Chapter 24 Sensor, Actuator, Computer Service 463
The number and types of sensors will vary with the
specifi c make and model car. Refer to the car’s service
manual for exact sensor types and locations. Sometimes,
the manual will show what sensors are used and where
they are located, Figure 24-3.
The shop manual will also have a wiring diagram for
the computer system. The diagram will show the color
codes of wires and number of connectors that are used to
feed data from the sensor back to the computer. This can be
very helpful when working on a computer system that you
have never worked on before, Figure 24-4.
Sensors and their circuits can sometimes cause com-
puter system malfunctions. As with other electronic com-
ponents, sensors and sensor circuits can develop opens,
shorts, or abnormal resistance or voltage values. When
your tests fi nd a problem, the sensor should be replaced
or the circuit repaired. Detailed in earlier chapters, sensors
can produce signals for the computer in several ways:
Voltage generating sensor—oxygen sensor and some
speed sensors produce an internal voltage during
operation.
Switching sensors—sensor simply acts as either a
conductor or an insulator to switch on and off with
condition changes; examples are a brake switch and
neutral safety switch.

Variable resistance sensors—sensor ohms change
with the condition to signal the computer by altering
current fl ow back to the computer.
Each type of sensor needs a slightly different testing
method. A voltage generating sensor requires a digital volt-
meter to read the small or weak voltage output. An ohmme-
ter and voltmeter are commonly used to check switching
and variable resistance sensors.
A reference voltage of typically fi ve volts is fed to
switching and variable resistance sensors. Then, when con-
ditions and sensor resistance change, the amount of current
fl owing back to the computer also changes. The reference
voltage is needed so that a signal returns to the computer.
If you disconnect the reference voltage wire from a
resistive sensor, an ohmmeter can be used to check the sen-
sor. However, the voltage drop across the sensor can also
be used to check resistive sensors with the reference wire
still connected to the sensor. You must use a service manual
and your own judgment to determine which testing method
should be used under the circumstances.
Engine Speed Sensor Service
A bad engine speed sensor will sometimes keep an
engine from running by disabling the ignition system. The
Figure 24-3. Sensors, actuators, and ECUs can be located in various places on a car. The service manual will give exact locations
and list what sensors, actuators, and computers or ECUs are used on the specifi c vehicle. (Nissan)
Knock
Sensor
Idle Air
Regulator
Fuel
Pressure
Regulator
Engine
Coolant
Temperature
Sensor
Fuel
Injector
Electronic
Control Module
Electric
Fuel Pump
Oxygen Sensor
Ignition
Coil
TPS
Intake Air
Temperature Sensor
Airflow
Sensor
Vacuum Control
Modulator
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