july2023 - 27

DIAGNOS TIC S
attempting to memorize hundreds of DTCs
or SPNs (suspect parameter numbers), FMIs
have 23 values. These describe how a circuit,
component, and/or system has failed. When I
do not have the manufacturer's FCAP or diagnostic
tree to follow, I use FMIs. The FMIs are
going to send you down the best diagnostic
path in order to verify your repair process.
Under each system, start
by grouping direct circuit
failures together. These FMIs
are 3, 4, 5, 6, 9, 19. This group
is based on actual circuit
failures and should be diagnosed
first. Nevertheless,
I have seen FMI 9 and FMI
19 derive from failed sensor
modules as well, like NOx and
QLS sensors.
You must understand basic
electrical theory when diagnosing
in this direction. The
logic within the module will
depend on how the circuit is
looked at. You may get FMI 3
(an out-of-range high) with
a two-wire sensor or FMI 4
(an out-of-range low) with a
three-wire sensor, and possibly
have the same cause, a
broken wire.
Next, group and diagnose
the components. This group
puts you toward a monitored
system component
failure
and/or components associated
with that system of
concern. Those FMIs are 2, 7,
8, 11, 12, 13, and 14.
On occasion, component
FMIs need to be diagnosed
first if you see a 2, 13, or 14.
For example, a VGT actuator
has an FMI 13, or " out of calibration. "
The cause is most
likely the actuator and will
possibly require calibration
during a new install. Some
FMIs, like 11,14, and 31, may
be manufacturer-specific.
Lastly, the third group is
System FMIs: 0, 1, 10, 15, 16,
17, 18, 20, 21, and 31. These
describe the system's inefficiency
and may provide the
level of condition.
For example, if the unit is in
derate and the DPF is plugged,
or has SCR system-related
failures, you'll get a red stop
engine. You will see active
FMI 0, 15, and/or 16 in the
system for " Data Valid But
Above Normal Operational
Range-(15 for least, 16 for
moderately, 0 for Most Severe
Level). "
If your ECM displays FMI 0,
the data your ECM is receiving
is valid and now meeting the threshold
of most severe level.
If you don't have the right strategy, discovering
this could take much longer and lead
to much higher stress levels. That's not good
for you or your customer. But if you approach
it with the right technique, diagnostics can
truly be fun. Just remember to look at the
bigger picture and narrow down the root
cause using the process of elimination and a
systematic method, and then make the necessary
corrections.
Getting those trucks out of the bay as working
right will provide you with that amazing
feeling of success and allow you to move on
to the next challenge.
CORROSION IS THE LEADING CAUSE OF TRAILER LIGHTING FAILURES
PROVIDING TOOLS FLEETS NEED
Lighting is one of the top three CSA violations that frustrate drivers and fleets alike.
Diagnostic testing and repairs should be performed according to TMC recommended
practices. Peterson is proud to introduce our Trouble Shooting and Repair Guide,
which includes TMC Recommended Practices now available as a PDF download on
our corporate website.
Our lights create safer roads for everyone.
Day or night, in any weather, we deliver
exceptional performance in the most
demanding environments and we're proud
to manufacture them in America's heartland.
Visit pmlights.com/guides
July 2023 | FleetMaintenance.com
27
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july2023

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