June 2023 - 21

p Figure 7 - 2012 Cruze misfire analysis
Photo courtesy of Scott Brown
the misfire although a P0304 was not
set. Since this motor used a " cassette "
style ignition coil assembly, I chose to
use the Picoscope secondary ignition
leads to establish a clean sample of the
KV waveform for each plug wire (Figure
6). We also wanted to use a current probe
to sample both the ignition coil power
supply along the fuel injectors and I
found that F9UA was tasked with protecting
both systems via circuit 5291
at the underhood power distribution
center and used one of my fuse loops to
connect a current probe. Additionally,
I back-probed the cylinder one ignition
coil ECM command signal as a reference
(Circuit 2121). This setup allows one to
analyze both ignition and fueling events
for each cylinder which is extremely helpful
during misfire analysis. As you can
see in the scope capture in Figure 7, the
current probe allows us to see the activity
of two circuits (fuel injectors and ignition
coils) within one channel, which is
a huge time saver. The reason for this is
so we can see whether the coil primary
circuit is being driven on the misfiring
cylinder and the reason for triggering
from coil one is so I can tell which cylinder
looks abnormal. My process typically
has me sampling each cylinder for
a few seconds starting with cylinder
one and moving in cylinder numerical
order. It's clear that the No. 4 cylinder
is misfiring because the secondary output
is non-existent. Zooming in on that
capture allows us to take a closer look to
verify the injector current and secondary
current are happening for the suspect
cylinder, which it was (Figure 8). With
that level of testing out of the way, we
were very certain that the ignition coil
was causing the misfire. Replacement of
the coil pack assembly solved the misfire
problem.Since a misfire can be caused by
several items including compression, we
performed a preliminary " ear-test " of the
relative compression prior to deploying
the scope and used it to quickly assess
the ignition and fuel injector's operation.
The ignition coil that we removed
from the vehicle appeared to be a genuine
OEM product, but a few markings
were different from the new OEM unit
we sourced. It's very possible that the
coil pack assembly the customer sourced
could have been counterfeit, but that's a
discussion for another day.
CASE STUDY NO. 3:
INTERMITTENT BLOWER MOTOR
OPERATION
A 2010 Toyota Tacoma arrived with a
complaint that the blower motor didn't
want to work sometimes and when it
was in failure mode, the client stated
that he could tap on the area below the
glovebox and the blower would work.
When we went to check the operation,
we found that the blower would work
every time we asked it to perform.
So, after performing a visual inspection
of the blower motor connections
we decided to take a quick look at the
blower motor current to see if we could
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June 2023

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