Topic: BCM .......... well, I do NOT need one

[snaponbob]

Well, I have learned not to trust the 3157 bulbs we have. Why? A bad bulb caused all the angst, and after about 3 hours of very in depth diagnostics, it turns out that one of the rear turn/brake bulbs was shorting internally. (Don't assume I am a drooling moron  that can't figure out a bad bulb, because a) it checked good, and b) I was digging for the complete truth.) This is not the first time, either. About 2.5 years ago the same thing happened. The car went to the dealership, and after seeing no output from the BCM, it was replaced. A couple days later the r.r brake/turn was dead again, so back it went. They installed ANOTHER BCM for not output from pin A12 (the output for that circuit) and bi-passed the wire from the BCM to the rear corner. A week later, same deal, so round three. Knowing the tech, I suggested that the only thing left was the bulbs. "No way". So, I shrugged and left in a loaner. Two hours later, new bulbs and problem solved.

So, what was causing the BCM failures? They were not failing. I discovered that when I disconnected the main harness plug from the BCM, it reset itself when the plug was reinstalled. Apparently, the BCM has a failsafe against shorts in the lights (at least on THAT circuit). New bulb from the parts store, reset the BCM, and all is good again. How 'bout DEM apples?

[DeepBlueGXP]

wild

[Sly Bob]

Wow, that was a heck of stumper! Good information though just the same. Thanks for the heads up.

[Gentleman Jack]

that is bizare! 

[snaponbob]

There is nothing average about Kappas.

[2kwk4u]

Glad you finally got it sorted out...  and that you don't need an expensive BCM.  It's nice to know that function is built INTO the BCM to save the type of costly repair it would have caused were it not (i.e. that bad bulb WOULD HAVE cooked your BCM if it didn't have the circuit to cut power in the event of a short).

[snaponbob]

The down side of this is that a bulb with an internal problem (vs. a broken filament) is hard to diagnose. In this case, a "professional" mechanic missed it three times. Had it not been warranty there would have been a bill for $350 for a BCM plus programming at $90 and all the labor, plus the second visit costing hundreds in labor to run the shunt of the wiring harness. All for naught. Seven years of troubleshooting USAF air traffic radars plus a lifetime of twiddling with motorcycles and cars has benefits. Add to that 19 years visiting mechanics as a Snap On dealer has convinced me that my best shot is at my house !!!! Now, if I could just be better when I am wearing a helmet. 

[scanner]

Snaponbob,  I'm very glad you have this issue resolved and explained the on-board protection. There is a lot of that now so the ECU doesn't not become an expensive fuse.  The BCM stores DTCs for a short condition - did you see that? Also switching the terminals left-right at a central point like the BCM connector is always a good diagnostic for wiring that can't been seen.   

So, if you will be doing most of you future repairs at home invest in a good scantool like Tech2. You'll need the scantool, 32MB flashcard with software and the CANdi module.  Local clubs might want to pool funds you can expect to pay at least $2500 used.

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[snaponbob]

Snaponbob,  I'm very glad you have this issue resolved and explained the on-board protection. There is a lot of that now so the ECU doesn't not become an expensive fuse.  The BCM stores DTCs for a short condition - did you see that? Also switching the terminals left-right at a central point like the BCM connector is always a good diagnostic for wiring that can't been seen.   

So, if you will be doing most of you future repairs at home invest in a good scantool like Tech2. You'll need the scantool, 32MB flashcard with software and the CANdi module.  Local clubs might want to pool funds you can expect to pay at least $2500 used.

Scanner

Not knowing what was actually wrong, I avoided swapping leads until I had exhausted other methods. I kept Snapon scanner, but have not updated it as it is fresh enough for my other vehicles and have HPT. Had I checked for codes, and if there was present for a short, I would still have to determined where it was. As it is, old school testing answered the questions. As for a Tech2, what you described is multiple thousands, and updates are not cheap. Remember, been there done that.  As it is, I can borrow newer equipment if needed.

[Critterman]

Damn Bob you sure like to have unusual problems, but glad you got it solved.

[scanner]

I wonder what the "professional" used?  Why spend all that time when a tool will tell you the BCM already determined which circuit is shorted?  Oh yeah, it could tell you a lot more and check more vehicles too.  This is part of why the dealer technician labor is "expensive"?  If this is where your "professional" works he or she needs training.

[Uranium-238]

Dunno about our buddy SOB, but at my hourly rate I could to a ton of work to my car before I'd reach $2.5k worth of my time.

[snaponbob]

I wonder what the "professional" used?  Why spend all that time when a tool will tell you the BCM already determined which circuit is shorted?  Oh yeah, it could tell you a lot more and check more vehicles too.  This is part of why the dealer technician labor is "expensive"?  If this is where your "professional" works he or she needs training.

