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Using Multimeters
We
list Radio Shack meters here, because everyone has a Radio Shack
store nearby.
If you have a local dealer, you may want to shop around for other
brand names and features.
To
successfully troubleshoot electrical truck problems, you will
eventually need a meter to help you determine what is wrong. Some
problems can be found with visual clues, while other problems
require
a meter. So go ahead and get yourself a multimeter now, so you
can do a good job. Multimeter simply means multiple meter functions.
For example, most multimeters include resistance measurement (ohm-meter),
current measurement (amp-meter), and voltage measurement (volt-meter)
capability. Many of the newer digital multimeters also include
a temperature measuring mode. Most meters are damaged due to
careless use or misunderstanding, so make sure that you understand
all sections of this topic before using a meter.
Multimeter
Selection
Today
there are many different multimeters to chose from. Analog multimeters
have physical meters with pointers which deflect to show the value
measured. Digital multimeters have digital readouts of the value
measured. Most of today's digital multimeters use liquid crystal
(LCD) displays. The older digital
multimeters used light emitting diode (LED) displays, but they
consumed a lot of battery power just to light the displays. The
LCD displays require only micro-amps (1/1,000,000 of an amp) to
display the results. They operate on such little power because
they simply reflect ambient light to make the display work.
The
analog and digital multimeters each have their own special advantages.
The analog meter works equally well in cold weather, whereas the
digital LCD meter works poorly or not at all in severe cold temperatures.
The liquid crystal material is really liquid, so it get thicker
with cold, and takes longer to change the display results. Storing
the LCD in a warm location before using in extreme cold, will
greatly increase its usability. The digital meter is much more
rugged because it does not have a jeweled meter movement.
Another
difference between the analog and digital multimeters is their
accuracy. The digital meters are much more accurate for complex
voltage measurements because they have a very high input resistance
(on the order of 1-million ohms). This eliminates meter loading
of the device being measured. The analog meters have a much lower
input resistance of 1000 ohms per volt, or 20,000
ohms per volt. This lower input resistance can affect voltage
measurements when the device being measured has high internal
resistance. Truck devices seldom have high internal resistance,
so the 1000 ohms per volt meter will do the job, and costs less
than the 20,000 ohms per volt meter.
Another
difference between the analog and digital multimeters is their
physical size. Analog meters are usually much larger than digital
meters. The smaller digital meter is easier to store, and easier
to use in crowded areas.
Another
difference between the analog and digital multimeters is a function
called auto ranging. To use an analog meter, you must have a general
knowledge of the expected measurement results. You must first
select a scale which will give you the most accurate reading,
and then make the reading. If the measured value is greater than
the selected scale, the the meter slams into its right hand limit
peg. Repeated slamming of the meter pointer will make the meter
inaccurate. So you must think before using the analog meter.
The
digital multimeters use electronic chips to determine the measured
value. These electronic chips are smart enough to auto range to
the proper scale without human intervention. When the measurement
starts, the meter automatically selects its own scale internally,
and displays the digital results.
The
last difference between analog and digital multimeters has to
do with auto polarity. With the analog meter, you must always
observe polarity before connecting the meter. You must connect
the positive  meter
lead to the positive side of the device to be measured, and the
negative meter lead to the negative side of the device to be measured.
If you do this wrong, the meter will slam into the left hand limit
peg. Once again, repeated slamming will make the meter inaccurate.
The
digital multimeter also has a positive and negative meter lead.
But the digital meter will simply place a minus sign (-) in front
of the measured result. Therefore it doesn't matter if you get
the polarity mixed up. The reading will still be accurate, and
no damage is done to the digital multimeter.
The
cheapest analog multimeter at Radio Shack costs about $15 and
the cheapest digital multimeter at Radio Shack costs about $25.
You can pay extra for better meters with more features, but those
extra features have little value when working on trucks. We recommend
the $30 meter at the top of this page, for it has the most features
for a reasonable price. Of course, if you do a lot of work in
cold environments, then an analog meter may be best for you.
One
last note about purchasing a multimeter. Take a half-hour and
read the booklet which comes with the meter. It is time well spent
because you will know how to protect your meter investment, and
that is a good thing. You will also know if protection fuses are
inside, and you will know what kind of batteries the meter uses.
