I'm talking about having them fixed by ONLY replacing the BCM and nothing else. If yes, what specific problems were you having?

The reason that I ask is that since late last year I've been chasing multiple issues in my race car and everything I've tried so far hasn't worked (new battery, checking and cleaning the grounds, looking for broken or loose wiring etc) and replacing the BCM is about the only thing I haven't tried yet. My initial hesitation with doing that is because I talked to one guy who seemed extremely knowledgeable on the subject and he said that actual BCM failures are extremely rare. I know that's just one guy but a couple internet searches seemed to mirror that thought.

In addition to all the ongoing problems I've been having all of this happened last night at the track in the span of about 10 minutes. I didn't realize that so much could happen in such a short period of time:

• car lost all power just as I was getting ready to race
• got out of the car to see if the battery connections were tight and the power came back on it's own
• coming back around and heading to my trailer the car went into limp mode along with service fuel system, service ABS and service vehicle soon messages on the DIC all at once
• also threw all the codes showing in the picture
• I wasn't able to connect to the OBD2 port initially but when I finally did, I was only able to clear the codes after several tries as the first few kept on saying there were none
• my radio now says "Locked" on the display and there's a red light on the dash
• 
  

 

Looks like a communication issue with the car...99.9999% of dealerships or indie shops wouldn't know where to start...if you can't communicate with the car that's is a symptom...ANY module if shorting the CAN bus to power or ground or even the Class 2 serial bus can cause these "U" codes as well as a broken wire on the bus (physical layer)...your problem seems intermittent but when it is occurring a DMM would be needed to diagnose this...a labscope sometimes is required when it's intermittent...you need to check pins 6 and 14 (CAN) and pin 2 (Class 2) with the DMM....below is a good CAN bus and Class 2 waveform on my C6 with my scope and the 60 ohm and 2.5 bias voltages on pins 6 and 14...green waveform is the 0-7 volt Class 2 bus and the blue and red are CAN high and low...CAN high (pin 6) going from 2.5 to 3.5 volts and CAN low (pin 14) going from 2.5 down to 1.5 volts...CAN high and low mirror each other...the CAN bus communicates with the low speed bus through the BCM (gateway)...it is ALWAYS a good idea to let us know what year is the car...as far as a diagnosis I'd be looking at the HVAC, IPC, TCM (pending DTC) and Airbag module since they are causing some of these U codes...you can try disconnecting each of these modules until possibly the symptoms go away...if a power or ground gets on the bus whether high or low speed you'll have all kinds of issues...an auto electric shop can usually diagnose this for you...the others can't !!...doubt this is a BCM issue.

 

All of your problem modules are on the low speed Class 2 bus except for the TCM...the Information Bus wiring diagram is from my 2008...BTW, if you were to disconnect the TCM you would have no comms with all modules downstream of it...the EBCM and ECM...without the ECM "online" the car will not start !!...you don't learn this stuff by "turning wrenches"...it takes education and I'm still taking classes on this stuff to this day...you won't know everything !!

 

Thanks for responding and it's a 2007 1LT.

I already bought an oscilloscope and a breakout box is next on my list as I don't believe there are any decent electrical shops close by. Plus, I'd really like to fix this on my own. I have my own MDI2 and a GM tech friend has a Tech2.

I noticed your OTC is currently $361 on Amazon. Any idea if one like this (or something similar) would also do the job? At the end of the day I know I'll need to spend whatever it takes to fix it, I just want to spend my money wisely.

 

A CAN breakout box isn't really necessary to use...you just have to probe the terminals on the DLC but getting under there on your back is sometimes a pain...the one you pictured is good enough and the $25.99 one on Amazon will work too...what scope are you using ??...what accessories do you have for it ???...I have a 4 channel PicoScope (4425A) and that one is all I'll need...the CAN bus information "packets" are VERY fast and you need a scope that can measure them in microseconds...millionth of a second...now if you need to bypass the TCM but still have the ECM communicate with other modules you just need to jumper the 2 CAN wires going into and out of the TCM connector...now the information bus is "fault tolerant" as you can put 12 volts or a ground on terminals 2,6 and 14 through a 12 volt test light...you can see how it affects your car.

 

I bought the one shown below but I'm guessing it may not be fast enough. Is there a reasonably priced one you'd recommend if the one I bought won't really help?

