Chapter 25 Computer System Service 409
Electrical Component Damage
Semiconductor devices, such as transistors and
integrated circuits, are very easy to damage. They
can be damaged by static electricity, voltage spikes,
heat, and impact shocks. Here are some things
to remember when working with semiconductor
devices and their wiring:
• When welding on any part of a vehicle, discon-
nect the battery ground and disarm the airbag
system. If welding within 12″ (304.8 mm) of any
control module, remove the electronic module
from the vehicle to avoid heat and radiation
damage.
• Never disconnect the battery cables while the
engine is running. In the past, some technicians
would do this to see if the alternator was work-
ing. This can destroy or weaken electronic cir-
cuits, causing failure in a short period of time.
• Do not disconnect or connect wiring, especially
the computer wiring, with the ignition key on.
This can cause a current surge that can damage
the computer.
• Make sure you do not reverse the battery
cable connections. This can destroy elec-
tronic components.
• Wear an anti-static wrist strap whenever you
handle static-sensitive components (remov-
able PROM chips, for example) to protect them
from damage.
• Use only high-impedance test lights and meters
when checking electronic circuits or their wir-
ing. A conventional test light or meter will
draw too much current and destroy electrical
components.
• Do not disconnect a scan tool from the data
link connector while the ignition key is on. This
could create a voltage spike that can damage
the computer.
Computer System Circuit Problems
Almost all electrical-electronic problems are actu-
ally basic circuit problems. A basic circuit problem
is caused by a problem in a circuit that increases
or decreases current, resistance, or voltage. For
example, a broken wire could stop current fl ow or
a charging system problem could decrease output
voltage and current fl ow.
Unfortunately, when a minor problem occurs in
a complex circuit like a computer control system, it
may not seem like a minor problem. For example, a
poor electrical connection in a positive feed wire to
a body ground may cause the computer system or
one of the systems it controls to lose power. This can
result in a shutdown of the fuel injection system, the
emissions control system, the ignition system, or the
entire vehicle.
You might think that any of these systems or the
computer itself is at fault. Several systems could
appear to have a problem. In reality, it is simply a
poor electrical connection in one wire that is causing
all the problems. It is important for an automotive
technician to remain calm when diagnosing electri-
cal problems. If analyzed properly, problems can
usually be found and corrected easily.
Locating Computer Problems
The most diffi cult aspect of making computer sys-
tem repairs is fi nding the source of the problem. To
fi nd the source of computer problems, you must ask
yourself the following types of questions:
• What could be causing the specifi c symptoms?
Mentally picture the parts in the circuit and
how they function. Trace through the circuit
while referencing a wiring diagram to fi nd out
Excess
fumes
Contaminated oil
Burned
oil
PCV system
pulls fumes
into intake
manifold
Computer
Trouble
code out
Fumes pulled
into combustion
chamber and
burned
Abnormal
oxygen
sensor
voltage
Figure 25-1. This example shows that a trouble code can
be tripped by factors other than a computer system problem.
Contaminated engine oil can cause excess fumes to be
pulled into the PCV system. When burned in the combustion
chambers, the fumes could trick the computer into sensing that
something is wrong with the oxygen sensor circuit.