Digitalmulti-Meter - Craftsman 28906 Professional Shop Manual

ELECTRICAL SYSTEM
DigitalMulti-meter
A DMM is the most useful tool to trouble-shoot
any electrical system. There is an amazing vari-
ety of DMMs on the market. Some are very
basic, others are tailored to specific industries,
and some high-end graphing meters function
like oscilloscopes. Even the most basic ones are
quite versatile. See Figure 7.31.
Uses:
Voltage: Set meter to read "Volts DC ( _ __ )" if using
an auto-ranging meter or to an appropriate scale (typi-
cally 20 Volts DC) if using a more basic model.
Connect the meter in parallel to the circuit
being measured, between the test point and a
known-good ground. Turn-on the circuit to be
tested, and read the meter. For most tests the
engine need not be running, but the key will be
turned-on.
If there is question about which end of the circuit
the electricity is coming from, the circuit may be
disconnected near the test point.
If the meter is connected with the polarity
reversed, a "-" will appear in front of the voltage
reading. It has no ill effects on the meter nor on
accuracy.
If the meter is set to Volts AC (~) the reading will
be much lower that expected, but no physical
harm will be done to the meter nor the equip-
ment being diagnosed. It may waste some time
though.
Amperage: Most DMMs have a very limited capacity
to test amperage (2-3 Amperes). When measuring cur-
rent flow, the meter must be connected in series with
the component to be measured. That means opening
the circuit and having the circuit go through the meter.
Some meters have an inductive "Amp clamp"
accessory that can be used without breaking the
circuit.
Testing amperage beyond the capacity of the
meter can burn-out an internal fuse in some
meters. The fuses can be expensive.
Resistance: Set the meter for the "W" scale.
Isolate the part of the circuit to be tested (discon-
nect it from the source of power).
Ohms are read on a scale of 0 to 1, with "0" indi-
cating no resistance and "1" indicating infinite
resistance.
Most auto-ranging meters will provide readings
on several scales. For outdoor power equip-
ment, the straight Ohm scale is most appropri-
ate. If a letter appears next to the W on the
screen of the DMM, it indicates different scales
of sensitivity.
"m" is micro-Ohms (.001 W), a more sensitive
scale that effectively moves the decimal point
three places to the left of its location for plain W
"K" is Kilo-Ohms (1,000 W), a less sensitive
scale that effectively moves the decimal point
three places to the right of its location for plain W
"M" is Meg-Ohms (1,000,000 W), is the least
sensitive scale that effectively moves the decimal
point six places to the right of its location for plain
W
A reading of "0" may be called "Continuity".
A reading of "OL" may be referred to as "No
Continuity".
Mistaken Ohm readings most frequently come
from bad technique. Poor connections between
the probes and the point to be read can throw-off
readings. False readings can be generated if the
technician touches both probes with their fingers
while taking the reading.
The meter has it's own power source to measure
resistance. Connecting the meter to a compo-
nent that has current going through it will dam-
age the meter (usually beyond repair).
23