462 Auto Electricity and Electronics
when diagnosing electrical problems. If analyzed properly,
problems can usually be found and corrected easily.
Sensor and Actuator Problems
As with other electrical and electronic components,
sensors, actuators, and their circuits can develop opens,
shorts, or abnormal resistance or voltage values. When
your pinpoint tests fi nd a problem, the sensor or actuator
should be replaced or the circuit repaired.
In most cases, a scan tool is used to fi nd the prob-
lem circuit and a digital multimeter is used to measure the
resistance in the circuit and the actual sensor output or
actuator input. Then, this value (voltage, resistance, cur-
rent, or action) is compared to factory specifi cations. If the
test value is too high or too low, you would know that the
sensor or actuator is faulty and must be replaced.
The shop manual will also have a wiring diagram, or
schematic, for the computer system. The diagram will show
the color codes of the wires and the number of connectors
that are used to feed signals from the sensors to the computer
and from the computer to the actuators. This can be very
helpful when servicing any computer system. The following
paragraphs discuss the most frequent circuit problems.
Poor Electrical Connections
Poor electrical connections are the most common
cause of electrical-related problems in a computer system.
Discussed in previous chapters, a wiggle test will help fi nd
bad connections and intermittent problems. Always check
electrical connections when diagnosing sensors and other
electronic components, Figure 24-2.
Poor electrical connections can be due to corroded
terminals, loose terminal ends, burned terminals, chafed
wires, and other problems. Dirt and moisture can get into
connectors, causing high resistance. Any of these condi-
tions can prevent a normal sensor signal from returning to
the computer. They can also prevent the control current
from reaching an actuator.
Vacuum Leaks
Vacuum leaks are frequently caused by deteriorated,
broken, or loose vacuum hoses. Some vacuum leaks can
upset the operation of a computer system and cause a wide
range of symptoms. Also, several engine sensors and vac-
uum actuators (vacuum switches) rely on engine vacuum
for operation.
Always check for vacuum leaks when they could be
causing a performance problem. For example, if the trouble
code indicates a problem with the manifold absolute pres-
sure (MAP) sensor, always check any vacuum lines leading
to the sensor. If there is a vacuum leak, the sensor cannot
function normally.
Air leaks, after an airfl ow sensor, can also cause
problems. The sensor cannot measure the air being
drawn into the engine, and an incorrect air-fuel mixture
will result.
Sensor Service
Sensor service involves testing and sometimes replac-
ing computer system sensors. Since sensor designs vary
and some can be damaged by incorrect testing methods,
it is important for you to know the most common ways of
checking sensor values.
Basically, a high-impedance VOM is used to measure
the actual sensor output. Then, this output (voltage, resis-
tance, or current) can be compared to factory specs. If the
test value is too high or too low, you would know the sen-
sor is faulty and must be replaced.
Figure 24-2. A preliminary inspection will help fi nd some prob-
lems. A—Check that wiring connectors and air inlet hoses are tight
and secure. B—Check and listen closely for leaking or missing
vacuum hoses. A vacuum leak will often make a hissing noise.
A
B
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