This is actually my second post about this I can’t find my first one for some reason and I have some updates. Long story short I started experiencing what seems to be a serial data malfunction where every module is throwing codes and warning messages on the dic at once. I fully charged the battery, checked fuses and removed the combs off of both star connectors to isolate the pcm and bcm with a jumper wire. None of this fixed anything. Assumed it was the bcm because jumping the pcm and bcm should have worked and I ruled out the pcm because I was able to communicate with it via a scanner. So I took it into a shop to let them replace the bcm because I was honestly over it at that point. Here’s a synopsis of everything that they did.


Technician Notes: tested all grounds and other modules on the serial bus, disconnecting the other modules made no change, believe the bcm is bad but cannot confirm without a new module to test. installed new module, no change. tested and cleaned all grounds related to modules on the serial network, no change, tested for shorts to power and ground at both the bcm and pcm, no issues found. removed and checked connections for fuse box, all good. inspected underside of vehicle for broken or melted wires and nothing was found, read multiple forum posts about issues similar to these and tested and tried every suggestion i read and nothing made a change. Isolating the serial network to the pcm and bcm also made no change. recommend vehicle be taken to someone who better knows the c5 corvette who may have more knowledge of the specifics of this exact vehicle and its electrical system as it seems to be very quirky and prone to issues similar to this.

So replacing the bcm made no change, I asked if they performed the bcm key relearn procedure and they did. So it isn’t the bcm, pcm, any of the modules, grounds are good, fuses are good, battery is good, what’s going on?? Last thing I can think of is maybe ignition switch? This is my first ever corvette and I am at a complete loss, any help is greatly appreciated.

 

Your original thread is here: https://www.corvetteforum.com/forums...l-of-this.html

 

Your ORIGINAL post was on 8/14…I am going to delete this duplicate !!…you can update your post there.

 

Alright

 

Did they check the connectors that connect the door wiring to the car, many people have posted that those connectors are prone to loose male/ female contacts. They are contained just inside the car past the rubber accordions.

 

Yes that was one of the first things that I did. I recently purchased a scope and discovered that the entire data bus is showing a 7 volt square wave as it should be. No module is pulling the data bus voltage up or down. However, I can’t jump the pcm or bcm for some reason it just says no comm. They only gain communication when I add the ipc to the data line as well. Had the ipc, bcm, and pcm communicating with eachother, neither of the 3 modules had any codes according to the dic and the problem still persisted. When I have just the pcm connected and I’m monitoring it on the scope it isn’t transmitting any data onto the bus, voltage is at zero. The bcm is putting out data albeit very slow, and when the ipc is added back on I’m seeing TON of information like 3x more than any other module, not sure if that’s normal. I installed a catback exhaust and intake manifold like a month before this all happened so I’m not too sure if that’s important at all. I’ve browsed a few dozen different threads across multiple forums documenting this issue and everything that ends up solving the problem for them isn’t solving the problem for me. So I’m just lost.

 

If you are seeing the 0-7 volt square wave on the data bus what is your problem then ??…if you jump the purple data line wire to each wire on each splice pack the scan tool should be able to communicate with that module…if you can’t check power and ground for that module.

 

Thats where I’m confused, how can I be having a serial data malfunction if the bus is good? I’ll attach a picture of what it looks like on my scope. I’ve tried jumping the pcm and bcm with the purple wire and I just get a “no comm” message for both modules. But the grounds to both of modules were checked already so??? I only regain comms with those modules when I add the ipc onto the network aswell. But like I said, when monitoring just the pcm on my scope it isn’t putting any data onto the bus it’s just hung down low at ground, is that normal? I’ll check grounds for both the pcm and bcm again tomorrow, I’ve verified that the bcm and pcm fuses are getting correct voltage, and when both splice packs are connected I don’t get any “no comms” messages to any module on the dic. I’ll keep at it because this is my first corvette and I don’t want to give up on it it’s really an amazing car, but man this is so weird.

 

With the key ON there should be that 0-7 volts on the PCM…you have to check powers and grounds on the PCM…have you done that ??…WHAT YEAR IS THIS CAR ??…you HAVE to put all this info in your profile so we know what we are dealing with !!…below is powers for a 2001…you have to check for power at the PCM connectors by back probing…2 ignition and 1 battery feed…Pin 19 C1 connector (pink) is battery feed and Pin 20 and 57 (orange) is ignition feed.

 

Check each ground (pins 1 and 40) at the C1 and C2 connectors with a 12 volt incandescent test light connected to B+ !!…you can remove both PCM connectors to check this…if test light illuminates ground is GOOD.

 

It’s a 1997 my apologies, I’ll get everything updated. I have not checked the connectors via back probing I will definitely do everything that you’ve suggested here when I’m off on Monday. Thank you for these helpful resources!

 

From reading this thread it seems like your PCM has a problem. If the BCM by itself is putting data onto the bus, but when you connect the PCM it stops transmitting data and you see no databus activity. That seems like it a PCM issue. The PCM should have signals on the bus. When you connect the IPC you see more data, that's expected because the IPC gets almost all of it data over the bus. What's not clear to me is what your actual problem is. Does it start, is the cluster just going crazy? You seemed to have power drain issues and low voltage. then no more. So when everything is connected what is the problem?

