Chapter 7 Computer Control Fundamentals 125
Copyright by Goodheart-Willcox Co., Inc.
increases or decreases. This results in a change in the volt-
age drop across the sensor. The ECM reads the variation in
voltage as an input signal. For example, the resistance in
some airfl ow sensors varies according to the position of the
resistor unit. The resistance of a manifold vacuum or EGR
position sensor varies according to movement within the
resistor unit.
In other sensors, changes in temperature affect the
resistance within the sensor. Therefore, temperature changes
alter the voltage drop across the sensor. The ECM reads this
change in voltage as a change in temperature. Intake air
temperature and engine coolant temperature sensors are
typical of temperature-sensitive, resistance-type sensors. A
typical coolant temperature sensor is shown in Figure 7-7.
Some vehicles are equipped with an exhaust temperature
sensor. This sensor is used to tell the ECM when exhaust
temperatures are high enough for the oxygen sensor read-
ings to be reliable, Figure 7-8.
Some sensors are called transducers. They consist of
an iron plunger inside of a small wire coil. Current fl ows
through the coil when the engine is operating, creating
a magnetic fi eld. When the plunger is moved by outside
force, such as a vacuum diaphragm or driver-operated
linkage, it changes the strength of the magnetic fi eld in the
coil. Changes in the magnetic fi eld cause a change in the
amount of current fl owing through the coil. The ECM can
read this as a change in position of the monitored device.
Figure 7-9 shows a typical transducer.
Atmospheric Sensors
Atmospheric sensors measure conditions outside
of the vehicle. Atmospheric sensors include those that
measure air temperature and barometric pressure. Air-
Figure 7-6. A variable resistor is used to measure the position
of a part, such as the throttle plate. It consists of a resistor and
a sliding contact. The position of the contact on the resistor
determines how much current can fl ow through the switch. The
ECM reads the change in current fl ow as a signal indicating the
change in position.
Material with high electrical resistance
Current must travel a long distance
through the high-resistance material
Current travels a short distance
through the high-resistance material
Sliding contact
Resistor construction
High resistance
Low resistance
Figure 7-7. Cutaway of a engine coolant temperature sensor.
As temperature increases, the resistance of the material in the
sensor decreases. (Ford)
Connector
Boot
Gasket
Bulb
Thermistor and
lead assembly
Bullet-type terminal
Figure 7-8. Some vehicles use an exhaust temperature sen-
sor to monitor when exhaust temperatures are right for accurate
oxygen sensor readings. Most manufacturers do not use this
sensor. (Subaru)
Sensor element
Tightening nut
Gasket
Connector