NEED HELP -- Bad BCM? 2010 Buick Enclave Intermittent No Com, No Crank, No Start

Need some confirmation or potential re-direction here on a customer's intermittent issue. Sorry in advance for the load of info.

Vehicle runs good when it does run. This is my third time checking it.
-First time could not reproduce issue.

-Second time battery was at 6V by the time I got to it next day (parasitic drain?), but luckily was still not cranking after hooking up jump box. CAN signals were very wacky looking, inconsistent and noisy. Scan tool was unable to detect or communicate with any high speed modules (ECM, TCM, BCM, FPCM, EBCM, VCI, Rear Diff Control Module, Headlamp Module); but, low speed modules showed up (SRS, Radio, HVAC, etc.).
+Disconnected battery and did some checks for 6 hours. Network resistance 60 ohms. Checked various module and harness connectors for corrosion or pin fitment issues. Found nothing, but after reconnecting battery, surprisingly car cranked and ran normal, CAN communication restored with good waveforms.
+Decided to try and simulate issue by disconnecting various modules. Only module that produced a similar issue when disconnected was BCM. No communication with any high speed modules, no crank no start. Could not condemn anything definitively, so cleaned battery terminals and told customer to call if it happens again.

-Third time (today), car has been driving fine for two weeks, then suddenly does it again after customer gets in to start car after a trip to store.
+Same issue, no comm, weird CAN signals (but a little different from last time.. image attached for this time).

I'm ready to call a BCM but not too confident. I just know the network and wiring checks out, and that the BCM is a sort of gateway module on this GMLAN network setup. The random parasitic drain I noticed and the fact the CD player wigged out on me once today randomly also make me think BCM. The truck apparently also has an aftermarket alarm installed, but it was completely unplugged when I found it. I am very suspicious of it still. Plan to cut it out if I replace BCM (customer bought from used car dealer, never knew it was even under the dash).


Thanks in advance,
Ken

I am always hesitant to diagnose a bad computer/module in most circumstances. Like you, I wouldn't want to make a mistake and diagnose a bad BCM when it could be another issue. One thing I have learned over the years is that GM has had an extremely bad reputation when it comes to ground issues. The first thing that I would look at are the grounds. Personally, I use Identifix for electrical diagrams and other troubleshooting help. One other thing I have found is that GM also runs many things through the BCM, such as the radio, and if it malfunctions it can give you a false indication that you have a bad BCM. I once had an S10 truck that had a bad radio which caused a no start issue. I know that companies like Identifix seem expensive, but they are their weight in gold when troubleshooting issues like you are experiencing, especially when they can save you time, which is money.

Alades -- thanks for the reply. You are correct about Identifix. I have been dragging my feet on investing in AllData or a similar service. Being mobile and independent, it is times like these that I need access to exclusive information.

As for the vehicle, I ended up purchasing information from Mitchell. I checked both grounds that run to the BCM. One was pristine, the other, which is the main battery ground, had a little corrosion and green crusties, but not so much that I would expect it to selectively knock out the high CAN bus (while leaving everything else on the vehicle functional). I cleaned it nonetheless. Power feeds were also good. Additionally, the BCM was replaced with a plug and play unit, and the aftermarket security system was removed.

Unfortunately, by the time I got back to the vehicle to do these things, the intermittent condition had corrected itself and it was cranking and communicating again. So I can not say whether we are in the clear until I hear (or don't, rather) from the customer again. I will report back here if there are any more developments.


Ken

An update for anyone following --

So after replacing the BCM, the vehicle no longer experiences the "CAN knockout" issue anymore. It has been over 30 days.

A few things to note:

1. After replacing BCM, customer did have issues with reverse cam no longer working, many short to ground codes in BCM, ABS light coming on randomly and sometimes odd shifting performance. I suggested we get a replacement unit sent out (what can make you feel crazier than calling a module TWICE?!?!). The second replacement and programming the brake pedal position sensor to the new BCM did the trick for the above mentioned issues.

2. I will say in retrospect it is possible that replacing the BCM was not the definitive fix. The B+ terminal was broken, loose enough to lift off by hand and unable to be tightened. However, I could not find a replacement cable assembly and the vehicle would run for days before the intermittent showed up, so I didn't think much of it (rookie overlook). Manufacturer flow chart pointed at the BCM as well, so I went ahead and replaced BCM first. During the second swap about 2 weeks later, I happened to have some battery shims on me and used those to bridge the gap. That loose connection could have played a factor. In fact, I suspect it did in a way that was complicated to prove, which could have led to false flagging of the BCM.

3. Unfortunately, the timing chain suddenly started to go now almost 2 months after the BCM fiasco. A bad break indeed for customer, but just another confirmation the GM is pure crap.

I hope this thread was informative,
Ken