 

When everything’s connected, I get a ton of error messages on the dic. Service Trac control, low oil level, reduced engine power etc. Also my gas gauge is stuck on E even though I know it has fuel in it, my coolant gauge is at max temp and my oil pressure gauge is at zero. When I attempt to start the car it just cranks and never starts, or sometimes (very rarely) it’ll actually start and then kill itself immediately. The week leading up to all of this happening I was having intermittent anti theft issues where I would park the car, come back and try to start it up and it would kill itself and the pcm would throw a p1626 code. After a couple of tries though, it would eventually start. That led me to believe that it was the pellet in my key. But it would do it with both of my keys so that was unlikely, and seeing as the bcm was replaced it’s safe to say It wasn’t that. Like I said this started a week before the symptoms that I have now, prior to this I had no issues.

 

Here is some info to look at related to the P1626 code. https://www.scribd.com/document/369604851/203783 . This document provides the troubleshooting steps to be used to isolate the problem.

 

I found some time to do what you suggested today, the test lamp lit up as it should when making contact with those grounds. Also the two orange pins had the correct voltage, and the pink pin had the correct voltage when the key was in the ON position. Also pulled the current codes and here they are

PCM - No codes
TCS - U1255 H C
RTD - No codes
BCM - No codes
IPC - No codes
RADIO - No Codes
HVAC - B0363 H
LDCM - U1096 H
RDCM - U1096 H
SCM - No codes
BO RFA - U1096 H | U1000 H C

Also double checked the PCM’s output on the bus and still nothing, not sure why I’m not getting a loss of communication with PCM code from any module.

 

Thanks for that, but it would only throw that code back when I was having that issue. Ever since this serial data issue started I haven’t gotten any codes from the pcm.

 

I think you're making this more of a problem than it is. You have a communication problem with the traction control module and the remote function adapter. The BCM and PCM arent throwing any faults and the other faults are "history" and should clear when you erase them

 

I just unplugged both the traction control module and the rfa module and cleared all codes. RFA and TCS obviously shows no comm, and every other module has no codes, problem is still there.

 

History codes may clear when you clear them, BUT.....Depending on what code it is, if the problem is still there, it may not trigger any MIL lights until enough key cycles are again made. But they could still be present. I have a couple HVAC codes that as soon as I clear them, if I run a DIC check, they come up as History again. Not H and C (current), just History!! IDK why for sure, but the C5 electrical system has its share of gremlins. Sometimes I think that no two cars are exactly alike electronically, which they should be. Even GM never figured out the locking steering column....

 

FIXED!!!!

The thing that was causing all of this madness was none other than the PCM. I bought a new one and had it programmed to my VIN, after performing the security relearn it started right up! What I still don’t understand is how did the PCM go bad without throwing a single code or displaying a “no comm” message on the DIC? And why did it go bad in the first place, do the PCMs in these cars just randomly crap themselves? Nonetheless it is fixed now and I want to thank everyone on this amazing forum that helped me out. In particular I want to thank C5 Diag and Bill Curlee, you guys were very helpful. I’ll now summarize everything that was done to the car, that way it’s all in this one post and it’s easier for someone in the future to follow along if they are having similar issues. I personally think this is one of the weirder examples of a serial data malfunction but maybe I’m just biased because it happened to me. Anyways,

The first thing that was replaced was the battery, this of course did not help. After browsing threads on the forum I checked the connectors inside the accordion between the doors and the car and gave it a shake, didn’t help either. After that I had it towed to a shop (I do not recommend) where they checked all of the grounds and replaced the BCM, when that didn’t solve the problem they gave up and told me I had to find a corvette specific mechanic, this was when I realized I would have to figure this out myself. I then removed the star connector with 4 wires to isolate those modules and that did not solve the issue. I then removed the top from both star connectors and attempted to jump the PCM and BCM, still didn’t solve the issue. Was skeptical of the PCM since the BCM was replaced previously, but was thrown off due to the fact that it wouldn’t throw a code or a no comm message and I wanted to be sure before spending so much money on a PCM. After this I went and viewed the data bus on an oscilloscope to see what was going on, and to my surprise the data bus was normal everything was communicating as it should’ve been 0-7 volt square waves. This threw me completely off and I was getting frustrated. Ended up checking fuses for both PCM and BCM and they were good. Last thing I did was check power and grounds to the PCM via the c1 and c2 connectors and all was good. Eventually came to the conclusion that even though it was weird that the PCM wasn’t throwing any codes, it had to be the culprit. Also my check engine light wouldn’t come on when the key was in the ON position so that helped as well. In hindsight it was pretty obvious that it was the PCM, but this was my first time ever dealing with electrical problems and I was, and still am very ignorant to a lot of stuff in relation to diagnosing these issues. But regardless, I hope this helps someone out in the future