Yamaha MM700A, Manual, Page: 29 / 282

GENERAL INFORMATION 23
Voltage
Ohmmeter
resistor
1.
Connect the positive meter test lead to the electrical
source (where electricity is coming from).
2.
Connect the negative meter test lead to the electrical
load (where electricity is going). See Figure 45.
3. If necessary, activate the component(s) in the circuit.
4.
A voltage reading of 1 volt or more indicates excessive
resistance in the circuit. A reading equal to battery voltage
indicates an open circuit.
Resistance
Resistance is the opposition to the flow of electricity
within a circuit or component and is measured in ohms.
Resistance causes a reduction in available current and
voltage.
Resistance is measured in a inactive circuit with an
ohmmeter. The ohmmeter sends a small amount of current
into the circuit and measures how difficult it is to push the
current through the circuit.
An ohmmeter, although useful, is not always a good in-
dicator of a circuit's actual ability under operating condi-
tions. This is due to the low voltage (6-9 volts) that the
meter uses to test the circuit. The voltage in an ignition
coil secondary winding can be several thousand volts.
Such high voltage can cause the coil to malfunction, even
though it tests acceptable during a resistance test.
Resistance generally increases with temperature. Per-
form all testing with the component or circuit at room
temperature. Resistance tests performed at high tempera-
lures may indicate high resistance readings and result in
the unnecessary replacement of a component.
Measuring resistance and continuity testing
CA UTION
Only use an ohmmeter on a circuit that has
no voltage present. The meter will be dam-
aged i f it is connected
to
a
live circuit. An
analog meter must be calibrated each time it
is used or the scale is changed. See
Multimeter in this chapter.
A continuity test can determine if the circuit is com-
plete. This type of test is performed with an ohmmeter or a
self-powered test lamp.
1.
Disconnect the negative battery cable.
2. Attach one test lead (ohmmeter or test light) to one end
of the component or circuit.
3. Attach the other test lead to the opposite end of the
component or circuit (Figure 46).
4. A self-powered test light will come on if the circuit has
continuity or is complete. An ohmmeter will indicate ei-
ther low or no resistance if the circuit has continuity. An
open circuit is indicated if the meter displays infinite re-
sistance.
Amperage
Amperage is the unit of measure for the amount of cur-
rent within a circuit. Current is the actual flow of electric-
ity. The higher the current, the more work that can be
performed up to a given point. If the current flow exceeds
the circuit or component capacity, the system will be dam-
aged.
Measuring amps
An ammeter measures the current flow or amps of a cir-
cuit (Figure 47). Amperage measurement requires that
the circuit be disconnected and the ammeter be connected
in series to the circuit. Always use an ammeter that can
read higher than the anticipated current flow to prevent