2003 LB7 - need Exorcism (or CAN BUS guru)
 

This is a second post - my first was about 8 weeks ago and I'm still fighting the same electrical/transmission problem and my local shop, dealer, GM can't seem to fix this one. HELP!

History: Chevrolet 2003 Duramax (LB7 with Allison 1000). 95k miles
No history of fuel system or transmission work or previous problems. No programmer.

Problem:
Over a 2 week period, after 20+ minute highway driving, when slowing down shifting was different than normal. Truck would shift like it was in tow-mode, however tow-mode indicator was not lit. No MIL indications at this point. Took to local shop for a fluid and filter change.

First Service: (local shop)
Transmission fluid drained and replaced and both filters replaced. No visible material on internal filter. Truck drove without issue for 5 days, then the same shifting problem came back after about 3 days. This time MIL came on. Codes read were P0700, U2104, U2105.

Code: P0700 Modules: ECM, TCM Description: Transmission Control Module (TCM) Requested MIL Illumination
Code: U2104 Description: Controller Area Network (CAN) Bus Reset Counter Overrun
Code: U2105 Modules: TCM Description: Lost Communications With Engine Control System

Second Service: (original GM dealer)
Technician diagnosed that TCM needed re-program. Reprogrammed TCM and did not see any other issues. Truck drove ok for 3 days, MIL returned with same 3 codes.

Third Service: (local shop)
Transmission pan removed and inspected for any pinched wires. No issue. One of the two batteries reportedly had a bad cell. Replaced both batteries and inspected all ground connections. Inspected all TCM/ECM connectors for PIN connector problems. No impact, same issue.

Fourth Service: (local shop) Replaced and reflashed TCM (purchased directly from Allison supplier) and inspected all connectors. After 2 days, same MIL and same codes returned.

Fifth Service: (technician from local dealer)
Tested wiring harness, connectors and “CAN BUS” voltages. Technician recommended replace ECM. ECM replaced and re-flashed. Same MIL codes returned. Technician recommends take directly to Allison service center to “make sure transmission electronics are normal”.

What next?
Take to Allison dealer? (I don’t think that there is a problem in the transmission. When MIL was not set, transmission shifts fine) Replace FICM? (this is the only other computer that could be bringing down the CAN bus). Replace Ignition Switch? (no other codes ever set) Replace NSBU? (door lock and unlock automatically and no illumination errors seen on PRND)

Any further recommendations? Does anyone know a reliable Duramax/Allison shop in the North Texas Area that specializes in this kind of problem?

I'm almost at wits end...

Since no one wanted to touch this one, I'll at least report status.
Allison tech line says that the U2105 is the smoking gun.

TCC Solenoid communicates to TCM on wires 29/32. A broken or intermittent short or break there would cause the TCM to report CAN bus errors?

Anyone seen that?

you most likely have a bad wire in there somewhere that breaks connection intermittently.. I would hate to try and trace that one! but it looks like you got some good info from the tech line there.

You could try to possibly make a jumper wire or two from one plug to the other and see if the problem goes away. If it happens so frequently I would do one of the wires at a time on the CAN bus system and see if you can find your smoking gun.

Also and this might sound a little stupid but I have seen it on other newer vehicles and that is....have you checked the alternator? Not that it is putting out the correct voltage but to make sure it is not putting out any stray signals....such as AC voltage. If there is any sign of AC voltage it can mess with all the electronics on vehicles throwing bogus faults.

So to sum it up.
1. Check alternator first...I guess it would be the easiest thing to start with.
2. Make up a jumper/bypass wire to eliminate the trucks stock wiring on the CAN bus system.

Good Luck finding the issue

Update - Fixed finally!!! It was a bad FICM. Everyone said that they DON'T go bad. Well, I'll tell you that mine did.

Original codes - P0800, U2104, U2105

P1800 showed up later and tests pointed to the FICM. Tech says FICM was taking down the CAN bus signals.

Good luck if you ever have to go through that one. I wound up with a 2nd ECM and TCM for future use though.

Cheers,

Man those can be some of the worse to chase down. Hopefully everyone that worked on this didn't charge you full bore since they didn't fix anything..... Thats what has made it hard for mechanics everywhere....

wow, thanks for the info, im sure this will help someone else out in the future

Above post had a typo - original code was actually a P0700 not a P0800.