Chapter 22 Basic Electrical Tests 355
with a test light or voltmeter. If power is reaching
the relay, check that the relay points are closing.
Check for voltage leaving the closed relay points. If
not, replace the relay.
Testing Fuses
A fuse can be checked quickly using a test light.
See Figure 22-36. The test light should glow when
connected to either side of the fuse. If the light glows
only on the feed side, the fuse is blown. If you fi nd
that the fuse is blown, you should check for a short
that may have caused the fuse to blow. However,
sometimes fuses fail after prolonged service, even if
there is no short in the circuit.
A voltmeter can also be used to check a fuse, but
it is not as handy as a test light. If the meter is digi-
tal and makes an audible signal when the voltage
changes, you can check the fuse without too much
trouble. Again, voltage should be present on both
sides of the fuse.
Testing Capacitors
Capacitors are commonly used to absorb fl uctuat-
ing voltage or current in a vehicle’s electrical system.
They can be used to prevent interference in the radio
caused by unsmooth dc current or switches opening
and closing. Capacitors were also used in older con-
tact point ignition systems to prevent contact point
burning.
Figure 22-29. Using an ammeter to check a circuit. If current is
too high, a low resistance or high voltage is indicated. If current
is low, a high resistance or low supply voltage is indicated.
Figure 22-30. A good set of test leads makes it possible to
probe all types of electrical connectors without damaging them.
To prevent damage, never insert a large test probe into a small
connector terminal. (OTC/SPX Corp.)
Figure 22-31. Testing a connector. A—Using a test light, you
can quickly check for power on both sides of a connector. If the
test light glows on one side only, a bad terminal connection is
indicated. B—Use a voltmeter to check the voltage drop across
a connector. A high drop shows a high resistance problem.
Figure 22-32. The resistance of all resistors in a vehicle must
be within specifi cations. After a period of service, resistor
values can change enough to upset circuit operation.
Power source
(12 V)
Motor
or load
~V
OFF
V
Ω
mA
mA
A
~
Ammeter
+ –
Bad
connector
0 V +12 V
0 V +12 V
~V
OFF
V
Ω
mA
mA
A
~
12.0V
A
B
Voltage drop
shows bad
connection
~V
OFF
V
Ω
mA
mA
A
~
4.3 kΩ