I have a 2004 Z06 that developed an electrical issue which appears to be BCM related. I spent countless hours reading every pertinent thread and tried everything suggested but came up short. I then gave it to an “expert C5” shop whom $2k later claimed to have fixed it. On the drive home, the issues were back. Disgusted and worn out I parked it... that was 2 years ago! I have since decided to give it one last go and hope with the collective knowledge of this forum we can figure it out.

The symptoms are so substantial I thought the source of the problems would be easy to trace. PCM, TCS, BCM, SDM, HVAC, LDCM, RDCM all NO COMM. IPC shows no codes. Reduced engine power, service engine soon, low fuel flashes. Oil pressure and fuel read zero, temp is max, but voltage works as does rpm. Weird thing is it starts! Sometimes it will start and die. Disconnecting and then reconnecting the battery usually will make it start again.

I have checked all fuses to PCM and BCM, unplugged the star connector to eliminate most other control modules, and checked the ground pack for corrosion. The ground wire that usually goes right above the starter I have relocated to the frame rail ground beside it but tried moving it back and it made no difference. I replaced and resynced the BCM and it didn’t fix the problem. I have checked the door accordion connections which were fine, but also bypassed them entirely with the star connector.

I am at a loss as to what to try next but since there is no comm across the board I figured it was a power or ground issue to the BCM. I am confused by how it starts when both the PCM and BCM have NO COMM.

Any suggestions?

Colby

 

Moved to C5 Tech.

 

With what your describing...all "NO COMMS" to the modules, a start and stall condition, SVS, REP, LOW FUEL, along with the oil pressure and fuel at zero and temp "maxed out" are ALL classic examples of a serial data bus problem...either 12 volts or a ground on the serial data line instead of the normal 0-7 volt square wave (pictured above) that you will see with a scope. Now if any module loses a ground for instance the power from that module can sometimes back feed through the serial data line (it takes the path of least resistance) looking for ground and there you go the exact symptoms you have...now you can use a voltmeter to check the serial data line but since a voltmeter only "averages" the voltage you might see around 0-1 volt that keeps dancing around on a good data line...(see my picture above of my good data line using a voltmeter... only 0.33 volts) if you have a voltmeter that has a fast min-max such as a Fluke 87 you may catch the voltage up at 7 volts and down near ground...a scope is best !!...you can hook up your voltmeter to pin 2 of the DLC (where your scan tool connects to) and a good ground...you can also use either pins 4 and 5 of the DLC...they are the grounds....when you have those issues if the data line reads either 12 volts constant or close to ground the data line is compromised !!.....if the issue is on a module when you remove the "comb" from the STAR connector (2 each) you normally can restore the data line and the car will start...you basically isolate the modules...remove the comb off the star connector with the 4 or 5 wires...if no good now remove the other comb with the 9 wires...YOU HAVE TO USE A JUMPER WIRE FROM THE BCM TO THE PCM to allow the car to start IF it is a module other than PCM and BCM....if still a "no start" you HAVE a problem with the BCM or PCM....now you have to check EVERY power and ground on those 2 modules... I will be doing a YouTube video on this issue soon and I took a quick video of what you'll see on the instrument panel...at the end look at the gauges (I did have a FULL tank of gas)...just like you had !!...I shorted the data line to power (it won't do any damage) and you can see the results. I attached the video below !!...another time I did this it also showed the LOW FUEL on the DIC...not this time though !!...oh, BTW what did you "EXPERT" C5 shop exactly do ??...I'm curious !!
EDIT: if I can remember my car DID start once when I either shorted the data line to power or to ground...it is usually a start and stall !!

 

EXCELLENT information! This definitely gives me a place to start. So tell me if this sounds like an adequate plan. I will remove both star connectors and jumper the PCM to BCM. If they aren't the problem then I should have COMM to PCM and BCM, but nothing else. I should also get my gauge functions back. If I still have NO COMM then the problem is either the PCM or BCM, correct? Assuming this is the case, I need to check my power and grounds to each, right? Do I also need to check voltages on the data lines? Can anyone help me identify which wires these are?

The shop wasn't very concise as to what they did. They said that troubleshooting electrical gremlins is incredibly time consuming and they eventually found a corroded ground under the dash from water intrusion. Needless to say I wouldn't recommend them but also don't want to flame anyone so I have left their name out of this thread.

