344 Section 3 Fundamentals of Electricity and Electronics
Tech Tip
Avoid using a test light probe to puncture the
insulation on wires in a computer or sensor
circuit. Moisture can enter through the hole in
the insulation. If the wire is to a sensor with
a weak or low voltage signal, this can upset
the small signal going to the computer, and a
computer system malfunction may result.
A self-powered test light, or continuity tester, has
an internal battery. It uses its own power source to
check for a complete electrical path, so the circuit’s
power source must be disconnected for testing.
Figure 22-13 shows how a self-powered test light
can be used to check for continuity.
Figure 22-14 shows how a test light can be used
to help fi nd shorts. The test light is substituted for
the fuse in the affected circuit. As long as the circuit
is complete, the test light glows. Wiggle the wires or
disconnect components or wires until the test light
goes out. When the test light does not glow, you
have found the source of the short.
WARNING
Test light probes are very sharp. It is
easy to jab the probe through your
skin when trying to penetrate the wire
insulation. Do not place your fi ngers behind the
probe when testing.
Figure 22-10. Jumper wires allow you to bypass a section of the circuit for testing. Be careful not to connect the battery voltage
to any ground. A—Jumper wires can be used to bypass a component or connect power to a circuit for testing. B—A jumper wire
provides an easy way to bypass a component or connect power directly to a component or section of circuit. In this example, if
bypassing the switch makes the motor run, the switch is bad. (Snap-on Tool Corp.)
Figure 22-11. A test light is quick way to check for battery voltage in a wire or component. A—A test light provides a way to check
for power or voltage in a circuit. B—With the circuit-powered type, the alligator clip is grounded. Then the test probe is touched to
the circuit at different locations to fi nd breaks in the circuit. (Snap-on Tool Corp.)
M
A
Flexible
insulation
Alligator clips B
Battery
Bad switch
Motor
Jumper wire
bypasses bad
switch
Fuse
Alligator
clip
Test
probe
Light
–
+
Battery
Start here if
more than
one component
is dead
Start here if only one
component is dead
To any
ground
Trace along circuit
until open is found
A B