Computer System Service 19.Computer System Service

by
Russell Krick

Publisher

The Goodheart-Willcox Co., Inc.
Tinley Park, Illinois
Permission granted to reproduce for educational use only


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 Preliminary visual inspection
 Computer system circuit problems
 Sensor and actuator problems
 Sensor service
 Actuator service
 Computer service

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 Involves looking for signs of obvious
trouble:
 loose wires, leaking vacuum hoses, damaged
parts, etc.

 When a computer system malfunction
occurs, the cause is often something
simple

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Contaminated Engine
Oil
 Something as simple as contaminated
oil can set a trouble code
 Fumes from the crankcase are drawn
into the intake manifold for burning
 these fuel-rich vapors can trick the
oxygen sensor into signaling a rich airfuel mixture, setting a trouble code

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Contaminated Engine
Oil

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Electrical Component
Damage
 Semiconductor devices are very easily
damaged by static electricity, voltage
spikes, heat, and impact shocks
 Here are some rules to remember
when working with electronic devices
and their wiring:

 Arc welding can damage on-board
computers
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Electrical Component
Damage
 Never disconnect the battery cables
while the engine is running
 Do not disconnect or connect wiring,
especially the computer wiring, with the
ignition on
 Make sure you do not reverse the
battery cable connections

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Electrical Component
Damage
 Wear an anti-static wrist strap
whenever you handle static-sensitive
components
 Only use high impedance test lights
and meters when checking electronic
circuits or their wiring
 Do not disconnect a scan tool from the
data link connector with the ignition on
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 Most electrical-electronic problems are
actually basic circuit problems
 increased or decreased current,
resistance, or voltage

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Locating Computer
Problems
 To find the source of computer
problems, you must ask yourself the
following types of questions:
 What could be causing the specific
symptoms?
 mentally picture the circuits

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Locating Computer
Problems

 How many components are not
working?

 if several components are not working,
something close to a common power
source or ground point is at fault

 Is the problem always present or is it
intermittent?
 if the problem is intermittent, the
conditions causing the fault must be
simulated
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Locating Computer
Problems
 Is the problem’s occurrence related to
heat or cold?
 electronic circuits are sensitive to heat

 Is the problem’s occurrence affected by
moisture?
 in most cases, moisture will not affect
sealed components, but it will affect wire
connections

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Stress Testing
 Using heat, cold, or moisture to
simulate extreme operating conditions
of components
 moisture is often used to check the
condition of spark plug wires

 cold and heat are often used to test
electronic modules

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Sensors, actuators, and their circuits can
develop opens, shorts, or abnormal
resistance or voltage values

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Problem Diagnosis
 In most cases, you can use a scan tool
to find the problem circuit
 Use a digital multimeter to measure the
resistance in the circuit and the sensor
output voltage or actuator input voltage
 compare these readings to factory
specifications

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Problem Diagnosis
 Wiring diagrams show the color codes
of the wires and the number of
connectors that are used in the circuit
 These diagrams or schematics are

helpful when servicing any computer
system

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Poor Electrical
Connections
 Common cause of electrical-related
problems in a computer system
 Caused by corroded or burned
terminals, loose terminal ends, and
chafed wires
 Always check connections when
diagnosing sensors and other
components
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Poor Electrical
Connections
 Testing for wiring problems inside a
harness:
 disconnect the wiring at both ends of the
harness
 ground the suspect wire on one end
 use an ohmmeter to check the wire’s
resistance
 if the resistance is high, repair or replace
the wire

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Poor Electrical
Connections

Testing for wiring problems
inside a harness
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Vacuum Leaks

Inspect for deteriorated, broken,
or loose vacuum hoses
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Vacuum Leaks
 Can upset the operation of the
computer system and cause a wide
range of symptoms
 Manifold absolute pressure sensor
hoses and intake manifold gaskets are
common locations of leaks
 Air leaks after the mass airflow sensor
can result in lean mixtures and stalling
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 Involves testing and replacing
computer system sensors
 For testing purposes, sensors may be
classified into one of two categories:
 passive sensors
 active sensors

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Passive Sensors
 Some common passive sensors
include:
 intake air temperature sensors
 coolant temperature sensors
 throttle position sensors
 transmission linkage position sensors
 EGR pintle position sensors
 manifold absolute pressure sensors

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Active Sensors
 Some common active sensors include:
 oxygen sensors
 crankshaft position sensors
 camshaft position sensors
 vehicle speed sensors

 knock sensors
 solar sensors

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