Hi,
I have very difficult situation with my 2008 C6 Corvette that has cost me many hours and thousands of dollars at a GM workshop, but without any success. The car is back in my garage now and I'm a bit desperate and trying with all sources of experts I can come to think of. I have a detailed description of the symptom and clear description of systematic test here below:

- The first symptom was that the the car suddenly refused to start. The first thing I did was to replace the battery with an almost new fully charged. That didn't help.
- Then checked the codes with my HP Tuners which showed this:


Sometimes I also got this when scanning:

... and sometimes this:


- In ACC mode the codes above are not raised, but switching to IGN mode (without trying to start engine) the codes show up.
- After that I took off, grinned metal surfaces and tightened the two groundings under the battery tray and the two at the front of the frame rails. Nothing changed.
- Next thing was to loosen all connectors where the can bus is passing, inspecting connectors, cleaned with compressed air and sprayed some "electronic cleaner" before attaching again. Nothing changes.
- With a Tech2 analyser I get connection correctly with all can bus connected units.
- The can bus signal looks like this with oscilloscope in ACC mode.

- In IGN mode the can bus signal looks like this.


- The following fuses in the engine compartment fuse box does not have any 12V in IGN mode. Don't know if it's because the ECM has disconnected power to these as a result of the DTC codes or if it's a fault by itself.
6 - O2 SEN (15A)
9 - PT INPUT/ETC (10A)
10 - MAN TRAN SOL (10A)
12 - ODD INJ (20A)
14 - EMISSION (15A)
16 - EVEN INJ (20A)
23 - RT HI BM (10A)
24 - LT HI BM (10A)

- There is no (< 40 mV) voltage drop between battery negative and ECM ground.
- In IGN mode the battery poles gives 12.08V while the ECM system voltage is 11.7V.

I have ordered a new engine compartment fuse box, but that is just a part of an "expensive guessing game".

Is there anyone out there that could possibly give some hint on how to proceed?

Thank you!

 

I didn’t even have to look at a wiring diagram to maybe figure this out…LOL !!…I am surprised a dealership couldn’t diagnose this…well I’m not surprised…they’re idiots !!…all those 5 circuits…O2’s, Emission (powers MAF and Purge Valve),even and odd injectors etc...are powered through Ignition Relay 44….swap that relay out with another 4 pin relay…horn relay is a good one to use…also with relay removed you should have 12 volts to 2 of the 4 pins of the relay…check that !!…those pins are “hot at all times”….if you were to jumper pins 87 and 30 (H14 to F13) at the relay those fuses should now be powered.I will also have to see what controls “Ignition 1 Relay Control” at the ECM…if the ECM is not grounding it the relay will not work…I’m impressed you have a scope…the waveform looks fine…they are supposed to mirror each other and CAN high goes from 2 1/2 up to 3 1/2 volts and CAN low goes from 2 1/2 down to 1 1/2…it operates on a 2 1/2 volt bias…I can’t see your voltage settings…you are able to communicate with all modules so it’s not a comm issue !!…diagram is from an O5…ECM does NOT remove power because of those DTC’s…and WELCOME to the Forum !!

 

Thank you C5 Diag for a quick reply!
I rushed out in the garage and swapped the two relays you suggested, but that didn't make any difference. Pin E5 is never grounded by the ECM, most likely due to the can bus error it detects. I think the root cause is the fact that the ECM detects a can bus error and refuses to activate relay 44.

 

Can you communicate with the ECM with the scan tool ??…if you can’t and that is the ONLY module you can’t talk to for a single module “no comm” you would be checking ECM power and grounds…that is the first step…if you were to jump the load side of the relay you will power up those fuses.

 

Yes, I can communicate with the ECM with my HP Tuners. However, I don't have a Tech2 tester, but according to the workshop that tried to help me, they could communicate with all units including the ECM on the CANbus. I cannot even think of any reasonable explanation to all this. It is so strange.

