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Multimeter tips for automotive
technicians
When looking for a digital multimeter (DMM), raise the
hood and take a look inside. Talk to your fellow technicians
and find out what works for them. You'll probably use the
meter for the next several years, so purchase one with more
power than you think you need. You'll grow into the capabilities
as time goes on. Also look for a product with the best warranty
and service. If something does happen to your meter, you
don't want to be without it for long.
Standard DMM functions
DMMs integrate a wide range of test functions in a single
package. Common measurements such as volts, the electrical
pressure in a circuit, are found on most meters. A DC volt
selection is the main test function used around a vehicle.
DC stands for direct current, flowing in one direction only.
Whatever leaves the battery will also return. AC stands
for alternating current and is found around the shop or
in the home. On the car, alternators produce AC voltage
and current, both of which are rectified using a set of
diodes.
Most accessories designed for use with a multimeter convert
measurements into a millivolt output. For instance, when
using a temperature adapter, the output is 1mV for every
degree of temperature measured. If measuring current with
a clamp, depending on the scaling factor, each amp equals
1mV. For these reasons, most DMMs have a millivolt setting
on the rotary selection knob.
Continuity, used to determine if a circuit is open or shorted,
and ohms test are common on most DMMs. When buying a meter,
make sure to find out what accuracy level and measurement
ranges the meter is capable of; in addition, determine if
the meter is manual or auto ranging. Auto ranging will automatically
determine the best range for the measurement and adjust
the meter accordingly. Some meters use an integrated chip
to ensure compatibility and accuracy of all functions. Remember
that you are not just buying an instrument; you are buying
years of research, experience and product knowledge from
that company too.
Advanced DMM functions
Data recording
Some circuits on the vehicle are designed to turn on and
off over time. If these circuits do not turn off when they
should, excessive current drain could cause the battery
to run down. MIN/MAX is a data recorder designed to record
a reading that has reached a minimum or maximum level. This
function will also indicate what the average measurement
is: for instance, enabling a fast look at oxygen sensor
trends. MIN/MAX operates at different speeds, depending
on the equipment used. The Fluke 88V, for instance, will
record signals at a speed of 250 microseconds (that is 250
millionths of a second)!
MIN/MAX
MIN/MAX is like having another set of eyes, ears and hands.
Set the meter to make the measurement (current, voltage,
temperature, resistance — it doesn't matter), then walk
away. After returning to the meter, simply replay the results.
If there's a current drain causing the battery to run down,
it will show up on the display.
Data logging
The newest technology allows some meters to send measurement
results directly to a PC via built-in infrared ports. The
interval, or time spent between readings, can be set from
just a fraction of a second to hours. The computer will
then recreate a graphical picture of the circuit's trends,
allowing in-depth analysis of these electrical signals.
The meter itself has enough memory to hold up to a thousand
readings that can be replayed without a computer.
Touch Hold
How many times have you tried to make connections in awkward
locations, making operating the meter and holding the test
leads at the same time next to impossible? The unique Touch
Hold function can be entered into at any time and will freeze
the results on the display. Simply rotate the rotary knob
to the desired measurement and press the AutoHOLD button.
Set the meter aside and touch the test leads to the connection
points, and then remove the test leads. The display will
capture the reading.
Relative or Zero modes
Some components found on today's cars have extremely low
resistance. If the measurement is made without subtracting
test lead resistance, the component could accidentally be
found defective. Relative or Zero will automatically subtract
one reading from another, and display the result on the
instrument. Set the rotary knob to measure resistance and
short the test leads together. Press the Relative or Zero
button and the display will indicate zero (the actual reading
will show up on the analog bar graph). Now the actual resistance
of any item under test will be displayed.
Frequency
Frequency is the number of cycles that take place per second;
the more cycles that take place in one second, the higher
the frequency reading. Frequencies are measured in Hertz,
which is the number of cycles per second. There are varieties
of sensors on the vehicle that create either an AC or a
DC frequency.
The AC frequency is typically created from a magnetic type
sensor, one with two wires. It does not require any outside
power to operate and will increase signal strength, depending
on the speed of the trigger wheel cutting through a magnetic
field. Air gap will also affect the output signal strength.
The greater the gap – the lower the output voltage.
While most service manuals suggest testing the AC voltage
signal on ABS wheel sensors, there is no specific relationship
between the voltage level and wheel speed. Air gap, wheel
speed and magnet strength all affect the voltage level.
That's why the ABS computer doesn't actually read the voltage
level; it reads the AC signal frequency. The frequency is
directly related to the wheel speed, and increases as the
wheel speed increases. The ABS computer compares the frequencies
from the wheel sensors, and uses them to maintain the wheel
speeds while braking.
DC frequency is created from a pulsed DC signal. There
are a number of DC frequency producing sensors in cars today
– from Hall Effect sensors to Karman Vortex airflow meters.
These sensors vary their signal frequency; voltage and dwell
readings remain constant, for the most part.
Duty Cycle
Duty cycle is a measurement comparing the signal on time
to the length of one complete cycle. As on time increases,
off time decreases. Duty cycle is measured in percentage
of on time: A 60 percent duty cycle is a signal that is
on 60 percent of the time, and off 40 percent of the time.
Another way to measure duty cycle is dwell, which is measured
in degrees instead of percent.
Pulse Width
Pulse width is the actual on time of a signal, measured
in milliseconds. Fuel injectors and EGR systems use pulse
width modulation (a constantly changing pulse width) to
vary on time. With pulse width measurements, off time does
not really matter – the only real concern is how long the
signal's on.
Temperature
Some DMMs have built-in temperature measurement capabilities.
Otherwise, separate temperature modules are available that
convert a temperature measurement into a millivolt output.
Select the millivolt input and simply make the measurement.
Temperature is a great diagnostic aid. For example:
- Measure and compare exhaust runners to find a cool running
cylinder, pointing to an ignition, fuel or compression
problem.
- If the brakes are pulling from one side to the next,
use a temperature probe to determine if a sticking caliper
is a problem.
- Wondering if a bearing or u-joint is going south? Use
an infrared probe to gain non-contact temperature information.
A dry joint or bearing will typically run a little hotter
than others; compare your readings.
- To find out if a heater core is working well, compare
the inlet to outlet temperature. Even a restricted radiator
will show up very quickly.
Current
Many DMMs have the ability to measure current directly,
but only up to 10 amps (20 amps for 30 seconds). This portion
of the meter is fused to protect against excessive current
loads. If high current measurements are required, such as
starting or charging circuit tests, current clamps are available.
Similar in operation to the temperature modules, current
clamps use Hall effect technology to convert the current
measurement into a voltage read by the meter.
Current clamps are designed to operate within a certain
"range of operation." Low current clamps have narrow jaws
and great electrical shielding to ensure accuracy at the
low measurement range. Other clamps have the ability to
measure as high as 1000 amps, but may only measure down
to 2 or 3 amps.
Before determining what type of clamp you should buy, think
about your application. Do you work with low current circuits
and perform a lot of diagnostics? If the answer is yes,
then look at the 80i-110s Current Clamp from Fluke. It will
measure from 50 milliamps up to 100 amps, with 10mA resolution.
If you need a clamp to measure starting currents, look for
a clamp with a higher upper range. You may find that more
than one clamp is required to fit all of your measurement
needs.
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