December 2021 - 38

ANALYSIS
As I scrolled my way through the
captured data, I had a realization. These
injector commands continued to vary as
the engine ran with no change in driver
input or load. Although not displayed,
the same variation was going on with
the ignition coil current ramps as well.
Something very odd was going on here.
I referred back to the available engine
data and saw no real change in any key
inputs affecting the calculation for injector
pulse width. I began to suspect that
the PCM may be malfunctioning, whether
due to an underlying fault or internal
to the PCM itself. I ruled out system
voltage/alt. output as a potential fault,
and it proved to be sufficient. I was left
with nothing to measure but the inputs
responsible for the timing and duration
of both the fuel injectors and the ignition
coil. These inputs are the PIP and the
SPOUT signals.
TRACKING THE ROOT CAUSE
The PIP is a camshaft position sensor/
crankshaft position sensor, and it is
internal to the distributor. It reports to
the PCM and is used to determine the
base timing and duration of the injectors
and coil. The PCM uses other inputs
to determine how to modify the injector
pulse width and coil control for timing
advance. The PCM will output a spark
output (SPOUT) to the ignition control
module to modify the timing of the coil
and this same modification can be seen
in the INJPW. The PIP input looked
unblemished, but the erratically operating
coil/injectors seem to follow the
modified SPOUT signal (not displayed).
I then decided to unplug the SPOUT connector
and prevent the modified signal
from reaching the coil. The coil dwell
began to normalize and operated with a
fixed dwell. However, the symptom still
existed and the injectors continued to
dwell erratically.
The good news is that I proved
the PCM's modified SPOUT signal
Photo courtesy of Brandon Steckler
elimination had the effect I anticipated.
This led me closer to believing the PCM
had an internal processing fault. The
bad news is that a " faulty PCM " is still
a hunch and at this point, I'm not ready
to commit. I still desire more evidence to
condemn the PCM, but how?
With my lab scope still connected to
an injector driver circuit, I recall scrolling
through the channel menu of the scope.
There, I found a selection to place the
channel in a domain to display the measured
INJPW in " milliseconds " over an
elapsed time (very similar to PicoScope's
math channels).
What I'm eluding to is a vehicle that
is at a steady state operation with no
change in load or fuel demand should
display the graphed data in a straight
line (fixed pulse width) as time elapses.
I hypothesize that the subject vehicle
will output an INJPW that is not a
fixed pulse width and will be reflected
accordingly as graphed data that varies
in amplitude as time elapses. Of course,
this is just the hypothesis, but perhaps it
will lead to a more confident diagnosis?
Since both injector control circuits
reported the same activity, I chose only
to monitor one circuit for the experiment.
I started the engine and began to collect
data for 10 seconds across the scope
screen. What I saw gave me great confidence.
Using the vertical cursors to measure
the difference in amplitude, there
38 PTEN DECEMBER 2021 VehicleServicePros.com
A neighbor complained his van would cut out
and then regain consciousness.
was a variation in INJPW of 3.7 milliseconds,
in only two seconds of elapsed time.
If this were truly called for, there would
have to be a significant increase in load.
And with the fuel system in open loop,
I could immediately rule out the feedback
system as the cause. It was time to
shut the door on this case and call for a
replacement PCM.
With the new PCM installed, the
same test was carried out and the graphed
data of INJPW exhibited a steady amplitude
with no change in load. Along
with that, the vehicle ran great, and the
exhaust no longer smelled. The HO2
sensor operation as well as that of the
CAT was restored and my neighbor was
doing backflips!
Sometimes it simply takes a different
perspective to allow you to see
what once seemed not so obvious but
is now clear as day. The functionality
of the tools we have in our possession
is not typically fully utilized.
I urge you as a technician to spend
some time with the tools you currently
have and increase that return
on investment. Being familiar with
your tools of choice could save you
time, make you money, and grow your
knowledge simultaneously.
Brandon Steckler is a technical editor
for Motor Age, a sister publication of PTEN.
http://www.VehicleServicePros.com
December 2021

Table of Contents for the Digital Edition of December 2021
