Yamaha MM700A, Manual, Page: 28 / 282

22
CHAPTER
ONE
Results may vary if using a meter not recommend by the
manufacturer is used. Such requirements are noted when
applicable.
Ohmmeter
(analog) calibration
Each time an analog ohmmeter is used or the scale is
changed, the ohmmeter must be calibrated.
Digital ohmmeters do not require calibration.
1. Make sure the meter battery is in good condition.
2.
Make sure the meter probes are in good condition.
3. Touch the two probes together and observe the needle
location on the ohms scale.
The needle must align with the 0 mark to obtain accu-
rate measurements.
4.
If necessary. rotate the meter ohms adjust knob until
the needle and
0 mark align.
ELECTRICAL SYSTEM FUNDAMENTALS
A thorough study of the many types of electrical sys-
tems used in today's vehicles is beyond the scope of this
manual. However, a basic understanding of electrical ba-
sics is necessary to perform simple diagnostic tests.
Voltage
Voltage is the electrical potential or pressure in an elec-
trical circuit and is expressed in volts. The more pressure
(voltage)
in a circuit. the more work that can be per-
formed.
Direct current (DC) voltage means the electricity flows
in
one direction. All circuits powered by a battery are DC
circuits.
Alternating current (AC) means the electricity flows in
one direction momentarily then switches to the opposite
direction. Alternator output is an example of AC voltage.
This voltage must be changed or rectified to direct current
to operate in a battery powered system.
Measuring voltage
Unless otherwise specified, perform all voltage tests
with the electrical connectors attached.
When measuring voltage, select a meter range one scale
higher than the expected voltage of the circuit to prevent
damage to the meter.
To determine the actual voltage in a
circuit, use a voltmeter. To simply check if voltage is pres-
ent,
use a test light.
NOTE
When using
a test light, either lead can be
attached
to
ground.
1.
Attach the negative meter test lead to a good ground
(bare metal). Make sure the ground is not insulated with a
rubber gasket or grommet.
2 Attach the positive meter test lead to the point being
checked for voltage (Figure 44).
3.
Turn on the ignition switch. The test light should light
or the meter should display a reading. The reading should
be within one volt of battery voltage. If the voltage is less,
there is a problem in the circuit.
Voltage drop test
Resistance causes voltage to drop. This resistance can
be measured in an active circuit by using a voltmeter to
perform a voltage drop test. A voltage drop test compares
the difference between the voltage available at the start of
a circuit to the voltage at the end of the circuit while the
circuit is operational. If the circuit has no resistanee, there
will be no voltage drop. The greater the resistanee, the
greater the voltage drop will be. A voltage drop of one volt
or more indicates excessive resistance in the circuit.