To capture the images presented, I simply connected the scope
up to the bypass and EST line and
started the engine. Ok, your saying well DUH...... But! That is
important as we will see.
Lets start at the beginning... The event we want to focus in
on is the Bypass going high.
When did Bypass go high (5v)?? When should it? When Dref
indicates an RPM of approximately
200-300 rpms or in other words, engine running.
Code 42 has two parts or two test stages. Center stage to this
diagnostic are Bypass and EST.
During the first stage of the diagnostic, the ECM monitors EST
while Bypass is low. There should
be no hi-lo transitions of the EST. Did we pass this part of the
test? Yep.... We know that EST at
this point should be a square wave with a peak of about 500mv but
even though it is flatlined, we still
pass the first stage of the diagnostic.
After the ECM applies 5 volts to the Bypass line, the second
stage of the Code 42 diagnostic runs.
With 5v applied to the Bypass line, the ECM expects to see hi-lo
transitions of the EST. Does it see
those hi-lo transitions at this point? Nope.... Blammo, DTC 42
sets....
Now we ask ourselves... could the EST line be open or shorted?
At that juncture, you will need to know
just where in the heck I am hooked up to, at ECM, ICM, etc....
But wait... lets let the next image help
us out.
Here we see that EST is neither open or shorted. But it is
sitting at a strange peak voltage... Should
be 5v peaks, not 4v peaks... Going back to the first image, just
where does the EST waveform come
from? It is generated by the ECM based on the Dref signal. Bypass
is applied when Dref pulses
tell the ECM that the engine is running. Anytime the engine is
cranking or running, we should have
Dref pulses.
So now it sounds like we have a valid and logical decision of
the ECM to fail the EST SYSTEM
and we have no EST signal coming out of the ECM when the engine
is first started. Sounds like a
bad ECM to me... Or it could be the ECM circuits are not
operating properly because the ECM
grounds, B+, or Ign wiring are not up to snuff. The circuitry
cannot function correctly if they are not
properly biased.
So I snoop around and find the battery negative cable dangling....
Its not connected to anything.
This thing is cranking (normal speed) using the body ground to
the battery negative. Yikes! So I drove
the truck back to whence it came from.... The line tech who had
just finished the valve job had
inadvertantly left the cable off. I told him to reconnect the
negative cable and holler at me if it was
still setting a 42.
Yep... you guessed it, he brought it back over <g> And
that is when I noticed that the Bypass line
was going to 5v anytime the key was turned on.... Scratch one ECM!
Now as to my comment at
the start; if I had just turned the key on and looked at bypass
then, I would have saved myself some
time. The instant the key was turned on, Bypass was applied
causing the ICM circuitry to switch and
EST was never pulled low. In realitiy we did not pass the first
stage of the diagnostic, it was never run!
Now here is the funny part to all this... this is the same
truck that I had just written about in March's
first article titled "Gravy Tune-up?". The line tech
who worked on it is an excellent tech, it was just
one of those things that happen in the crazy atmosphere we try to
function in... <g>
Two of the guys (in the Forum) both guessed correctly using
the same thought process that I did.
Like me, they focused on why the EST signal was missing. Sure,
that was a fairly easy way to narrow
things down, but darn.... I missed something that I shouldn't
have.
Read the forum to see what that was....