Before this turns left real bad, I will explain that a Tech2 may indicate a short in a circuit, but not WHERE the short is, or WHAT the short is. The same holds true for almost any code no matter which module on the car is being scanned. A code is an indication that something is wrong in a system, and it is up to a tech to figure it out. If there is a downstream O2 sensor code, it does NOT mean that the downstream O2 sensor needs replacement, only that there is a problem in that circuit.

As far as spending $2500 for a diagnostic tool, there is no economic sense to do so for the average at home guy spinning wrenches. An inexpensive scan tool, the internet, and some common sense can fix a lot of stuff. An expensive tool does not make a tech. A tech uses a tool to do a better job. I have sold to guys with a sleeve full of ASE certs, and could barely balance a tire. Theory is one thing, application is another. Tools don't fix ANYTHING, people do.

To cap this off, GM says one needs a Tech2 to energize the circuit I was working on. Guess what? The four way flashers were just fine.

[snaponbob]

Damn Bob you sure like to have unusual problems, but glad you got it solved.

Hey, problems are just places to install solutions.

[scanner]

I agree with all of above.  Just wondering why the professional thought the BCM was bad again when it should have indicated a short.  It's beginning to sound like the professional didn't have a scantool that could read the BCM. 
Anyway the advantage of the on-board diagnostic system is that they can often tell what circuit has a problem.  Prior to DTCs electrical diagnostics were done with symptoms and tech knowledge.  A professional these days will use symptoms, on-board systems, tools and personal technical knowledge. The tuners are getting good at that now too, they can use all of these and keep the on-boards happy - that's a powerful skill set I'm happy to see.

Happy Mother's day to all the moms!     

[snaponbob]

I agree with all of above.  Just wondering why the professional thought the BCM was bad again when it should have indicated a short.  It's beginning to sound like the professional didn't have a scantool that could read the BCM. 
Anyway the advantage of the on-board diagnostic system is that they can often tell what circuit has a problem.  Prior to DTCs electrical diagnostics were done with symptoms and tech knowledge.  A professional these days will use symptoms, on-board systems, tools and personal technical knowledge. The tuners are getting good at that now too, they can use all of these and keep the on-boards happy - that's a powerful skill set I'm happy to see.

Happy Mother's day to all the moms!     

JUst as all doctors are not equal, neither are those in other professions. The assumption that there was a code in the BCM can not be verified. Frankly, I never heard that a BCM could be reset in the way mine did by removing and installing the main plug. I started to see a pattern, and then verified it.

In all the years I sold tools, I saw that the biggest weakness of dealership techs (as a group) was that too many used the parts counter as a diagnostic tool (swapping parts until the car came right) and the manufacturers made it worse by approving warranty repairs one step at a time. On the other hand, independent shops didn't have the luxury of endless parts, and the best shops had the best diagnosticians. For me, 7 years of of repairing airport radar equipment (in the USAF) taught me to assume NOTHING and go step by step. Even then, diag tools such as o'scopes could only POINT one towards the problem, and testing zero'ed one into the culprit.

[scanner]

Assumption? Cannot be verified? 

The BCM has to be interrogated with a scan tool to verify the DTC. The condition described is in the BCM on-board diagnostic design. That is not an  assumption, just a declaration of a match between the fault condition and the design. This was offered here as helpful before the root cause was given.   

The assertion that it cannot be verified is not correct. Using the right tool at the right time will determine the result of the design - most likely a DTC with FTB indicating the failure mode, in this case a short.   From the symptoms first presented one could not know if the fault is a short, an open or a BCM output driver. The BCM has DTCs for each vehicle corner and an FTB to state the failure type, open or short. If it was not tested that is okay, but it does not mean it cannot be verified.  Those are facts.

Now my opinion. The systems engineers have done a great job designing failsoft systems that minimize impacts of faults and provide professional technicians with more diagnostic information than in the past decade or before.  In the past a condition like this could pop a fuse and take out all the signal lamps and leave the technician with no indication what branch of the signals circuits caused the short.  Things really are better, the engineers made it so, technicians should make use of the system advantages.  Owners already paid for it and the performance is good.     

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[snaponbob]

No doubt that the failsafe did its job. As for any assumptions, the last occurrence was over two years ago, the Saturn store is gone, and I don't have a Tech2 nor will I be buying one. Basic troubleshooting resolved the issue, the cost was a couple hours of my time, and $3 for a pack of two bulbs. Now, if YOU want me to "verify" anything, send me your credit card info and I will take it to a GM dealership. As it is, the car is properly repaired, this thread was started to share what was learned about the BCM so that other owners can benefit from NOT going through a lot of hassle, and the assertion that I needed a Tech2 is simply wrong. Proof? Re-read this paragraph. Additionally, tools don't make the mechanic, BRAINS do.

