128 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
a shutter or rotating magnets are used to affect the magnetic
fi eld. Interrupting the magnetic fi eld creates a voltage fl uc-
tuation in the circuit, which is read by the ECM.
In some systems, the input signal is generated as a
byproduct of the operation of the ignition system. In these
cases, the signal generated by a Hall-effect switch is used to
trigger the coil discharge and also as an input to the ECM.
These sensors are discussed in more detail in Chapter 8.
Crankshaft-position and camshaft-position sensors are
installed near the crankshaft or camshaft, Figure 7-16. A
disk with a series of magnetized teeth is installed on the
crankshaft or camshaft. As the shaft rotates, the disk passes
near the sensor, creating a magnetic fi eld. This magnetic
fi eld creates a voltage signal to the ECM. The ECM can pro-
cess this signal to determine engine speed. The ECM can
also determine the crankshaft and camshaft position and,
therefore, the position of each piston by reading the pattern
of voltage signals.
Many speed sensors are referred to as 3X, 7X, 18X, 24X,
or other numbers followed by an X. These numbers indicate
how many signals the sensor sends during one crankshaft
revolution. For example, the camshaft-position sensor in a
6-cylinder engine may be an X3 sensor. One rotation of the
camshaft produces six signals from the pickup. Since the
camshaft turns at one-half of the engine speed, the camshaft
makes one half turn for every complete crankshaft revolu-
tion. The pickup, therefore, produces three signals during
one crankshaft revolution. These sensors are always refer-
enced to crankshaft revolutions even if they are operated
by the camshaft.
Light Sensors
Some sensors use light emitting and collecting devices
to measure speed or position rotating parts. A typical light
sensor contains three major components. Light is produced
by a device called an emitter. This light is directed at a
light-absorbing device called a detector. A rotating device
called an interrupter periodically blocks the light beam
between the emitter and detector.
Emitters can be LEDs or infrared emitting diodes
(IREDs). The detector is a light-sensitive semiconduc-
tor, often called a photodiode or phototransistor. Current
fl ows in the detector whenever the engine is running. Light
striking the detector affects the amount of current fl owing
through the semiconductor. The ECM monitors the current
fl ow and reads variations as an input signal. The interrupter
is attached to a rotating part of the vehicle, such as the
Figure 7-13. Parts of an engine coolant temperature sensor.
These sensors are normally installed in the intake manifold or in
the thermostat housing. (Chrysler)
Probe
Threads
Wiring
terminals
Figure 7-14. Rotating teeth create a magnetic fi eld in the pickup
coil. The pickup coil is composed of a small wire that responds
to changes in this magnetic fi eld. A small voltage is produced
that can be read by the ECM.
Tooth
S
N
Pickup coil
Module
Figure 7-15. The Hall-effect switch shown here is a wire coil
with current fl owing through it. Current produces a magnetic
fi eld around the coil. When the rotating shutter breaks the mag-
netic fi eld, the voltage of the current fl owing through the coil is
changed. This is sent as a signal to the ECM.
Shutter
S N Module
Figure 7-16. The camshaft-position sensor used with a cam-in-
block engine is often mounted directly behind the coolant pump.
The camshaft-position sensor for an overhead camshaft may
be installed on the front of the cylinder head, as shown here.
(University of Toyota/Toyota Motor Sales USA)
Camshaft-position sensor