Many people just rip the meter out of the packaging and start
jamming leads into electrical circuits. Then after the smoke clears,
they wonder why the meter doesn't work.
Voltage Measurements (volt-meter)
Measuring
voltage is one of the easier things you can do with a multimeter.
When you measure voltage, you are simply placing the meter across
the voltage to be measured as shown above. First, ensure that
the meter is in the voltage mode. If you try to measure voltage
while in the current or resistance mode, damage to the meter will
most certainly occur. Then select the correct voltage scale,
 which
on trucks will usually be the 20 or 50 volts scale (the lowest
scale which is greater than the maximum expected voltage of 14
volts). With auto-ranging digital meters, don't worry about voltage
scales.
Now figure out which side of the device to be measured is the
positive voltage side, and connect the positive meter lead. Connect
the other meter lead (the negative lead) to the other side of
the device to be measured. With a digital meter, the positive
side of the device doesn't matter since the meter will auto-polarity
adjust the readout accordingly.
Most
truck electrical devices use the truck chassis as the negative
lead. For those devices, simply measure the voltage by connecting
the positive lead to the device, and the negative lead to the
truck chassis ground.
Current
Measurements (amp-meter)
Measuring
current is another quick way to damage your meter. So pay attention
while make current measurements. Fortunately, there are very few
situations where current measurements are important in fixing
truck problems.
When
measuring current, the amp-meter must be placed in series with
the device to be measured just like in the diagram above. Just
like a device fuse, the amp-meter must be in-line with the power
source. Since most truck devices use the truck chassis for the
negative return path (chassis ground), the amp-meter must be placed
into the supply (positive) side of the device.
Selecting
the proper current scale is very important. You must have an idea
of what current is expected, then you can select a current scale
which wont damage your meter. Remember, all the current the measured
device draws, will pass right through the amp-meter. If the measured
device draws 20 amps and your amp-meter is set to a 10-amp scale,
then the meter will probably be damaged. Always ensure that
the selected current scale is greater than the current draw required
by the device.
Once
you have selected the proper current scale, you must be certain
to observe polarity of the meter leads. The electron current flow
must enter into the negative lead of the amp-meter, and exit the
positive lead of the amp-meter. Do this backwards, and the analog
meter will probably be damaged. Most all trucks connect the negative
battery terminal to chassis ground. For those truck devices, the
negative amp-meter leads connects to the device, and the positive
amp-meter lead connects to the positive battery supply voltage.
When
you have finished your measurement, remove the meter from the
circuit and restore normal circuit connections. DO NOT leave
the meter in current mode. Place the meter in its off mode
or its voltage mode as soon as the meter is out of the circuit.
If you later try to measure voltage while in current mode,
damage to the meter will most certainly take place.
Resistance
Measurements (ohm-meter)
Measuring
resistance has one important requirement. The device being
measured must have NO power supplied to it. Trying to measure
the resistance of a powered device, will probably damage the ohm-meter
portion of your multimeter. Disconnect the device from all power
sources before you attempt to measure resistance of the device.
Next,
select the resistance range that will give you approximate mid
scale meter deflection on an analog meter. Notice that the meter
scale is non-linear. This means that a 100-ohm meter scale, will
NOT read 50 in the mid-scale position. Instead, the mid-scale
position would read more like 4-ohms. At the very left end of
the ohms scale you will see a symbol which looks like the number
8 laying on its side. This is called the epsilon symbol, and it
represents infinite resistance, which we call an open circuit.
The right side of the ohms scale represents zero resistance, and
we call this a short. On a digital meter, simply select a range
which gives you a displayed reading. Many digital meters which
don't have auto-ranging, will display "OV" (over range)
when the resistance exceeds the selected resistance scale. When
this happens, just select a lower resistance scale. For auto-range
meters, the range selection is done automatically.
With
analog meters, you must now calibrate the resistance scale. To
do this, just short the meter leads together, and then adjust
the zero ohms adjust knob on the meter for full scale deflection
of the meter. Adjust until the meter pointer is pointing to the
zero ohms mark of the scale (full deflection to the right). Since
your meter leads are shorted together, there is zero resistance
between them. Now the ohm-meter has been compensated to read zero
ohms while using the ohm-meter battery voltage inside your multimeter.
As the ohmmeter internal battery gets used up over time from resistance
measurements, the battery voltage will decrease and require a
new zero adjust as described above, to compensate for the weaker
battery voltage. When you can no longer adjust for zero ohms,
it is time to replace that battery.
