december2021 - 56

ANALYSIS
(profile ignition pickup-
described shortly)
* GREEN = Bank 1 Injector driver
at PCM
* BLUE = Bank 2 Injector driver
at PCM
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 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 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.
56 Fleet Maintenance | December 2021
december2021

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