Conclusions? The 3057 style bulbs suck !!!!!!!!!!!!!!

[Critterman]

Mechanics are few and far between now days, but you can find technicians near and far.  Even more common are those who throw parts at a car until it is fixed.

If you find a mechanic, treat him like your favorite brother, remember him at Christmas and his birthday.

[Gentleman Jack]

Mechanics are few and far between now days, but you can find technicians near and far.  Even more common are those who throw parts at a car until it is fixed.

If you find a mechanic, treat him like your favorite brother, remember him at Christmas and his birthday.

Critterman,
My Birthday is in April, and I will give you my address closer to Christmas.  JimmyBaja's Birthday is sometime between April and March, don't forget him...

GJ

[sunnygator]

Mechanics are few and far between now days

My grandfather was a "true mechanic" and yes, I've never met anyone since that had his knack for anything with spark plugs.  He used to fix Model As and Model Ts -  the test drive was making up a "steep" hill in Brooksville, FL (insert joke here about "hills" in FL).  He could simply fix just about anything, and create what was needed.  For example, his well pump and irrigation system for his orange grove he created himself (he was also a part-time farmer) and it was powered by an old Ford Windsor V-8. 

Dandy - as he was known to his grandkids - was one special man.  I miss him.

sunnygator

[scanner]

Yes, to read a DTC a scantool is required on all Kappas. No need to re-read anything on that.
I'd suggest re-reading the posts on May 3.

Also a scantool is not always required to fix a car.  It can be helpful and sometimes it's required by design, e.g. crankshaft variation learn.  If I go back to the original thread & post, (not this one) it seemed that the BCM was going to be changed - many thanks for continuing the testing to find the real cause. It surely couldn't have my suggestion to look for a short on May 3, just 1.5 hrs after the posting that it must be the BCM?

[/quote]

No brake light, no blinker, and no 4-way. There are two circuits;  the low watt circuit is for running lights and all other high watt outputs are the other circuit. I have running lights.

[I have just returned from gathering the G/M p/n, and checked pins A9 (left brake/turn signal) and A12 (right brake/turn signal). These are also the 4 way outputs. A9 energizes, A12 does not. Looking at the GM wiring diagram, it just must be the BCM.

If this circuit has effected past BCMs I wonder if there is short to ground at times.  The BCM has the ability to store DTCs for this, see B3951-02, you'll need a good scantool to see this...  A short to ground could be a current condition too.  That is, if the circuit is shorted low and you back probe for voltage there will not be any or very low, causing it to look like no BCM output.  If you have the terminal tools swap left & right and retest.  If the BCM is toast find out why, they don't just die, something likey killed that channel unless there is some indication of a common problem with the BCM.
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I'm sorry that I failed to state that the BCM will remember intermittent faults as long as 49 key cycles ago.  So if the problem is current the BCM states the current DTC, and a History DTC.  When the fault is not present the DTC is only History.

Many thanks for the determination and skill to find the real cause.  I hope all readers find the dialog helpful - that is my goal too.  I will try help with anything electrical, even if this means challenging dialog. No credit card number required. Oh well if anyone rathers that I only lurk as a newby just let me know.

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[Critterman]

I have to have exact addresses and dates, you can publish them here no one will notice

[snaponbob]

Lurking is a matter of choice. Lecturing down to forum members is why a lot of us came over here. I have had all sorts of suggestions from this and two other forums when I first started looking for a BCM. At the risk of sounding rude, I was looking for a BCM, and not much else. WHY was I looking for a BCM? Because, IF there was a need for one I could choose a used one for cheap or a new one. If I felt the need for a scan tool, I know where to find one from a $200 paper weight, to a $10K hand held, and everything in between. I am not a troglodyte with a roll of duck tape and a paper clip. When I step up to my toolbox, there is nothing in there that I don't know how to use - and there is plenty in there. Correction - I bought a paint gun and haven't learned to use it yet ---------  on my 70 GT6. On THAT I just need lots of extra electrons because Lucas systems never had enough. 

[Sly Bob]

Don't just be lurking Scanner, this exchange is helpful for everyone here in the community. The resources here that you and everyone else here can offer are invaluable.

We aren't all class "A" mechanics but each of us has something to offer here. Those that have chosen to just lurk and not get involved are losing out.

Thanks to everyone here on KP. Had you all been lurkers, I would be like the Solstice owner at work that runs his tire pressure a few pounds under the maximum on the side wall and torques his wheel nuts to 135 foot pounds.

He did anyway until a few days ago when I spoke to him and set him straight.