This car once had a Stage I TTI twin turbo kit on it, which is why I had relocated the ground above the starter to the frame rail (to keep it from melting). This kit had a lot of hot stuff around a lot of wiring. I have since switched to a supercharger setup. I have looked and not found any melted wires, but suspected this may have been the source of my problems.


Colby

 

The first thing I'd do is check voltage from Pin 2 to either a good ground or Pins 4 or 5 of the DLC with the key ON and you are seeing stuff like in the video...you'll have to lay on your back under the steering wheel...see if it reads 12 volts...or something below a volt like in my pic on a good data line... or close to ground...if 12 volts or close to ground go got a problem !!...Are you using a scan tool that can see ALL the modules ??...if you pull off both combs you should be able to communicate with the modules that are not BAD !!...the one you can't communicate is the bad module...see if you can comm with the BCM and PCM...you only need to jumper the BCM and PCM to see if the car will start...it will not start because with that comb removed the PCM and BCM can't talk...if they can't talk to one another the car won't start !!...on my '01 the PCM and BCM data lines are light green and dark green wires. Or instead of going to the DLC you can remove BOTH combs are see what your voltmeter reads on each pin...it SHOULD NOT be 12 volts OR ground....should be dancing around .3 to .5 volts like mine key ON. YES, if the scan tool can't see the BCM or PCM if data line is OK you have to check power and grounds (no more than .3v cranking or .1v running when checking grounds)...if PCM or BCM isn't powered up it won't communicate but with your symptoms you have problems on the data line I believe.

So in simple terms:
1) Yes, remove BOTH star connector combs and see if you can comm with all the modules...if BCM and PCM are good you should be able to see them on your scan tool or check the light green and dark green wires (PCM/BCM) for either 12 volts or ground (bad data line) with DVOM if no scan tool...if car starts jumpering those two modules BOTH PCM and BCM are good...YOU ONLY JUMPER TO ALLOW THE CAR TO START !!
2) WATCH THIS VIDEO !!...the seat control module was shorted to power under the seat !!....this is a kind of common problem...with clogged drains "the udders" water from at the base of the windshield can find its way into the car and into the seat module or even into the BCM on that side !!

 

MAJOR DEVELOPMENT!!! Over lunch I tried replicating what you did in the video. I thought I had already tried removing the Star 2 connector jumper in the past, but once I did I had my COMM back! Gauges work, no REP. NO COMM only on TCS (which confuses me as its not on that connector pack. I will check the fuse tonight.), LDCM, RDCM and SCM. I don't have a scan tool or fancy meter, so I'm using a Harbor Freight special. I checked the voltage and resistance on each pin. .3V on all but Pin A which was 1.3v. All were infinite resistance when measured to ground so I don't think any are shorted to ground. FWIW, I don't have my stock seats anymore; they were replaced with racing seats. So that harness may be pinched somewhere and causing a short? Can I jumper each Pin one at a time to Pin A and see if it 1) restores COMM to that module or 2) crashes the system and gives me back my NO COMM to isolate which module is giving me the problem?

I am so psyched that I have almost narrowed down the problem, but am now beating myself up for parking it for two years instead of just asking the forum for help!

Colby

 

OK, so if you removed the star connector #2 (one with 4 wires) and the comms returned you're almost there !!...will the car start now ??...on that star connector Pin A which on my car is the data line (purple wire)...is that where you were reading 1.3 volts ??...you may have caught the voltage at a higher value...what are the other voltage values on that connector other than Pin A ??...check the brown/white wire...that is the SCM....EBCM is on the other star connector. Yes, you can add in one module at a time to see if you can get that COMM back and communicate and YES you may have pinched a wire somewhere under the seat !!...it's happened before !!...did your issue first appear after you changed the seats ??....the no comm on the EBCM will have to be addressed later if no comms...it may be only a power or ground issue and not affecting the data line...no biggie !!...here is a pic of a 2002 data lines !!

 

Yes, 1.3V was the data line (purple). The other three were .3V. This was on the 20V setting on my multimeter. The car started and ran fine, but it did before as well with the occasional start/stall. Ok, I will jumper each one and see which one is the culprit. No, I replaced my seats well before the onset of problems but if it's the culprit I will chase down that harness and see what's up. I'll report back my findings!

