Before getting started on the fix, let me apologize for not being clearer…
This truck would start and run ok, but would set the MIL and be hard to start due to the starter drives
failure to stay engaged to the flywheel.
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It is always wise to look for any relationships between all the concerns noted on the repair order. Notice here that there is a starter problem as well as the MIL being on. The two DTCs set, have to do with the CKP and CMP sensors performance. So how does the PCM monitor their performance? |
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The Ignition system used on this truck is a DIS system.
The crankshaft has a reluctor wheel on it consisting of 6 notches spaced
every 60° with a 7th notch spaced 10° away from one of the other
notches. This 7th notch allows the PCM to determine the crankshaft
position. The CKP sensor is a hall-effect switch, which produces a 7x square
wave that goes directly to the
PCM. The CMP sensor (a hall-effect switch) is also mounted in the block near
the ICM and its output also goes directly
to the PCM. |
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So, how does the P0341 (CMP sensor performance) DTC set? How does the PCM monitor its performance?
Whenever the PCM receives an intermittent signal from the CMP, the CMP Resync Counter will increment. If
too many resyncs are found within a specific amount of time, the DTC will set. That is how the book explains it.
Doesn’t really explain it at all, does it?… The PCM is not looking at the time frame between CMP pulses and
trying to second guess when a failure occurred, it is
monitoring CMP pulses against the CKP sensor
pulses.
Now we also had a P0335,
Crankshaft Position Sensor Circuit error. Knowing a little now about how it
works,
we should have an idea of how the
PCM flags a failure. Should we not have 1 sync pulse per 6 normal pulses?
We also know from the data stream,
that the PCM is in charge of ignition coil ‘syncing’ because of the CKP Active
Counters parameter. Please not
that this does not always mean the CKP signal goes directly to the PCM.
Now lets put this all together and
think about what could happen or is likely to happen if the starter drive keeps
letting go while we are attempting
to start the engine. Get the picture yet? No? Ok, then lets go back and review
the Freeze Frame and Failure
Record data for the two DTCs that set.
Remember, all of them had the same data.
So what does that mean? How about
they set at the same time? Now review the data
I flagged an hour for diagnostic
time on the RO and then cleared the DTCs and I/M flags. Next, I pulled the
truck into my stall to replace the
starter. It took a few tries due to the starter drive flaking out on me, but I
finally got it in my stall and
replaced the starter. Would you believe the MIL came on again? If you haven’t
guessed by now, all that was wrong
with the truck was that the starter needed to be replaced. What really
ticked me off is that the writer
didn’t think I should be paid my one hour diagnostic time for the illuminated
MIL concern. Since he wanted the
truck ready for the customer sometime this century, he lost the tug of war…
This was a quick and easy job. My
first thought when seeing the codes was, “no way!”. Once I got over that… <g>
it was a matter of looking at the
conditions that the codes set and when (all at one time) they set. Like every
other
technician out there, I make
mistakes and misdiagnosis from time to time; but if you take a step back and
look
at the big picture before you go
for those trouble charts and enable criteria… you can save yourself some grief.
And just think, you’ll have more
energy to choke the life out of that brain dead service writer who doesn’t
think your time is worth anything…
<g>
Scope shots of the CKP and CMP
signals will be forthcoming…