Chapter 22 Basic Electrical Tests 347
can occur if you accidentally send too much current
through a meter. Use an inductive pickup for higher
amp readings.
The bar graph on a digital-analog multimeter
serves as an analog display that can show rapid
changes in readings. The bar graph functions much
like the needle on an analog meter. If a circuit or
component has an intermittent open or bad connec-
tion, the bar graph display moves back and forth to
display this rapid change in the reading. The digital
display changes as well, but it may not be able to
keep up with the reading changes. See Figure 22-23.
Some older meters are not auto ranging and must
be switched to the correct range. For example, the
meter might have a kilohms setting, which means
each reading must be multiplied by 100 to fi nd the
value in ohms. If the reading is in megohms, you
must multiply the reading by 100,000. Various volt-
age and current settings are also provided on some
meters that are not auto ranging.
Figure 22-19 shows a few voltmeter readings. This
meter has function settings for dc volts and ac volts.
The volts setting is used for most measurements in a
vehicle’s electrical systems. The volts might be used
to check the signals from speed or position sensors
that generate a small ac signal indicating engine or
driveline speed.
Figure 22-20 shows some ohmmeter readings
from a digital multimeter. Note the multiplier
shown on the far right of the display screen. Some
ohmmeters must be calibrated to zero before you
can make accurate resistance measurements. The
calibrate function compensates for battery condition
and provides accurate measurements. To calibrate
the ohmmeter, touch the two meter leads together.
The meter should display a reading of zero ohms. If
it does not, press the Zero button to recalibrate the
meter to zero. See Figure 22-21.
Figure 22-22 shows a multimeter set to make
current (amp) readings. To avoid blowing a fuse in
the ammeter, make sure the possible amp reading
is below the rating of the ammeter. Meter damage
Figure 22-16. These are the methods normally used to connect a voltmeter, ammeter, and ohmmeter to a circuit. A—A voltmeter is
normally connected in parallel with the circuit. B—Power must be disconnected from the component being tested so the ohmmeter
can use its battery power to measure resistance. C—A conventional ammeter is wired in series with the circuit to measure current.
D—An inductive ammeter clamps onto the outside of the wire and reads the magnetic fi eld around the wire.
~V
OFF
V
Ω
mA
A
mA
A
~
~V
OFF
V
Ω
mA
A
mA
A
~
~V
OFF
V
Ω
mA
A
mA
A
~
12.6V
0.50A
~V
OFF
V
Ω
mA
A
mA
A
~
12.6A
A
C
B
D
Parallel
connection
Multimeter set
to read volts
Circuit power
disconnected
Multimeter set
to read ohms
Series
connection Multimeter set
to read amps
or milliamps
Meter connects only
to test section of circuit
Meter clamps
on circuit wire
Multimeter set
to read amps
or milliamps