march2022 - 32

DIAGNOSTIC TECH
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Figure 3: This graph displays a comparison of peak tire resonance between a tire pressurized to specification and one that is 25 percent lower.
of the wheel speed sensor data. To
explain how this is calculated let's
look at an excerpt from Honda's OE
service info: " Peak resonance values
are learned during calibration
and become the reference values for
pressure monitoring. When using
the frequency resonance method,
the TPMS module compares the peak
resonance values between all four tires
and the two front tires. If the present
values have dropped below a 25
percent pressure threshold, the low
tire pressure/TPMS indicator turns
ON. Resonance frequency is corrected
based on the outside air temperature
sensor information, because resonance
frequency is changed by the outside
air temperature. "
To better understand this relationship,
look at Figure 3.
Because of this, they can tell if just
one or all four tires are low as long as
the system is calibrated properly. With
these systems you MUST read service
information and perform the calibration
exactly as written. In one document
published by Honda they state
that the system could enter calibration
on a new car when the pre-delivery
service is performed, if the tires were
not set to the correct pressure, then
the vehicle will learn the pressures at
say 65 psi instead of at the tire placard
pressure. Honda says to perform calibration,
the tires must be set to the tire
placard pressure only and the calibration
should be performed any time the
tire pressure is adjusted, or the tires
are rotated or replaced. The tires must
also be the correct size, same make/
model on all four corners, and tread
depth must also match (there is no
tread depth spec difference given but
it states tires must not be different
tread depths).
Now, let's look at diagnosing these
systems. If the vehicle has a fault,
first we need to look at the tire pressures,
tire sizes, and tread depths. A
variation in tread depth could potentially
be a problem. Documenting
things like this during routine service
is a good idea as a CYA procedure
as well. I use the Autel TBE200 and
ITS600 to document this information
for the customer.
After tires are checked, we need to
make sure that the ambient temperature
sensor is reading correctly and
make sure there are no ABS or VSC
system faults. To do this we would use
a tool like the Autel MS909 to complete
a system scan of all the modules.
32 Professional Distributor I March 2022 I VehicleServicePros.com
Then we would find the ambient sensor
PID in live data. If the vehicle has
an outside temperature read out that
can be used as well to verify. After
this it's a good idea to take the vehicle
for a short drive and graph the wheel
speed sensors to verify they are reading
correctly. In the Autel MS909 you
would go into the ABS module, select
all 4-wheel speed sensors, and then
select the graph merge option to look at
all the sensors together on one graph.
I prefer graphing data in comparison
to looking at numerical data because if
there is a drop out, it's easier to catch.
Sometimes the factory scan tools
offer faster refresh rate and better
graphing so if you have access to those
tools then that is the best option for
graphing data. Through the research I
did for this article (thanks to my good
friend Chris Roberds) I was also alerted
to a problem where a steering angle
fault can turn on the low tire light. If
the steering angle is off more than two
degrees while driving straight, the tire
light may turn on. According to several
SAE documents and patents on indirect
TPMS, these systems can measure
steering torque and decide if a tire is
low. The documents explain this in
detail, but the manufacturers are vague

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march2022

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