Chapter 7 Computer Control Fundamentals 127
Copyright by Goodheart-Willcox Co., Inc.
Mass Airfl ow Sensors
Many engines have a mass airfl ow sensor (MAF), which
precisely monitors the amount of air entering the engine.
The ECM uses the mass airfl ow sensor input to determine
how much fuel to inject to match the amount of air entering
the engine. Three types of airfl ow sensors are used: heated
element, air valve, and Karman vortex.
The heated element MAF consists of a wire or fi lm
made of temperature-sensitive, resistance material. This
element extends into the intake air passage and contacts
the stream of incoming air. A heated element sensor is
shown in Figure 7-12. The ECM directs a reference voltage
through the element. As current fl ows through the element,
it heats up and its resistance increases. Since the reference
voltage from the ECM is constant, increasing resistance
decreases the amount of current fl ow and, therefore, the
voltage returning to the ECM. Airfl ow through the air pas-
sage removes heat from the element, reducing its resistance
and increasing the voltage signal. Therefore, the voltage sig-
nal goes up when airfl ow is increased and goes down when
airfl ow is reduced. The ECM reads the voltage variations
and calculates the amount of air fl owing into the engine.
The air valve MAF consists of a moveable fl ap, attached
to a rheostat, that extends into the airstream in the intake air
passage. The fl ap is spring loaded so that airfl ow is opposed
by spring pressure. As airfl ow into the engine increases and
decreases, the fl ap moves. The rheostat receives a refer-
ence voltage from the ECM. Variations in airfl ow cause the
fl ap to move, which in turn moves the rheostat contact arm.
This increases or decreases the voltage signal to the ECM,
which reads it as changes in airfl ow.
The Karman vortex MAF consists of a metal-foil mir-
ror, LED, and light receptor (photo transistor). The light
produced by the LED is refl ected from the metal-foil mirror
into the photo transistor. The metal-foil mirror is placed in
the intake air passages, where the incoming air causes it to
vibrate. Increases and decreases in airfl ow change the rate of
vibration and, therefore, the amount of light that reaches the
photo transistor. The photo transistor changes this light into a
voltage signal that can be read by the ECM as airfl ow rate.
Mass airfl ow sensors are not used on some multiport
injection systems, nor on throttle body injection systems.
The vehicle computer systems monitoring other sen-
sor inputs and calculating the amount of air entering the
engine. These are known as speed-density systems. In a
typical speed-density system, the ECM computes airfl ow
from inputs provided by the MAP, engine speed, throttle
position, vehicle speed, and intake air temperature sensors.
This process is discussed in greater detail in Chapter 9.
Engine-Coolant Temperature Sensors
To properly adjust the fuel mixture and ignition tim-
ing and to operate the emission controls, the ECM must
know engine temperature. In addition, the ECM relies on
engine temperature to decide whether to place the system
in closed or open loop and to turn the radiator fans and air
conditioning compressor on or off. Since the temperature
of the engine coolant is a good indicator of overall engine
temperature, every vehicle has at least one engine-coolant
temperature (ECT) sensor. A few systems have a second
sensor to more closely monitor engine warm up.
The engine-coolant temperature sensor consists of a
heat-sensitive, resistance material. Unlike most resistors,
the material’s resistance decreases with increases in tem-
perature. The ECM sends a reference voltage to the sensor,
which passes through the resistance material before return-
ing to the ECM. When the coolant and sensor are cold,
the material has a very high resistance. Electricity fl owing
through the sensor meets this resistance and the voltage sig-
nal returning to the ECM is low. As the coolant warms the
material, the resistance decreases. This increases the voltage
signal returning to the ECM. The ECM reads these variations
in voltage as engine temperature. A typical engine-coolant
temperature sensor is shown in Figure 7-13.
Engine Speed and Position Sensors
Some sensors develop a voltage signal by creating or
varying a magnetic fi eld. If a magnetic fi eld is used to create
the voltage signal, no outside voltage source is needed. In
other sensors, a voltage signal created in the ECM is modi-
fi ed by the operation of the sensor.
Magnetic-fi eld sensors use a magnetic fi eld generated
by moving parts. In a typical system, a toothed wheel passes
near a small pickup coil, creating a magnetic fi eld. This
magnetic fi eld produces a small voltage signal to the ECM,
Figure 7-14. Some sensors create a signal by interrupting
an existing magnetic fi eld. As shown in Figure 7-15, either
Figure 7-12. Mass airfl ow sensors (MAF) are installed in the air
intake duct between the air fi lter and throttle valve. (Cardone)