diagnosticsupplement-1223 - 12

ESSENTIAL TOOLS
lines, to reduce line capacitance.
Note, however, that unstable readings
can also be caused by loose motor-generator
cables, which can set performance
DTCs, and/or prevent a motor-generator
from operating. Check fastener torque at
both ends of the cable (inverter side and
transaxle side), and don't trust resistance
readings until they become stable.
HIOKI RM 3548 MILLIOHMMETER
The Hioki 3548 milliohmmeter is a popular
resistance meter designed for automotive
use. It's the tool that both Toyota and
Tesla specify for their service centers.
Hioki also offers a range of leads for specialized
resistance measurements, in addition
to the standard L2107 alligator-clip
leads which come with the RM 3548.
PICO MT03 MILLIOHMMETER
The Pico MT03 is a relatively new product
that uses a laptop to display test results.
Test results are easy to print or send as a
digital file. The MT03 is also unique in
that all three phase-to-phase resistances
of a stator winding can be tested at the
same time, instead of individually. Results
are easy to see, with resistance readings for
each phase pair displayed on the laptop
along with green or red indicators that act
as " go-no go " or " pass-fail " gauges.
Which meter is best? It's close enough
to be a matter of personal preference. The
MT03 leads have less contact area than
the Hioki's, and may take a little more
care to stabilize the reading, but it's not
a dealbreaker. On the plus side, many
technicians prefer the MT03, citing its
ability to measure all three phases of a
motor-generator at once, as well as the
graphically displayed results which can be
easily saved to share with the customer.
The Hioki 3548 can also save measurements;
however, readings must be saved
to the meter and then uploaded to a PC.
USING A MILLIOHMMETER TO ESTIMATE
APPLIED FASTENER TORQUE
In the shop, we apply torque to bolts and
measure that torque to create the clamping
loads required by the OEM. While a
torque reading is not a direct measurement
of the strength of the clamping load, the
torque applied correlates with the required
clamping load.
Another way to " measure " bolt torque,
if the vehicle manufacturer provides a
specification, is to measure the electrical
resistance of the bolted connection, for
example between the bolt head and HV
busbar. Such measurements test the limits
of milliohmmeters, as the resistance specification
is often much less than one milliohm,
and may be stated in micro-ohms
(µΩ), or millionths of ohms.
Why would this kind of measurement
be useful? Well, most hybrid and electric
vehicles have multiple high-voltage bolted
connections. Correct torque of these connections
is essential to maintaining the
integrity of the connection as high-voltage
current flows through it. Incorrect torque
can produce heat that is easily enough to
melt copper or aluminum and lead to
component failure or worse.
A common diagnostic step when
tracking down high-voltage circuit issues
is confirming proper torque so as to rule
out an issue caused by an incorrectly
torqued high-voltage connection. If the
vehicle manufacturer provides resistance
specifications, such connections can be
tested using a milliohmmeter with pintype
probes, such as the Hioki rather than
alligator clips.
This kind of measurement is especially
useful when bolted HV connections
use single-use bolts that must be replaced
if they are loosened, such as high-voltage
connections in many Tesla vehicles. Keep
in mind that getting an accurate measurement
can be a challenge, and may only be
possible if (1) the connection surfaces are
first cleaned with isopropyl alcohol, and/
or (2) the user presses down on the pin
probes to ensure that they are getting a
sufficient bite into the area under test.
If the meter's readings are unstable, the
measurement is invalid.
Most OEMs do not yet require meaAlligator
clip leads are used when measuring phase-to-phase motor-generator resistances.
Pin probes are used to measure the resistance of bolted connections, including ground points.
Photo courtesy of Jack Rosebro
surements in the micro-ohm range, but
with the increased uptake of electric vehicles,
they may become more common in
the future. We shall see.
12 DECEMBER 2023 VehicleServicePros.com

http://www.VehicleServicePros.com

diagnosticsupplement-1223

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