Chapter 22 Basic Electrical Tests 349
Infi nite resistance, shown with an ∞ (infi nity) sym-
bol, means the circuit path is not complete. Some-
thing is preventing current through the tested circuit
or component. There is no continuity with an infi nite
resistance reading. Zero resistance, or no resistance,
means a perfect electrical path exists between the two
test points. This shows that there is continuity and a
resistance of 0 in the tested circuit or component.
Figure 22-28 shows an example of using an ohm-
meter to check a resistor pack. The wires from the
switch and to the blower motor have to be discon-
nected so that accurate resistance readings can be
taken on the resistors in the pack. By comparing
your readings to specifi cations, you can determine
the condition of the unit.
Amperage Measurements
Amperage measurements are needed to check the
condition of various components. Figure 22-29 illus-
trates how an ammeter reading might be used to
check the condition of a motor. The measured values
can be compared to known good readings. A high
current draw might point to a problem producing
a drag on the motor, such as bad armature bearings
or a short in a winding. A low current measurement
might point to a problem that is preventing normal
current, such as burned motor brushes, poor termi-
nal connections, or a bad ground.
Rules for Using Multimeters
As you work with multimeters to diagnose electric
or electronic circuit problems, keep these rules in mind:
• Make sure the test leads are connected prop-
erly. An incorrect connection may damage elec-
tronic components or the meter itself.
• Set the meter to read a high enough value. If
you have the meter set on 0 to 10 amps, for
example, and you try to measure 100 amps,
you might burn up the meter or blow its fuse.
• Use a digital or digital-analog meter to check
computer circuits.
Figure 22-19. Sample voltage settings and readings. A—The meter can be set to read volts dc (straight line) or volts ac (curved
line). B—The meter displays ac or dc in the display window, depending on which type of voltage is being read. This is an ac voltage
reading. C—This is the dc voltage reading. D—This is the dc voltage reading in millivolts.
Volts dc
Volts ac
A
C
B
D