46 Auto Heating & Air Conditioning
Voltmeter
The voltmeter section of the multimeter is connected
to read voltage from a circuit. To read the voltage at an
electrical connection, connect the leads to the positive side
of the circuit and a ground. If necessary, select the proper
voltage range, then observe the reading.
The voltmeter can also be connected to read the
voltage across a connection as current fl ows through it.
If the connection has high resistance, current will try to
fl ow through the meter, creating a voltage reading. Voltage
higher than the specifi ed fi gure means the connection must
be cleaned or replaced.
Ohmmeter
An ohmmeter can be used to check electrical resis-
tance values. Ohmmeters can also be used to check for
complete circuits. To make an ohmmeter check, turn on the
multimeter and set it to ohms.
Caution: Electronic components can be
damaged by careless use of multimeters.
Always check the manufacturers’ literature
before testing any electronic part.
Most modern digital ohmmeters will select the correct
range automatically. Attach the leads to the wires or termi-
nals to be tested. When checking wires or relay contacts for
continuity, the resistance should be at or near zero. Other
parts, such as motor or solenoid windings and temperature
sensors should have a specifi c amount of resistance. If the
reading is zero or infi nity, the part is defective. The resis-
tance of temperature and sunload sensors should change
with changes in temperature or exposure.
Ammeter
The multimeter can usually check amperage (or amps).
The amp setting of the multimeter is called an ammeter.
Most multimeters have two separate ampere lead ports,
one for low amps and one that can usually measure up
to ten amps. For measuring greater amperage fl ows, many
modern ammeters can be equipped with an inductive
pickup. The pickup is clamped over the current carry-
ing wire. The pickup reads the magnetic fi eld created by
current fl owing through the wire and converts it into an
amperage reading.
Oscilloscopes
Oscilloscopes have been used in diagnosing gasoline
engines for years. With the addition of electronic controls
on HVAC systems, they can be used as a diagnostic tool to
check the waveform patterns from sensors and outputs. The
newer oscilloscopes are small, hand-held devices that can
be taken on road tests, Figure 3-14. With additional probes
and adapters, they can be used for a variety of diagnostic
tasks. The latest scopes have on-board memory functions,
which can be used to capture and store waveform patterns
for comparison to good patterns.
Belt Tension Gauges
If the compressor belt is loose, it may squeal when
the compressor clutch is engaged. If the belt is too tight, it
will quickly wear out and may damage the compressor and
clutch bearings, water pump, or the engine. To correctly
Figure 3-13. Multimeters contain several electrical testers, such
as voltmeters, ammeters, and ohmmeters. (Fluke)
FLUKE 88
AUTOMATIVE METER
Figure 3-14. Oscilloscopes can be used to diagnose problems
in computer circuits. (MAC Tools)