What Would You Do? No Communication With BCM
Posted to Technical Discussion Forum on 5/29/2012
4 Replies
It's been one of those weeks already. My students usually
follow my instructions and directions in conjunction with
service information, quite well. In many years as a
technician, the only module that I had an issue with during
diagnostics was when the old GM CAMS machine was hooked up.
That is eons ago!
A more interesting aspect of something that went a little
sideways yesterday was also worthy of discussion as follows.
Since we're looking at fuel supply of return and returnless
systems, one of the ways to provide hands-on learning is to
create some system failures that prevent the fuel pump from
functioning. From some simple issues with fuel pump relay
circuits to instill the fact that a relay socket can be a
useful multi-purpose test location, to shorting the Hi speed
CAN bus on the '07 Silverado C truck (current style GMT900).
This is easily accomplished at the rear termination resistor
on the bus, which is a simple plug in 121ohm resistor in a
connector body [2008 Chevrolet Silverado 1500 LT,
ECM/Inputs/Outputs Photo] as opposed to most other
termination resistors that are inside modules.
With a couple of strands of copper wire wound across the
resistor terminals, the desired no crank, no start, no
communication issue was ready to go. Since the truck was at
the end of the lot and the weather looked "iffy" with rain
clouds looming, I swapped the resistor for a non-bugged one
and drove the truck under the canopy at the shop entrance
and reset the bug. Team "A" tackled this diagnosis while two
other teams performed diagnostics on other vehicles. Being
the coach and observer/evaluator, I move from one group to
another leading, prodding and poking, to promote learning
and critical thinking, at the point when floundering or a
loss of forward direction is noted.
Back to team "A", they are usually pretty bright and an
indicator of whether a given activity is reasonable for the
level of learning that has taken place. They quickly
discovered a loss of communications and performed straight
forward bus diagnostics, determining that the High speed CAN
bus lines were physically shorted together. They looked at
the schematic and determined that opening a specific
connector would separate the bus and may be useful in
identifying where along the bus, the short was.
Once the connector was opened, communication to modules
upstream was restored. This is a base model truck with only
a relative handful of modules. A few minutes later, the
installed fault was identified and rectified. I left them to
verify the fix and reinstall the bug for the next team.
However, I was quickly summoned back to the vehicle, where
no communication with the BCM was being shown in other
modules, no crank and no PRNDL. The team retraced every
step, performed test after test and identified the bus
measurements for resistances and voltage etc. All were in
spec.
Their diagnosis identified a failure of the BCM, but I was
not satisfied that just calling the BCM on these results is
the final step. What could we do to go further without
shelling out hundreds of dollars? How about stepping back
and refocusing with a little thinking outside the box for a
few minutes?
These students know full well that swapping modules can be
asking for trouble, so that is not an avenue that they feel
comfortable exploring. Let's assess the situation a little
further. The network has been properly restored, no fuses
have been blown, all connections to the BCM are secured and
their claim is that they were never touched. However, I'm
not nearly as gullible as they might think, since the BCM
was removed from its bracket and left dangling during the
bus diagnosis! After all, if they had not disconnected the
BCM connectors, why was it not still clipped in its
bracket?!
Let's re-scan the system and also focus on the low speed
LAN, since the BCM is the gateway and active on both
protocols. Hmm, its alive and healthy over on the low speed
bus, no DTCs are stored anywhere. So, in summary, there
should be nothing wrong, but only the BCM is not
communicating on the HS, while modules downstream are just
fine.
At this point, I'm inclined to suspect module lock up. So,
just for the heck of it, the students disconnect the
battery, remove the BCM, disconnect all seven connectors and
verify no terminals are damaged. With all reconnected, the
result is the same. At this point, the students are
proclaiming their diagnosis to be accurate, but I'm still
not giving in. We have another truck on the lot, a fully
loaded 4x4 with the same BCM, but we're not supposed to swap
modules are we?. However, what do we have to lose at this
point?
One student proposes removing the module from the running
truck and trying it in the non runner. Let's revise that
approach just a little. If the non runner truck shows no
indication of any other system failures than the BCM
communication issue, what harm is likely to be inflicted by
installing a non-communicating module into the running truck
to verify the failure? With the BCMs ID'd as being the same
P/N, I don't care if the truck starts or the module
programming and setup is different. All I'm interested in is
whether the BCM is able to communicate on the HS LAN bus.
The students are prepared to take some measurements, but my
directive is to see if the engine runs first. It fires right
up with the proclaimed failed BCM!
Now the students want to take the other module and plug it
into the non runner, but I have them reinstall the original
back into the previously bugged truck and it fires up.
Neither vehicle exhibits any DTCs and we have salvaged a BCM
that in some way was simply "hung up".
In summary, I'm no advocate of "swaptronics", but I do like
to exhaust all avenues before making the final call. I
carefully weighed the options, the diagnostic steps that had
been limited to using only a digital multi-meter, no blown
fuses or other obvious component killers.
If you refer to the BCM replacement document 1741198, you
will read precautions regarding module programming and
setup, no start conditions and more, following a BCM
replacement. Do such precautions scare you? I will always
consider such statements, but take them under advisement. My
approach to reactivating the class vehicle, did not put the
donor vehicle network at any risk and in this case, thinking
outside the box, saved the cost of a new box!
My recommendation is to think through unusual problems
beyond the published information and be informed enough to
weigh any risks based on prior testing and results.
There are many who will simply call a BCM failure, replace,
reprogram, set up and move on to the next job quite happily.
When you read posts by some of the current crop of "top
guns" from independent shops or dealers do you realize or
consider that these people are generally "think outside the
box" types who do not simply "roll over" and accept
published information as "gospel"? If you don't understand
their way of thinking, step back and re-read some of their
posts and learn how they come to some conclusions that you
may be afraid to forecast. These individuals are
accomplished performers, full of self-confidence in their
abilities and system knowledge that goes well beyond
published information.
Are you skilled and confident enough in your abilities to
make judgement calls that allow you to accomplish every
diagnosis successfully, or do you consider your diagnostic
skills to be limited or inhibited by the quality of
available service information? Believe me, there have been
times during my career as a technician when I've been lucky
more than skilled, but I was extremely confident that the
outcome in this particular scenario was going to be a good
one.
Regards,
Martin from British Columbia
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