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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.


Martin from British Columbia

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