diagnosticsupplement-1223 - 10

ESSENTIAL TOOLS
Old tools, new tricks
With the increase of electric vehicles pending, the need for
a milliohmmeter may be in your shop's future.
By Jack Rosebro
I
t's been more than 20 years since some
of us first started using milliohmmeters
in hybrid and EV repair, generally to
measure phase-to-phase resistances
of motor-generators on first-generation
Prius hybrids and the vehicles that came
after it. Briefly, a milliohmmeter is a precision
ohmmeter that can measure very
small resistances in the thousandths of
ohms (milliohms), and in some cases,
millionths of ohms (microohms), as we
will see. It does this, in part, by (1) employing
four-wire Kelvin leads, which use two
wires per lead to separate test voltage
and test current, and by (2) generating a
higher current than the average DVOM.
It's a tool that might not be used regularly,
but when you need to measure resistances
of less than an ohm, and measure
them precisely, nothing else will do.
USEFUL FEATURES IN
A MILLIOHMMETER
These days, there are many milliohmmeters
to choose from. In this article, I'll discuss
functions to look for when choosing
one, using the Hioki 3548 milliohmmeter
as well as the relatively new laptop-based
Pico MT03 milliohmmeter as examples.
Note: Milliohmmeters are often used
to measure resistances of high-voltage
components after the components have
been de-energized. Care must be taken
to follow all safety protocols as directed
by the OEM before measuring.
Three features come to mind when I
think of what I need in a milliohmmeter:
1. ACCURACY
The Hioki is accurate to ±0.08 milliohms;
the Pico is accurate to ±0.5% ±100µΩ. Both
10 DECEMBER 2023 VehicleServicePros.com
Hioki 3548 showing 152.99 milliohms of U-to-V phase resistances on a 2013 Toyota Prius C.
Photo courtesy of Jack Rosebro
of these meters are accurate enough for our
automotive needs.
2. BATTERY-OPERATED
Some milliohmmeters are designed to
be used for bench-testing components,
rather than on-the-car testing. They run
on 120V and usually require an extension
cord to reach the vehicle. Battery-operated
milliohmmeters, on the other hand,
are portable as well as generally built to
be more sturdy and resistant to damage
than bench units.
3. AMBIENT TEMPERATURE SENSOR
Component resistance specifications
in the milliohm range typically refer to
resistance at a given reference temperature,
usually 68 degrees F. If the component
being tested is not at the reference
temperature - and it probably isn't - the
measurement reading must be adjusted
for ambient temperature, using a rather
cumbersome equation that can sometimes
be found in service information, if it is
used. This creates a potential for error if
the calculation is made incorrectly. Both
the Hioki milliohmmeter and the Pico
milliohmmeter come with ambient temperature
sensors. These sensors, along
with some circuitry, enable the meters to
compensate for temperature during testing
and to calculate the resistance that would
be present if the resistance had been measured
with the component at the reference
temperature. Let the machine do the math.
Tip: Keep in mind that a milliohmmeter's
ambient temperature sensor (if equipped)
measures ambient temperature, not component
temperature. If the component being
measured has not cooled sufficiently, your
readings will be high, as heat increases
electrical resistance. Toyota specifies, for
example, that low-ohms measurements be
made only after the vehicle has been turned
off for at least eight hours.
http://www.VehicleServicePros.com
diagnosticsupplement-1223

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