A & C.
10. ______ is the term used for an electronic system that
performs a job formerly done by a mechanical or
hydraulic system.
11. A(n) _____ can be thought of as a constantly
recurring cycle of causes and effects, the purpose of
which is to maintain a certain condition, even when
other conditions are constantly changing.