 

If you can communicate with the ECM and look at “live data” you don’t have a comm issue with the ECM…did you remove the relay and check for power at 2 of the 4 terminals at the relay with a test light…this is crucial !!…do you see a P0685 stored ??…like I said if you can jump pins 87 and 30 at the relay and those fuses shoukd be powered up…you need to concentrate on the control side of the relay….I think years ago I diagnosed a P0685 and it was a bad ECM….you may have a bad ECM too…i believe that owner changed his fusebox too…I would also remove the ECM’s C1 connector and see if you have continuity between F14 at the relay and pin 10 (white/black wire) at the ECM looking for an open…the driver may be closing but if you have a broken wire between the ECM and the relay the coil side of the relay will not work…get what I’m saying ??

 

Thank you C5 DIag for all your input. It's late here in Sweden right now, but I will definitely go through all your recommended steps first thing tomorrow. I am still confused by the fact that I get U2105 and U0100 when switching on ignition despite the Tech2 tester could communicate with the ECM (according to the workshop that had my car).

Thanks again!

 

You’ve looked at the high speed can and it looks ok…did you check low speed (Class 2) ??…on your scope you should see a 0-7 volt square wave at pin 2 at the DLC with Ign ON…12 volts or at ground no good !!

 

I do have signal on the low speed (Class 2) pin 2 at the DLC and I can read the full tune from the ECM/TCM, scan for error codes etc. I have replaced the relay 44 and measured E5 at the fuse box once again. It is never pulled to ground by the ECM. It's still a mystery why U2105 and U0100 shows up when switching to IGN.
I think I'll go for a new E38 ECM and try to flash it with my HP Tuner (Write Entire).

 

Have you checked for power with a test light at the load and power side feeds of the relay ??…they are H13 and H14 in the schematic….did you remove the ECM connector and checked for continuity between F14 of the relay and E5 at the ECM ??…we know E5 isn’t being pulled to ground but you must make certain the wiring between those 2 points are good…the relay didn’t need replacing because you swapped relays and the fuses were still not being powered.

 

Yes, H13 and H14 at relay 44 both have 12V. I have measures the connection from the relay socket pin F14 to the ECM X1 connector pin 47. Seems it boils down to either a broken ECM or CANBus failure making ECM unwilling to activate ignition relay 44. I have ordered a Tech2 instrument (clone model) to see for myself the status of all the CANBus modules.

 

I’d try jumping pins 87 and 30 (H14 to F13) and make sure those fuses are being powered…you HAVE to follow a process checking these circuits…I just left a message with a former co worker of mine that owns an auto electric shop…just waiting to hear back what closes that output driver at the ECM…now you can “be that ground” by removing the relay and taking some thin copper wire and wrapping it around pin F14 and then reinstall the relay…with a 12 volt test light connected to ground if you were to touch it to the copper wire the relay will operate…do you have any trouble codes stored right now with this ??

 

Keep in mind that you can’t install/flash a complete operating system on a E38 ECM with HP Tuners.

 

Does HP Tuners have bidirectional control ??…if it has bidirectional control you can try to activate that ground under output controls…a Tech 2 has “output controls” that can command the driver to close…if it can’t the ECM is bad !!…maybe you can get your hands on one !!

 

Thank you C5 Diag!

I followed your instructions and yes, all those fuses are being powered. I also put back the relay and removed pin 47 from the X1 connector and grounded it manually. The relay operated and the related fuses were all powered. At this state (with IGN on) I cleared the DTC codes and read them again. I still get CAN Bus error, but only U2105.

 

Yes, so I heard from some other sources as well. However, some others says you can, despite the HP Tuners omit two segments of the ECM when performing a "Write Entire". Don't really know who to trust, but I'll prefer to go the safe route. Just ordered a Tech2 (clone version) that is supposed to support SPS etc. Now I'm googling for some info on how to perform a safe E38 replacement the right way.

Thank you!

 

I doubt it has, but I'll check. I'll wait for my Tech2 to arrive and make that test you suggested.

Thanks again!

 

My friend who owns the auto electric shop says the ECM may be looking for an input from the start button…he’ll look into it…with the Tech 2 you’d want to find those “output controls” function which allows you to activate those drivers like fan relays, fuel pump relays etc…

 

Super !!…well I guess the ECM isn’t controlling it so let me look up that U code !!!

 

The 2105 is a loss of communication with the ECM…if relay 44 isn’t working the load side of the relay does not power the ECM as you can see in the schematic….for any no comm you would want to check the ECM’s power and grounds but it flagging the code because 12 volts isn’t getting sent to the ECM…I “think” !!