Colby

 

Check the other NO COMM...the TCS (EBCM)...pull off its comb...it is a light blue wire...see what that one reads...if you have a direct "short to power" on that wire you will see 12 volts !!... now you said you CAN communicate with those 3 modules on star connector 2 ??....the left and right door control modules and seat module ??...don't know if your HF scan tool can comm with all modules but if you can view "live data" on that module it's good !!...now like I said even if the EBCM is bad you can jumper the PCM and BCM and the car should start and run fine !!

 

NO COMM with all modules on Star 2 connector as the jumper has been removed. But with jumper on Star 2 removed I have COMM with all modules on Star 1 except EBCM. If EBCM had 12V it would be contaminating the signal to Star 1 and I'd still have the problems I had before (NO COMM to all modules), correct? Next step will be to jumper each module on Star 2 to see which one causes the system to flip out.

Colby

 

Removing BOTH "combs" isolates all the modules from the data line...with the scan tool if you jumper from the purple data wire to each of the module wires you will now be able to communicate via the scan tool...the EBCM if bad (short to power or ground) would only affect the data wire on that star connector because the comb is removed...the comb (jumper) ties them all together !!...yes jumper the modules on star connector 2 (4 wire)...hope these videos help !!

 

So my cause for rejoice was short lived. After work I came home to try jumper each Star 2 connector to purple but before I could all of my symptoms came back even with Star 2 disconnected. I then also disconnected the Star 1 jumper and still have all the same symptoms. However, with both Star connector jumpers removed AND NO JUMPER BETWEEN PCM AND BCM the car still starts and runs. My understanding was that it shouldn’t be able to start. I tested the purple wire at both Star connectors and the DLC for both 12v and short to ground... nothing. So with ALL modules isolated I still am getting data line corruption. I don’t have a scan tool and am only using a multimeter and the DIC on the IPC. Interestingly, with both Star connector jumpers out I do have COMM with the IPC. So with all modules ruled out perhaps the IPC is bad? What are the symptoms of a faulty IPC?

Colby

 

I jumpered the IPC/PCM/BCM to see if I still had my gauge cluster issues and to isolate any other modules from interfering with the signal from the PCM and BCM. Sure enough the gauges worked! So I created a fourth jumper wire and went down one at a time to find out which module was corrupting the data signal. None replicated my issue. So I plugged the comb back in and everything on Star 1 worked except for HVAC and TCS! I plugged in the Star 2 comb and my door windows worked! I was baffled, but this explained how the shop “fixed” it. They played around and it started working so they quickly handed me the bill and got it out the door. Sure enough, a few minutes later my gauges were still behaving but the windows once again didn’t work. So I took the Star 2 comb out and used my multi jumper and jumped both door wires to the purple wire and both doors worked again! Then it dawned on me, the common variable was the comb. It didn’t look corroded but I cleaned it up with sandpaper anyways and you doors worked again! I then did the same to the Star 1 connector and my TCS and HVAC came back online!!!! Fingers crossed but it appears this mystery is finally solved! A HUGE thanks to “C5 Diag” as I never would’ve been able to solve it without his help! He went so far as to call me and try things on his own car to see how it responded to better understand what my car was doing! I am forever grateful!

Colby

 

Colby,
So it was just the comb huh ??...I guess looks can be deceiving but hopefully this is a PERMANENT fix. I would suggest however using Deoxit to clean the pins of both STAR connectors and then use to Stabilant 22 as a contact "enhancer"...Stabilant 22 is a little on the expensive side but once the pins and combs are coated you probably never have to worry about those issues ever again.

 

Hey guys i have an 03 z06. Recently the windows have stopped working, my security system wont work, my seat wont move and my radio wont work. Everything that ive read on here makes me think i have a bad bcm and ive tried just about every possible solution that ive found on here and none of them have worked. Should i just buy a new bcm and reprogram it? And does anyone know how complicated it is to reprogram yourself? Thanks

 

nick.rynn, this thread is almost 5 years old, and I doubt you'll get any response. Better to start a new thread. Just sayin'......