352 Section 3 Fundamentals of Electricity and Electronics
A voltage drop test can also be used to check a
connector. See Figure 22-31B. A good connector has
little voltage drop. A high voltage drop indicates
poor terminal contact.
Testing Resistors
Various types of resistors are used in various
automobile circuits. After a period of use, resistors
can break down and change resistance. This can
upset the operation of the circuit. An ohmmeter can
be used to test actual resistance. See Figure 22-32.
A voltmeter can also be used to check the resistor’s
voltage drop. If the resistor is not within specifi ca-
tions, it should be replaced.
Testing Variable Resistors
A good variable resistor should exhibit a gradual
change in resistance as you turn its shaft or move
its slide. A bad variable resistor might be open (infi -
nite resistance) or shorted (zero resistance). It may
also be stuck at a specifi c resistance value, or its
resistance may not vary properly with shaft or slide
movement.
In most conditions, an ohmmeter is the most
accurate way of checking a variable resistor. The ser-
vice manual usually gives resistance specifi cations
for different variable resistor positions. If test val-
ues differ from specifi cations, the variable resistor is
bad. See Figure 22-33. You can also check a variable
resistor by testing voltage drops, but specifi cations
are not given this way.
Testing Switches
A good switch blocks current in one position and
passes current freely in the other position. A bad
switch may not turn on and off, or it may have high
resistance when on. A switch can be tested using a
test light, voltmeter, or ohmmeter.
If the switch is still installed, a test light can be
used to check for power in the two positions. A volt-
meter can also be connected across the switch. It
should show full voltage drop when the switch is off
and little or no voltage drop when the switch is on.
An ohmmeter can be used if the switch is removed
or isolated from the circuit. The switch should have
zero resistance when on and infi nite resistance when
off. See Figure 22-34.
Testing a Transducer Switch
The internal resistance of a transducer switch
changes with a change in a condition. Examples
include temperature switches, position switches,
and pressure switches. When a transducer switch
fails, it stays in the same position (on or off) even
though conditions change.
An ohmmeter is normally used to check a tem-
perature switch because the leads to the switch can
be disconnected easily. See Figure 22-35. Measure
the unit’s internal resistance when cold and hot
Figure 22-24. Voltage drop measurements check resistance
without requiring you to disconnect components. If the closed
switch has a high voltage drop, the circuit has a high resistance
problem.
Figure 22-25. A component’s resistance determines its voltage
drop, so components that have equal resistance have equal
voltage drops. Total circuit voltage drop equal voltage applied.
~V
OFF
V
Ω
mA
mA
A ~
10.3V
Fuse
block
Load
Switch
closed
High voltmeter
reading equals
high resistance
12 V
0.5 Ω 0.5 Ω 0.5 Ω
Voltage drop
Total resistance = 0.5 Ω + 0.5 Ω + 0.5 Ω = 1.5 Ω
Total voltage = 4 V + 4 V + 4 V = 12 V
4.00V 4.00V 4.00V
~V
OFF
V
Ω
mA
mA
A
~
~V
OFF
V
Ω
mA
mA
A
~
~V
OFF
V
Ω
mA
mA
A
~