340 Section 3 Fundamentals of Electricity and Electronics
If you are not familiar with the circuit, use a dia-
gram to trace the circuit with the malfunction. For
example, if the right headlight is not working but
the left one is, you could narrow down the problem
to the circuit feeding current to the right headlight.
You have, through logical thought, narrowed down
the source of the electrical problem to one section of
the circuit. If both headlights fail to function, then
anything in the complete headlight circuit could
be the cause. Further tests, probably at the power
source to the circuit, would be needed to narrow
down the cause.
Perform the Proper Tests
The next step is to perform the tests needed to
check the specifi c section of the electrical system.
If one headlight does not work, you might check
the bulb fi rst. If the bulb is good, you might test for
power to the bulb socket. Again, you must think of
how the circuit operates to select the next most logi-
cal test.
Use proper testing methods to verify your
assumption about what might be wrong in the cir-
cuit. If tests prove that your fi rst assumption is not
causing the trouble, proceed to the second most
logical place to test. When you are testing circuits,
remember the following:
• If only one component does not work, start your
tests at that component. Chances are the prob-
lem is in the inoperative component or in the
section of the circuit leading to the component.
• If more than one component does not work, start
your tests at the power source for the circuit. A
blown fuse or loose power feed wire could be
preventing current in the whole circuit.
If you have diffi culty fi nding a problem, refer
to the service manual troubleshooting charts and
circuit diagrams. These diagrams are specifi c to the
vehicle’s make and model and will give you ideas
for fi nding the source of the problem. Always think
about the symptoms and problem while you are
working. Visualize current through the circuit, as
well as each connection and each component. A new
testing method or another potential problem source
may occur to you at any time.
Make and Verify the Repair
Following the steps described in this section
should help you determine exactly what the problem
is. After you have found the source of the problem,
make the repair. This might involve replacing a bad
component, soldering a broken wire, or repairing a
faulty connector.
Finally, verify the repair by making sure the
circuit is operating normally. If needed, test-drive
the vehicle to make sure road shock and vibration
do not affect the repair. Then you can be confi dent
that the problem will not recur after the owner
drives away.
Types of Circuit Problems
The four basic types of circuit problems are open
circuit, high resistance, intermittent trouble, and
short circuit. This section of the chapter defi nes
these problems, lists their causes, and explains how
each can cause different symptoms depending on
the circuit confi guration.
Open Circuit
An open circuit is a complete break or discon-
nection in the circuit. An open circuit, or “open,”
stops current through a section of the circuit. See
Figure 22-3.
Open
wire
Open fuse
Switch open
internally
Poor terminal
connection or open
Open load
Bad ground
Figure 22-3. These are some of the sources of opens or breaks in a circuit. In a series circuit, everything after break is “dead.”