Chapter 22 Basic Electrical Tests 343
Tech Tip
When you are looking for shorts or opens,
wiggle or move the wires while you are
testing. This might move the conductor
enough to produce the problem. It might
also correct a problem temporarily so you
can detect the trouble. Always look for wire
damage. Check for cut or smashed wire
insulation and loose connections. Any of these
can cause a malfunction.
Using Jumper Wires
A jumper wire is a piece of wire with alligator
clips on both ends, as shown in Figure 22-10A.
It is commonly used to bypass components or to
apply voltage to a component or section of a circuit.
Figure 22-10B shows a good example of how a jumper
wire can be used to check a switch. You might use
the jumper wires to bypass the switch if symptoms
point to an open switch; for example, if turning the
switch on does not feed current to the load (motor).
If attaching the jumper around the switch turns the
motor on, the switch is bad.
WARNING
Never use a jumper wire to bypass a
high-resistance component, a fuse,
or a circuit breaker. This could allow
excess current and cause an electrical fi re..
A jumper can also be used to connect voltage
directly to the load. If the load begins to work, some-
thing is wrong in the circuit leading to the load. If
jumping directly to the load does not make it work,
the load itself or the ground may be at fault.
Using a Test Light
A test light provides a quick and easy way of checking
a circuit for power or continuity. See Figure 22-11A.
Most test lights have a sharp probe for touching
conductors and an alligator clip for grounding. As
previously discussed, test lights can be either circuit-
powered or self-powered. Circuit-powered test lights
are more commonly used in auto repair.
A circuit-powered test light uses the circuit voltage
to illuminate the tester bulb. Figure 22-11B shows
how to use a circuit-powered test light to check for
power. The alligator clip is connected to any good
ground. Then the test probe is touched on different
points in the circuit. If the test light glows, the cir-
cuit is good up to that point. See Figure 22-12A. As
soon as the test light fails to glow, you have found
the problem area.
Sharp test light probes can be used to puncture
wire insulation to check for power. See Figure 22-12B.
This is useful when you cannot access a connector
for your tests. The test light can also be inserted into
the back of a connector so that the probe touches one
of the metal terminals in the connector. The connec-
tor can also be unplugged to check for voltage.
Figure 22-9. The location of the short in a circuit affects the
symptoms that appear. A—A short after a load cuts power to
the second load. However, the fi rst load still works. Current
increases slightly because of less resistance in the circuit.
However, current may not increase enough to blow the fuse
because of the resistance of the fi rst load. B—Short before a
grounded switch. The load stays on all the time, even with the
switch open. Current bypasses the switch to ground. C—In this
wire-to-wire short, the short feeds current across two branches.
Even with one switch off, current leaks over to the other
branch. Only when both switches are off will the bulbs turn off.
Bulb off
Bulb has necessary
circuit resistance
Bulb on
A
Fuse
OK
Short after Load
Switch open
Short to ground
before switch
Bulb cannot
be turned off
B Short before Grounded Switch
Both bulbs still glow
Short between
two wires
Switch open
C Wire-to-Wire Short
Switch closed