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Hey Everyone- Currently battling a handful of very interesting problems on my 2001 C5. Currently can get the car to crank and it will turn over for a second but will only stay running if I go WOT, and even then it will just sit at 1.5k RPM's. I am getting a "Reduced Engine Power" on my DIC, so I tried to pull codes to diagnose this but sure enough i'm not getting power to my obd2 port so nothing was showing up on either of my scan tools. I then held the option button and tapped the fuel button four times to pull up codes on the DIC, literally every single module came back with a no COMM message If anyone could give me a good place to start on the fact that it's in this kind of limp mode and the fact that my serial data doesn't want to communicate with any part of the car. The previous owners of this car did not give it nearly enough love and I just finished replacing the diff, this car is newly in my possession. If I can't figure this out i'm likely just going to throw it on a Holley terminator ecu and harness as I need this thing running before the drift season starts up again. Any help is greatly appreciated, thanks!
So definitely a seriel data buss issue, problem maybe as simple as bad electrical connection between either the driver or passenger side door and car. There is a connector in the accordion tube between door and car that develops bad electrical connection usually the drivers side. There is also a way of pinpointing what module is the problem and that is done by accessing the two connectors that the seriel data buss is distributed. The connectors are located by the BCM. 830A
Thank you so much for the quick response mmartinez. That would make a lot of sense as both sets of controls for the windows, mirrors, and elocks do not work; however I was probing for power and ground at the connector on the drivers side but didn't test for anything serial data related. Would I test for this through ohms resistance or would I need a specialty tool to read serial data? And are these connectors located near the BCM or plug into it? Once again thank you very much for the help.
If power or ground to the doors is bad that in itself will cause the seriel data buss to crash. You do need a scan tool or oscilloscope to view the data pulses on the seriel data line. The two connectors I mentioned are beside the BCM and have a metal jumper comb that connects the seriel data buss wires, and you can remove both combs and connect just the PCM and BCM with a wire jumper which will eliminate all other modules, a internet search will explain how to do this.
Ok, let’s try this:
1) Locate the 2 Star Connectors to the left of the BCM and remove the buss bar or “comb” circled in the last picture from star connector #2…it has 4 wires on it…see if the car starts…if no start go to next step.
2) Now remove the comb off the other star connector leaving both combs removed…make a jumper wire and jumper the BCM to PCM…light green to dark green wire….if car starts you have a module other than the BCM and PCM on star connector #1 pulling the data bus down….if car still doesn’t start the module pulling the bus down is either the BCM or PCM…go to next step.
3) Take your jumper wire and connect from the purple wire (data bus wire) to the PCM (dark green wire)…now with your scan tool see if you can communicate with the PCM…if you can now communicate the PCM is good and issue may be the BCM…to see if the BCM is indeed bad take the jumper wire and connect the jumper now to the BCM (light green)…with a scan tool that can communicate with ALL modules (NOT a code reader) and if you can’t communicate we can “assume” BCM is bad…for single module “no comm” you want to check the powers and grounds to that module.
If car starts when removing the comb off star connector #1 or 2 you will need to see if the bus is shorted to power or ground with key ON…video shows using scope but with DVOM if you see the voltage fluctuate from .2 to .4 volts that module is communicating….you will see 12 volts (shorted to power) or near 0 volts (shorted to ground).
Wow, thank you so much for this C5 Diag I cannot thank you enough. I will try this today and will relay my findings, seriously thank you so much; people like you make this world continue to spin
No problem !!…you can actually ground out or put 12 volt power to the data bus (pin 2) to recreate what you’re seeing…it won’t damage the modules because they are “fault tolerant” !!…if you disconnect the battery and with an ohm meter going between pins 2 to 4 you should see around 8,800 ohms (8.8K ohms)…if less than 350 ohms the data bus may be shorted to ground.
So now that I have fully read and taken notes your comments, I think I have a grasp on what you're saying. So do the Combs in the connectors allow all the modules to communicate with one another and can I jump BCM to PCM with a back probing kit? Correct me if I'm wrong but my understanding of this process is that we are essentially singling out the modules in the vehicles by separating them through removing the combs in the connectors, therefore allowing us to run them independently and test them? If I'm wrong, please do correct me but I'm just trying to get a better idea of this as I've done a good bit of wiring but I'm really trying to get a better grasp on CAN. I will be off work by around 3ish and will send pictures of my findings. Your knowledge is absolutely priceless, and I cannot thank you enough.
The comb connects all the modules together so all modules can communicate…the BCM and PCM have to be connected together to allow the car to start…if one or the other can’t communicate the car won’t start….if you have 2 back probes just insert into those 2 terminals…a good class 2 is a 0-7 volt square wave on a scope but on a DVOM you will only see the voltage fluctuate…for network communication the key has to be on…this is not CAN but the older and slower Class 2 Serial Data network…the C6 such as on my 2008 has a high speed CAN and the Serial Data network.
Awesome, thank you so much for the explanation. So I just pulled the cover to reveal the BCM in the passenger side footwell and sure enough I am missing the Comb for the star connector with four wires (I believe you said that was Connector #2). I will now try to jump the BCM to PCM and will let you know how that goes, will update soon.
If that comb was missing the car should start anyway !!…the comb may be in there but the grey cover may be missing…you would only have no comms with the modules on that Star Connector.
That makes sense, however no there is definitely not a comb here and it looks like someone has messed around with the wiring in the BCM trying to tap into it. After jumping the PCM to BCM it will start for a second but will stall out shortly after. There is also no comms to my scan tool or my Tech II. I will keep following your instructions, looks like someone seriously messed around with a lot of the wiring around the BCM.
That mess is something I can’t help you with !!…this summer C&S Corvettes which is a highly respected shop in nearby Sarasota Florida called me to help them sort out a similar wiring issue…my response to Lyle was “NO THANKS” !!
I do not blame you for that response! So I somehow ended up figuring it out... Looks like previous owner used the yellow wire at the top of my photo with the red wire coming out of it to rob IGN power for the radio, I ended up removing that nonesense and sure enough that yellow wire was essentially split into two! I re soldered the wire and heat shrinked it, sure enough the thing fired right up and I could finally drive! It was a shot in the dark but it worked Thank you for all your help you are a gem
C5 Corvette Reduced Engine Power DIC
If anyone could give me a good place to start on the fact that it's in this kind of limp mode and the fact that my serial data doesn't want to communicate with any part of the car. The previous owners of this car did not give it nearly enough love and I just finished replacing the diff, this car is newly in my possession. If I can't figure this out i'm likely just going to throw it on a Holley terminator ecu and harness as I need this thing running before the drift season starts up again. Any help is greatly appreciated, thanks!
Thank you for all your help you are a gem