Chapter 7 Computer Control Fundamentals 129
speedometer cable or camshaft. Depending on the design,
the interrupter blocks the light beam in one of two ways, as
shown in Figure 7-17.
In Figure 7-17A, the interrupter is equipped with small
mirrors spaced at intervals on the disc. When light from the
emitter strikes a mirror, it is reflected into the detector. As
the disc turns, the mirror moves out of position and light is
no longer reflected into the detector. This causes a change
in current flow in the semiconductor. Further disc move-
ment causes the next mirror to line up with the emitter and
collector, again changing current flow.
In Figure 7-17B, the interrupter disc is constructed with
shutter blades at intervals on its outer edge. When one of
the shutter blades passes between the emitter and detector,
the light beam is broken. This causes a change in current
flow in the semiconductor. When the shutter blade passes,
the light beam is restored and current flow changes again.
Ion Sensors
Ion sensors can be used as speed sensors, triggering
devices, and misfire monitors. The main component of the
ion sensor is a capacitor that sends a low voltage signal
across the spark plug electrodes. This current passes across
the spark plug gap without producing a spark. The plug
electrodes are used as probes to determine how much
carbon is present in the cylinder. Carbon is a good conduc-
tor. If the gases in a cylinder have a high carbon content,
more current will flow across the plug electrodes than if the
carbon content is low. The signal produced by this current
flow is analyzed by the ECM to determine:
p
Whether complete combustion has occurred.
Unburned hydrocarbons have less resistance than
exhaust gases.
p
Whether spark plug knock has occurred. An explosion
in the cylinder creates more unburned hydrocarbons
than normal combustion.
p
The position of the camshaft. In this case, a Hall-effect
camshaft sensor is not used.
p
If cylinder misfiring has occurred.
The main job of the ion sensor is to monitor cylinder
combustion to determine whether the combustion process
has occurred properly. The ion sensor is also part of a more
accurate method of determining camshaft position and
checking for misfires.
Throttle Position Sensors
The throttle position sensor (TPS) measures the amount
of throttle opening using a variable resistor or transducer.
These sensors are mounted on the outside of the throttle
body.
The throttle position sensor is sent a voltage signal
by the ECM. The voltage is modified by the variable resis-
tor in the TPS. The altered voltage signal from the TPS is
interpreted by the ECM as throttle position. This process is
shown in Figure 7-18.
Some throttle position sensors are simple on-off units.
These sensors usually tell the ECM that the throttle is in the
idle position. Note that the idle switch is part of the throttle
position sensor in Figure 7-18. Some throttle position sen-
sors are adjustable.
Knock Sensors
Knock sensors contain a small crystal that reacts to pres-
sure changes. See Figure 7-19. Many knock sensors modify
an incoming voltage. However, some actually produce a
small electric current. The crystal is sensitive to certain types
of vibrations, such as those produced by engine knock. When
engine knocking is creating a vibration, the crystal modifies
or creates an electric current that is picked up by the ECM.
The ECM interprets this signal as the engine knocking.
Figure 7-17. Typical light sensor systems. The signal resulting from either system can be used as speed or position inputs to the
ECM. A—Reflector module. Light from the emitter hits the mirror on the rotating disk and bounces back to the collector. B—Interrupter
module. The shutter assembly blocks the light path.
Mirrored
surface
E
D
Shutter
A B