For
most truck devices, lead polarity doesn't matter. The meter has
its own internal battery which performs the resistance measurement.
So polarity is not normally an issue. One truck device, the alternator,
has devices called diodes (or rectifiers) inside which can affect
resistance readings if the meter leads are not connected properly.
Measuring diodes is covered in the next section
of this topic.
DO
NOT leave the meter in the ohm-meter mode. Place the meter
in its off mode or its voltage mode as soon as the resistance
measurement is completed. If you later try to measure voltage
while in the ohm-meter mode, damage to the meter will most certainly
take place.
Diode
Check & Beep Indicators
Many
of the newer digital meters include diode check and beeper modes.
The beeper mode is easy to understand, so we will discuss that
first.
When
you are checking for continuity, this means that you expect a
very low resistance to be measured. For example, if you are testing
a fuse (out of the circuit, no voltage present) then you
would expect about zero ohms. If you were checking the continuity
of a wire, then you would also expect about zero ohms. The beeper
mode places the meter in the ohm-meter mode, but instead of displaying
the resistance value measured, it simply beeps when a low resistance
is detected. Some meters display the resistance value and beep
at the same time. In any case, the beep tone is a quick and dirty
indication of continuity. Just touch the meter leads to the device
to be tested, and the audible tone confirms continuity.
Diode
checks is another whole ball of wax. If you really need to do
this, then read on. If you don't need to perform diode checks,
then skip to the next section.
A
diode is a special device which allows current to flow in only
one direction. Alternating current (AC) such as in your home,
reverses direction 60 times per second. This alternating current
has less power loss during transmission through those long distance
power lines you see along the highways. Less loss to the power
company means more profit for them, and smaller electric bills
for you (yeah right)! Since the truck does not have to move electricity
very far, the higher losses of direct current (DC) are negligable,
and the DC devices are cheaper to make than AC devices.
Sometimes
diodes are used for idiot lights in cars, and they may also be
found in some newer truck circuits. When a meter specifies diode
mode, this just means that the meter circuits will consider the
requirements to make valid diode checks. A good diode will test
with low resistance in one direction, and much higher resistance
when the meter leads are reversed.
The
alternator in your truck also generates AC because AC can generate
more current at slower engine speeds. The AC generated by the
alternator must be converted to direct current (DC) in order to
charge your truck battery. Inside the alternator are groups of
diodes which rectify (allow current to flow in one direction only)
the alternator AC and convert it into DC. The DC is what your
truck battery and electrical devices need.
The
alternator is actually 3-AC generators in one. This is called
3-phase, and the diode clusters add all 3-phases together and
rectify the current into a single DC output. The 3-phases is one
of the reasons that an alternator can put out more current at
lower engine speeds than a generator.
When
one or more of these alternator diodes shorts or opens, then the
current generating capacity of the alternator is reduced, and
it can not keep the truck battery fully charged. Sometimes, shorted
diodes inside the alternator may cause discharge of the truck's
battery.
To
test the alternator diodes, the alternator must first be disassembled.
Then the manufacturer's test procedures must be followed to determine
if the diodes test good or bad. Many repair manuals devote much
attention to this subject and should be referenced when testing
alternator diodes.
Battery
& Fuse Replacement
All
multimeters have batteries inside, and many of them have fuses
inside. The battery powers the ohm-meter circuits of the multimeter,
and for the digital meters, the battery also powers the meter
circuits. For the analog meters, a weak battery will prevent zero
ohms calibration while the meter leads are shorted together.
For
digital meters, the digital display will not work when the battery
is weak. Some digital meters have a separate battery for the resistance
measurement circuits.
Fuses
are inside some meters to protect the meter from stupid mistakes.
When the mistake is executed, the protective fuse blows, and must
be replaced before that mode can be used again. Some meters have
two or more fuses inside. The more the meter costs, the more fuse
protection it has.
Some
stupid actions which blow these fuses are:
-
Current
measurements exceeding the meter capacity
-
Voltage
measurements exceeding the selected scale
-
Measuring
voltage while in the resistance mode
-
Measuring
voltage while in the current mode
This
concludes our multimeter topic. Your multimeter should serve you
for a very long time if you follow the procedures described above.
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Repair Topics.
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