I am sorely aware of how hard it is to get up under the dash as I've got a Plan B in place for the next race. I've jumpered some of the connections between SP205 and SP208 to take several of the systems out of the loop to try and find out which one is bringing all the others down with it by removing the combs from each connector. I can't remember everything that will get removed but at a minimum it disabled the instrument cluster, door locks and windows when I tested it. If you think it'd be useful information let me know and I'll check to see exactly which circuits I jumpered.




I'm admittedly out of my element here as I'm more of a wrench turner but I'm always up for a challenge. Well, that's not quite true but I really don't have a choice in the matter at this point.

Update: found the crude drawing I made. SP205 is on the left.

 

Is having an oscilloscope a nice to have or need to have for this kind of troubleshooting?

 

Best to use if intermittent !!...if hard fault a DMM will suffice.

 

I've had my Pico for almost 15 years so I don't know which may be good...an automotive scope is your best bet...unfortunately those 99.9999% of dealership and indie mechanics don't use one and that is why they just replace parts or they have no diagnostic training and yes you don't learn diagnostics "wrenching"...a scope is absolutely required on diagnosing some issues...below is the low speed Class 2 bus for your 2007...if you are using the DMM with a DMM on pin 2 you should see a fluctuating voltage of 0.2 to 0.8 volts with the key ON...if you see 12 volts short to power...near 0 volts short to ground.

 

Oh it's very intermittent which is part of the challenge.

A few weeks back I thought it was fixed after cleaning several grounds, so I took it to a grocery store parking lot since the car isn't street legal. I drove it around for a solid 30 minutes including some hard simulated launches and it was flawless. While getting ready to make my first pass at the next race it went haywire with a bunch of different codes while the car was literally just sitting there idling.

Not sure if it's relevant but I just realized that the problem seems to always happen right after starting it. With Wednesday being the one exception, if I can get past the first few minutes it seems to run okay.

I'll check Amazon to see if I can find an oscilloscope with quicker response time that isn't crazy expensive.

 

No "cleaning the grounds" isn't going to fix this...the Pico 2000 series would work for you...you can download their software for free and practice using the scope...you can pick one up on EBay.

 

Just found this one new on Amazon which looks like it should work. Thanks.

 

Pico is the best !!...like I mentioned go to their website and download the software and get used to it.

 

Will do, thanks.

 

 

So when all is said and done, armed with a DMM, a back-probe kit, a breakout box, an oscilloscope, enough perseverance and time spent, should I (in theory at least) eventually be able to fix this on my own? Or is this one of those "it's above your pay grade" situations where someone with advanced knowledge and experience would be required?

As I said I'm not an electronics expert but believe I could at least garner enough knowledge to eventually figure it out.

Or...would you be interested in possibly helping me out remotely via video conferencing, obviously being compensated for your time?

 

If it's not the CAN wiring (physical layer) your best bet is to disconnect each of those modules on the class 2 bus one at a time and see if your problem resolves itself...if no good you can jumper the CAN wires at the TCM (I believe it is a 16 pin connector) and see what happens with the TCM not in the loop...I'm just wondering if the car will start because the PNP switch is bypassed...it is one of the inputs needed for the car to start.

 

It might at least qualify as a person of interest.

While letting the car idle this morning I noticed the Service Active Handling and Service Traction messages came on shortly after starting it. I then started the timer on my phone and it did it seven more times in a 20 minute span. The potential relevance of this is that I had my HPT interface connected at the time and several years ago I had a flaky OBDII extension cord which would do something similar.

Later in the afternoon I let the car idle with just the breakout box attached. For a full twenty minutes no warning messages appeared but when I connected the HPT again it happened twice in about 8 minutes. I was going to let it go longer but figured I annoyed my neighbors enough by that point.

Think there's any connection there?

The oscilloscope arrived today so the plan for tomorrow is to get it set up and see if there's any correlation between when the warning messages appear and what the scope shows me.

I also checked the resistance across pins 6 and 14. It was 62 ohms which isn't quite the expected 60 but I don't think that's an area of concern.

 

May be a connection...pins 4 and 5 at the DLC are chassis and signal grounds...hook your test light to B+ and the test light should illuminate probing both of those grounds...you can use pin 16 of the DLC as a power source...grounds are G104 and G201...62 ohms is fine !!

 

Don't have a test light but the DMM showed 13.1V when jumpering 16-4 and 16-5.